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J. Phys. D Appl. Phys. 25 (1992) -23.Printed in the UK

I REVIEW ARTICLE1 Magn etic ch arac terization of recording I1 mediaR W Chantrellt an d K O'Grady#t Physics Department. University of Keeie, Keele, StaffordshireST5 5BG, UK* School of Electronic Engineering Sc ience, UCNW, Bangor, Gwynedd LL57 1UTU KReceived 1 May 1991, in final form 2 September 1991

Abstract . Magnetic recording media are important technology materials whosebehaviour remains very poorly understood. However, magnetic measuremen tsform a very important part of t h e practical charac terization of recording media. Thisreview outlines the current techniques of magnetic characterization and considersth e effects of current research developments.

1. IntroductionMagnetic recording has been around in some formsince early this century, although most of the growthhas occurred during the past 2&30 years. The fieldnow encompasses a diversity of applications such asthe consumer marke t of audio and video recording, inaddition to the storage of digital information on rigidand floppy disks. This varied range of materials andtechniques is often referred to as m agnetic informationtechnology ( MI N T ) . Magnetic storage of information, inwhatever form, has many innate advantages, par-ticularly in terms of erasability coupled with long-termstability of the stored data, and is l ikely to remainviable for the foreseeable future. However, far frombeing a mature and established technology, MINT ispresently facing enorm ou s challenges, particularly asregards the increased requirements of informationstorage density and faster access times.

These challenges are presently being met by dev-elopments in particulate recording media, althoughmany alternative materials and techniques are underconsideration. These include thin film media and mag-neto-optic recording, and the vertical hloch line tech-nique which has considerable theoretical promise. It isnot possible to cover the many aspects of thesematerials in a brief review therefore this review willconcentrate on the properties and physical problemsconfronting the development of existing media. It isnot possible, h owev er, to consider the physical pro per -ties and characterization of the medium in isolationfrom the recording process. For this reason we startwith a n overview of the analogue and digital recordingprocesses, which provides the necessary background0022-37271921010001+23 $03.50@ 1992 OP Publishing Ltd

for the full consideration of the scientific problemspresented by the media themselves. It should also heacknowledged that although the physics of recordingmedia is a central problem in the field, magnetic rec-ording spans many scientific and engineering disci-plines, and covers areas outside the scope of thisreview. There are, however, many excellent hookscovering the whole of the field of magnetic recording,particularly those by Jorgensen (1980) and Mee andDaniel (1987).2. The recording processA recording system consists of a transport mechanismfor the medium and one or several transducers bymeans of which information is t ransferred to and fromthe medium. In addition to this there is also externalcircuitry to process the information. In this review weshall consider only the physics involved with thestorage and replay of the information, which dependscrucially on the recording technique. It is useful toconsider in detail the most common techniques, whichare broadly categorized as satu ratio n recording of digi-tal information and analogue recording of audio andvideo signals. Each of these techniques makes manydifferent demands on the recording medium and so itis necessary to consider the behaviour of recordingmedia in the specific context of the intended appli-cation.2.1. Digital recordingThis is the most easily visualized form of recording.The medium, in the form of a rigid or floppy disk

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R W Chantrell and K OGrady

L O G I C and W R I T ET I M I NG C I RCUI lS A M P L I F I E R S

Mot ion O f m e d i m

!

Region 1 n,M M 2 Region 2.

recorded signal

replayed signal( b )

Figure 1. (a ) Schematic of the digital recording processusing a ring head. (b ) Schematic write and read waveformsfor the digital recording process.

carrying a thin magnetizable film is rotated at highspeed bene ath a magnetizing hea d, as is shown in figurel(a). Another important configuration is the standardreel-to-reel transport of magnetic tape. This is a rela-tively inexpensive form of digital information storage,but it doe s not have th e rapid access tim es off ere d bythe rigid or floppy disk a nd is only useful fo r long-termstorage and back-up facilities. In all cases the storageof a binary digit is achieved by means of a currentpulse in the recording head. This consists of a gappedtoroid of soft magnetic material , wound with an ener-gizing coil to which the current pulse is applied. Theresult is a fringing field in the medium, which can bemade larger than the coercive force, thus causing themedium to be magnet ized. O n readback , the recordedinformation gives rise to an induced voltage producedby the Hux changes in the head. The write and read2

waveforms a re of the form shown in figure l (b ) ; elec-tronic processing is applied afterwards to the read sig-nal in order to retrieve the original squar e pulse. Th edigital recording process has been investigated ana-lytically by Middleton and Davies (1984) and Potter(1970) and many others.Magnetically th e process is similar to th e acquisitionof the remanence following the application of a non-saturating field, the isothermal remanent magnet-ization (IRM) to be defined later, although importantly,the process take s place in a very short t imescale, ofte nless tha n a microsecond. A further complication is thespatial variation of the head field throughout themedium, which is taken into account in the vectormodel of the recording process (Ortenburger and Pot-ter 1979, Po ttcr and Beardsley 1980, Beardsley 1982).The se models are l imited by a n imp erfect under-standing of th e recording medium itself, fo r reason s tobe discussed fully later. Similar considerations alsoapply in the case of analogue recording. Before con-sidering analogue recording it is importan t t o poin t outsome of the purely mechanical problems of magneticrecording which arise because of the na ture of thehead/medium interface. Consider, for exam ple, aWinchester disk, which may have a rotational speed inexcess of 3000 RPM. The Hying height of the head overthe medium is of the order of tenths of microns, sothere is an obvious requirement of a clean surface forthe medium. T he m otion actually creates an a ir bearingeffect which reduces we ar, but on th e oth er han d intro-duces a spacing loss which is an imp ortant l imit on theresolution of recorded data.In the case of ta pes the re is often con tact betweenthe head and medium, resulting in considerable prob-lems of wear, which can be m inimized by th e inclusionof lubricants in the tape formulation. There is also apressing need t o k eep th e surface roughness as low aspossible. Such tribological problems f orm an im portantarea of study which can make a vital contribution tothe further development of recording technology. Forthe most part, however, this review will concentrateon the magnetic behaviour and characterization of re-cording media, since in this area considerabie progressis possible on a fun damen tal level.

2.2. AC bias recordingAC bias recording, although somewhat complex as amagnetic process, is an extremely neat solution to theproblem of maximizing the linearity and response ofthe medium in analogue recording. The problem isillustrated in figure 2 . Curve a here shows the iso-thermal remanent magnetization ( I R M ) as a function ofthe applied field. This measurement will be describedin more detail later, but is essentially the DC responseof the system. I t can be se en that th e re spon se is verynon-linear. The first attempt to linearize the responseinvolved the application of a DC bias field Hd to ensurethat the medium was operating in the optim um range.

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Magnetic characterization of recording mediaI Q

P

8

7--Z 6x-i\rI

3

2

1

0 12 I 6 8 IQ 12 1I I 6 I8 20applied fie ld / n e

Figure 2. An experimental compar.son 01 th e tsothermal(curve a) and anhysteretic (curveb) remanentmagnetization, showing th e extended linear region lo r th elatter case.

The technique of AC bias is a later and much moreeffective method. Essentially the technique involvesthe application of a large, high-frequency bias fieldwhich is reduced to zero in a timescale over which thesignal does not vary appreciably. The closest physicalmeasurement to this process is anhysteresis, in whichthe signal field remains completely static while the A Cfield is reduced. Anhysteretic measurements are alsoshown in figure 2 (curve b). In low fields the responseis very linea r, and charac terized by a large value of theanhysteretic susceptibility xi.. In practice, as the tapeleaves the region of the head the signal and AC fieldstrengths reduce simultaneously resulting in a re-duction of x. by perhaps a factor of two. This,however, remains the most successful method of rec-ording analogue information.The techniques of analogue recording utilize twoformats. Audio recording uses a stationary head,whereas in video recording the head is rotated at anangle to the tape transport direction (helical scan re-cording). This makes optimal use of the whole area ofthe tape for recording, as is necessary in order to storethe high information density required for recordingvideo signals. A similar technique is used in the rotarydigital aud io ta pe (RDAT) system.

AC bias recording is a very important technique,but on e whose physical basis is very poorly unde rstood.This is essentially because of the strong dipolar inter-particle interactions which are known to dominate theprocess. A further complication is the lack of under-standing of the noise spectrum of a recording mediumwhich is also strongly dependent on interactions. In

Figure 3. Schema tic hysteresis loop defining t he primarymagnetic quantities used for the characterization ofrecording media.

fact, the interaction problem is central to the fun-damental physical investigations of these materials, aswill become clear later. A further difficulty is that ingeneral tapes a re characterized in terms of their staticmagnetic properties, whereas anhysteresis and the ACbias recording process ar e clearly dyriamic phen om ena.The relation between the static measurements and thepractical (dynamic) response of the medium duringthe recording process is still very much an unsolvedproblem. Real progress in this area will only be poss-ible given a much better fundam ental understanding ofthe physics of recording media. Th e current high levelof activity in the field is very promising in this respect.

3. Basic m aterials requirem entsWe shall shortly go on to consider the physics of themost common types of recording media. Before doingthis, however, it is worthwhile stating explicitly thebasic specifications for a useful recording medium inorder to provide a link with the technical aspects ofrecording and additionally to introduce the primarymagnetic quantities by which a medium is charac-terized. The basic physical attribute of a magneticmaterial which makes it usable for information stora geis non-equilibrium behaviour. This can be consideredas introducing a memory which is clearly essential toany information storage technique. In the case of amagnetic material the non-equilibrium behaviour isrepresented by the hysteresis loop obtained by meas-uring the magnetization M as the applied field H iscycled.Figure 3 is a schematic hysteresis loop illustratingthe primary magnetic quantities. These are the coer-civity H c , the saturation magnetization M , and rem-anen t magnetization M,. From this we can also definethe squareness of the loop S, =M,/M,. A useful re-cording medium requires a large enough value of M ,coupled with a large value of squareness, the actualvalues depending on the details of the application. Therequirements relating to the coercivity are somewhatmo re stringent. Ideally H , should he large, in order to

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R W Chantrell and K OGradyresist demagnetizing effects, although clearly increas-ing H , makes the write process more difficult. Thepractical value of H , depends upon the application.The smallest values of H , tend to be in low-densityapplications. Here, coercivities range from -300-400Oe and tend to increase with increasing infor-mation density, up to values in excess of 1000 Oe asthe specification fo r video recording.No magnetic material has a perfectly well definedfield at which magnetization reversal tak es place, sincethis would result in a square hysteresis loop which isnever observed in practice. It is per hap s useful to con-sider a recording m edium as comprised of characteristicactivation volumes, each of which has its ow n 'switch-ing field' at which it reverses its state of magnetization.This param eter depen ds on detailed considerations ofthe material and the reversal mechanism as will bediscussed later. What is important in th e p resent con-text is the fact of a distribution in these intrinsic coerc-ivities, know n a s th e 'switching field distributio n' (SFD) .The SF D naturally tends to reduce the squareness ofthe hysteresis loop, and a convenient measure of thewidth of the SFD is obtained from the Williams-Com-stock (1971) construction which is shown in figure 3.The parameter (1 - S*), which characterizes th e widthof th e SFD, is essentially a measure of the slope of th ehysteresis loop at H =H,:

This param eter has been recommended (Koe ster 1984)as the hest single para me ter for the cha racterization ofrecording media on a practical basis. Other methodsfor measurements of the S F D , based on remanencecurves will b e discussed in detail later.The SF D has a considerable effect on t he analoguerecording process as shown by Ko este r et a1 (1981) whofound that the high-frequency output was drasticallyreduced by increasing (1 -S*) in a series of tapes pre-pared by mixing cobalt modified iron o xides. Th e opti-mum bias current was found to decrease at a rate of-5.6 dB per unit (1- S*). Th e origin of this effect isascribed to the intrinsic spread of coercivities associ-ated with (1 -S*) coupled with the overbiasingphenomenon to be discussed later. Generally a small(1- S ' ) value is required for good recording proper-ties. In digital recording the information storage den-sity is limited by the minimum transition width asdetermined by demagnetizing effects. Essentially, themaximum demagnetizing field for a thin medium withan arctangent magnetization transition is H d =-16/2nwhere a is the transition width (here, and in the restof the paper, the symbol I is used to represent themagnetization in EM U cm-) ( M =4x0). Consequentlythe important parameters are the coercivity H , filmthickness 6 and remanence I,. Potter (1970) has shownthat the minimum transition width is (for a squarehysteresis loop with 1 - S* =0)4

a=61r/Hc. (1 )More recently, Bertram (1986) gave a more generalrelation for a medium with a finite value of (1 - * ) :

a = d ( l + * ) + [ ( d ( l - S') ) 2 + - ] Ir6d ' I 2 (2)

where Q is a param eter related to th e head field and dis the head to medium separation. G enerally speakingequation (2) is dominated by the second term with onlya weak dependence on (1 - S*). I t should be notedthat if equation (2) predicts a smaller transition widththan the demagnetization limited case (equation (1))the latter should he chosen. T hu s in saturation digitalrecording the relevant static properties are I, and H,.From the previous discussion it is is clear that theprimary characterization of recording media should bein terms of the static properties, a nd this is certainly th ecase in practice. However, there are many additionalimportant magnetic and electromagnetic propertieswhich characterize a medium. For the remainder ofthe review we shall concentrate on introducing thoseproperties and relating them to, on th e o ne h and, th erecord/replay process itself, and on t he o the r theunderlying physics of the medium. Before this,however, we describe briefly the preparation of par-ticulate and thin film recording media, which intro-duces many important factors relevant to theircharacterization.

nQ n Q K

4. Preparation of magnet ic record ing media4.1. Particulate mediaElongated particles consisting of a single magneticdomain are the most common magnetic recordingmedia in use today. A wide variety of particles areused and in this section only a brief description of th epreparation techniques commonly used is possible. Ofcourse, much of the detail of the particle preparationprocesses is proprietary to the major manufacturersand :hi ieadir should bc iiiidci iio ilktsion as :o :hclevel of care and sophistication that is involved in thebulk preparation of the highly uniform particles whichare in current use.To prepare particles which have good magneticproperties and dispersibility, direct precipitation tech-niques should ideally h e use d, which will result in p ar-ticles which have smooth surfaces and are highlyuniform in terms of size, anisotropy, etc. Unfortunatelymany of the particles which have the appropriateproperties cann ot be prepared directly an d m ore elah-orate techniques are required. These techniques arereviewed below.4.1.1. Preparation of elongated y-Fe,O, particles. Th emost widely used material fo r all recording applicationsconsists of elongated gam ma ferric oxide particles. T heparticles used are between 0.4 and 0 . 8 p m long with

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Magnetic characterization of recording media

Feso , no 0i ,,.",4Na OH I

D - l F e O l O HG o e t h i t ei r l h o r h o m b i r I

Figure 4. Th e principal stages in th e production of elongated y-Fe203particles lor recording media.

aspect ratios between 6:l and 12:l. Th e basic reactionto form the particles is shown in figure 4.Needle-shaped iron oxyhydroxide FeOOH is grownon precipitated seeds from a solution containing ironsalts (typically FeCI2). Usually orthorhombic a-FeOOH, synthetic geothite is used. The dehydrationand reduction processes usually require temperaturesup to 700C which can result in the particles sinteringtogether. Accordingly the FeOOH particles ar e usuallycoated with chemical complexes often incorporatingother ele men ts such as zinc, nickel or tin which inhibitsintering and are also instrumental in determining thesize and the aspect ratio of the particles.It is this step in the preparation which is vital tothe morph ology, dispersibility and magnetic propertiesof the final product. Thus the precise nature of theadditives and the method of their utilization is pro-prietary to the manufacturers and is not revealed.The reduction process is usually undertaken in ahydrogen or hydrogen rich atmosphere produced usinga variety of reducing oils. T he final controlled oxidationof the synthetic magnetite to the defect spinel y-Fe20 ,is undertaken by heating to between 300 and 400C.This stage of the process much also be carefully con-trolled to avoid the transformation of y -F e2 0 3 toweakly magnetic a-Fe203. he final value of the coer-civity can be enhanced at this stage by failing to com-plete th e oxidation proces s leaving a final product witha composition (Fe,O,),(Fe,O,), --I (synthetic bethol-ite).The final product is mechanically compressedbetween rollers to reduce the porosity of the particlealthough care must be exercised at this final stage toavoid damaging the particles which results in a wellknown reduction in the coercivity. During this finalstage initial dispersants and other chemicals may beadded to coat the particles thereby assisting the event-ual dispersibility.

In this process it is essential to recall that theproperties of the final product are essentially deter-mined by the initial seed formation process. In recentyears synthetic lepidocrocite (y-F eO OH ) has also beenused commercially as the precursor for y- Fe2 03.The eventual product is a brown (tan) free-runningpowder with coercivities which range from 260 to38 5 Oe. For particles which include a limited amountof Fe 30 a the coercivity can be as high as 425 Oe . Ingeneral the final value of the coercivity for the particleswhen dispersed and coated on to the s ubstrate can differfrom the original powder by up to 20 O e.For a full review of the preparation of y - F e 2 0 3particles the re ader is referred to Bat e (1980). Table 1summarizes the typical properties and applications ofy - F e 2 0 3particles.

4.1.2. Metal particles. In a purely historical context itis interesting to note that the first magnetic recordingtape was coated with elemental iron particles pro-duced by the thermal decomposition of iron penta-carbonyl. However, small particles of the ferro-magnetic elements are highly reactive and in fact maybe pyrophoric when exposed to air. They may also beattacked by the dispersants or binders used in the tapecoating process. Accordingly they were not used untilthe recent increase in demand for media with highmoment and coercivity for applications such as digitalaudio tape and 8 mm video.Elongated metal particles can be prepared by twobasic routes: either the reduction of oxides or saltsof the metals or the decomposition of organometalliccompounds or complexes. The f orm er method usuallyinvolves the reduction of any of the precursors of y-Fe,O, particles using hydrogen or othe r organic reduc-ing agents. Alternatively iron, cobalt or iron-cobaltalloy particles can be prepared by the borohydride

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R W Chantrell and K OGradyTable 1. Typical properties and applicationsof y-Fe203particles. Source: Bayer UK technical datasheet.

Particle Aspect M, (=4nls) CoercivityApplication length (pm) ratio (Gauss) (Oe)Audio and computer tape, floppydiscs 0.7 6:l 4000 285Lo w noise audio and video tape 0.5 10:l 4100 385Computer tape, floppy discs 0.6 8 : l 4000 305Professional audio tape 0.4 7:l 4100 300-380

reduction of solutions containing the metal salts (Oppe-gard et a1 1961). The latter process includes carbonyldecomposition, formate decomposition, etc, but theseprocesses are expensive due to the cost of the pre-cursors and have not found wide application.The key to the preparation and use of metal (usu-ally iron) particles lies in the passivation of the particlesurface. Due to the reactivity of the metal, organiccompounds are in general unsuitable and other metalor oxide coatings are preferred. For example Aonuma(1975) incorporated chromium and potassium sulphatein the borohydride reduction of metal salts to inhibitoxidation and produced a tape with H , =1000 Oe anda loop squareness of 0.81.The preferred and currently the only utilized tech-nique for the passivation of iron particles for recordingis the controlled oxidation of the particle surface, leav-ing a core of metal which comprises approximately50% of the particle volume. The oxidation reduces theeffective value of the specific magnetization of the bulkmaterial to around 150EM U g-' but still results in aremanence of double that of a fully oxidized particle.As is the case for y-Fe203 he coercivity is entirelycontrolled by the particle elongation. However, due tothe high moment, particle alignment during the coatingprocess is greater resulting in a loop squareness whichis generally higher than that achieved for y-Fe203andcomparable to the values obtained with C r 0 2 section4.1.4).4.1.3. Cobalt-modified iron oxides. The substitution ofcobalt ions for iron gives rise to a large increase inth e magnetocrystalline anisotropy of iron oxides. Thecobalt saturated oxide COO Fe 203, cobalt ferrite, isunique amongst the inverse spinel ferrites since it hascubic anisotropy with three easy axes along the cubeedges. Cobalt ferrite has a large intrinsic anisotropyconstant, K , , of 2 x 106ergcm-3 so it is hardly sur-prising that cobalt inclusions in y-Fe20, can raise thecoercivity to the region of 1OOOOe or greater ifrequired. The inclusion of cobalt into iron oxides canbe achieved by a variety of techniques. y-Fe203 canbe doped uniformly (body doped) with cobalt by add-ing cobalt salts to the solution before the precipitationof FeOOH or by precipitation of CoOOH onto theFeOOH particles after formation. The remainder ofthe process is as described in section 4.1.1. Theresulting particles exhibit multi-axial or isotropic mag-netic behaviour and are in general spherical or have6

axial ratios

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Magnetic characterization of recording mediaTable 2. Typical properties and applications of cobalt modified iron oxides. Source: Bayer UK technicaldata sheet.Application Particle Aspect M, (=4n/.) Coercivitylength ( p m ) ratio (Gauss) (Oe)High bias audio tape, Betamax video tape 0.45 10:1 4400 595VHS video tape 0.4 1O:l 4400 630High-density data tape, video tape 0.4 8 : l 4300 760-980

Table 3. Typical properties and applications of CO2.Sou rce: DuPont product information.Particle Aspect M, (=4n/,) CoercivityApplication length (pm) ratio (Gauss) (Oe)

Data tape 0.4-0.6 1O:l 4390 47C-670Video tape 0.3-0.4 1O:l 4390 470-620Video tape 0.4 1O:l 4390 485-540

gives examp les of various particles which are availableand their typical properties.4.1.4. Chromium dioxide. At the present t ime elon-gated particles of chromium dioxide (CrO,) are secondonly in im portan ce to y-Fe,O, as particles f or recordingmedia. CrO, particles have found application in videorecording and high-density data recording media,although Co-doped y-Fe,O, particles ar e now findingapplications in these areas. The common use of CrO,arises because it is possible to grow particles in theform of needles with very parallel sides, excellent mor-phology and an absence of dendritic growth.There are several methods available for the prep-aration of CrO, particles, but the main technique usedis the hydrothermal reduction of CrO, (Swoboda et al1961). The trioxide is mixed in equimolar ratio withwater and heated to between 400 and 50 0C at a press-ure of around 5 x lo6Pa. The reaction takes place ina double-walled vessel with an inner surface of plati-num or glass in order to prevent reaction with thevessel.

Th e size and axial ratio of the p articles is controlledby the addition of small quantities of metals, the mostcommonly used being antimony and tellurium (typi-cally 0.15%). These additions result in perfectly crys-tallized, highly uniform particles between 0. 3 and0 . 4pm i n length with an axial ratio of 1O : l . Iron isalso of ten added to increase th e crystalline anisotropy.Finally, since C r 0 , is an oxidizing agent, the particlesare coated with ,hromium oxyhydroxide (C rO O H) .In hulk production the particles have coercivitieswhich range from 400 to 700Oe, and are used in arange of applications. T he re is one unusual applicationof CrO,: since it has a very low Curie poin t (-125 " C )it can h e used to make th ermo rema nent copies of videotape material by heating the tape, for example by laserheating, whilst in contact with a (high Curie point)master tape carrying the signal to be copied. Table 3

gives som e typical properties a nd applications of CrO ,particles.The properties are controlled by varying the dia-meter of the needles, which is achieved by carefulcontrol of additives and preparation conditions. It isalso noteworthy that due to the near perfect nature ofth e crystallites, the particles ar e relatively easy to alignduring the tape production process, giving squarenessratios of up to 0.9 which is higher tha n can b e achievedwith iron oxide media and which results in a higheroutput f rom the tape.4.1.5. Barium f errite particles. Barium ferrite is uniqueamong recording media since the particles exhibitstro ng crystal hab it, reflecting th e HCP rystal structure,and exist as hexagonal platelets. Due to their crystals t ructure the particles exhibit strong (uniaxial) mag-netocrystalline a nisotropy. F or pure barium ferrite thisresults in coercivities in the range 2-3 kOe. For prac-tical application the coercivity must be reduced. Thisis achieved, with little effect on the saturation mag-netization, by doping with cobalt and titanium to pro-du ce partic les of c om pos ition BaFe, , _,Co,Ti,O,,which, when x =0.75, have a coercivity of 900 Oe .The particles can be prepared by the glass crys-tallization method (Kuho et a1 1982) in w hich a mixtureof BaO , Fe ,03 and B, 03 is melted and rapidly cooledbetween metal rollers. Dopants are included in themelt in oxid e form . The b oron p rovides a glassy matrixin which the barium ferrite is formed. After heating tocrystallize the barium ferrite particles, the boron andoften excess barium is removed with hot acetic acidleaving hexagonal platelets of B aO . 6Fe,O 3 approxi-mately 0.1 pm in diameter and less than 0.05 pm thick.Despite the presence of CO and Ti the easy axisof magnetization remains the hexagonal axis of theparticles, giving rise to particles which have opposingcrystalline and shape anisotropies. Consequently theparticles have a positive temperature coefficient ofcoercivity of aro und 4.8 O e "C-', in contrast with alloth er recording media particles, for which H , decreaseswith increasing tem perature . Fu rtherm ore the plateletaspect of the particles means that potentially they canbe incorporated in tapes for perpendicular recording.However, the dispersion and orientation of these par-ticles in difficult, and to date they have found limitedapplication in flexible disc media, although devel-opment of high-density tape products for video anddata applications is expected.

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R W Chantrell and K OGrady4.2. Production of magnetic recording tape backing 1011The production of a magnetic recording tape is essen-tially a two stage process. Firstly a dispersion of th epigment (particles) is produced by grinding the par-ticles usuallv in a mixture of solvents and disoersants., ~lubricants, anti-wear agents, anti-static agents andbinders. Secondly the dispersion is coated onto(usually) a polyester film and treate d in such a w ay asto produce a very smooth continuous coating with ahigh gloss and more importantly free of voids. In thissecond stage a magnetic field is applied, to align theparticle easy axes whilst the coating is wet.4.2.1. Preparat ion of particle dispersion. The particlesare dispersed by ball milling or, more usually, sandmilling in a combination of solvents possibly includingseveral from M E K , MIK, cyclohexanone, tetrahydro-furan, dioxane, or several othe r com pound s. Includedin this stage of the process are usually a 'cocktail' ofdispersants which may be anionic, for example sul-phonates; cationic, such as qu aternary a mm onium saltsor imidazolinum salts; o r amphoter ic , such as betainesor amino oxides. The particles are milled typically forseveral hou rs with th e dispersion quality mo nitore d byviscosity measurements or other techniques. Once theparticles a re dispersed, cross linked polymers or otherbinders are incorporated together with lubricants suchas silicone oils or hydrocarbon oils or more recentlysolid lubricants such as polyfluor hydrocarbon s or car-bon black are used, the latter having the advantageof reducing the dielectric constant of the medium. Inaddition other anti-static agents are added to producea resistivity less than 10R cm-*. Other wear resistingagents, e.g. alumina or carborundum, are alsoincluded. The final dispersion is then diluted withappropriate solvents and resins to produce a 'lacquer'with the correct viscosity for coating onto t h e polyesterfilm.4.2.2. The coating process. Coating technologies inmagnetic recording and other technologies are highlyadvanced versions of an essentially very simple process.The polyester film or web over half a inetre wide ispassed through rollers and fed into a region where thedispersion is simply poured onto its surface across thefull width of the web. The web then passes beneath aknife edge which leaves a very uniform thin layer ofthe dispersion o n the film.

If required, the particles are aligned as the web

doctor

d l r pe r don + kni fe

I- b lbarking mlicoating ro11

metering roll

dispers ion

pressure- f adI C 1

Figure 5. The tinal coating process by which theparticulate dispersions including binders and lubricants islaid down on the medium. (a)Gravure coating. (b ) Knifecoating. (c)Reverse roll coating.

Currently it is at this point th at the characterizationof the medium begins. Samples of tape are routinelytaken for a range of detailed analyses of their magnetic,recording, adhesive, morphological properties and itis a tribute to the formulation chemists and coatingengineers that they can rela te failures in the final prod-uct 10 any one of the many ingredients or processes inthe tape production procedure. Of course the precisedetails of the formulations used are proprietary asindeed are the precise coating techniques used. Infigure 5 schematic representations of the most commoncoating techniques are show n.4.3. Thin film recording mediaasses by an array of permanent magnets and thendried. The drv or almost drv coatine is then pressedbetween he aie d rollers in- a carefully controlledenvironment to produce a very smooth, high glossfinish. This pressing or calendering also serve s to mini-mize voids in the tap e.

The tape is then split by knife blades and woundonto spools, the oute r few thicknesses being discarded.Often the tape, particularly if it is to be used in digitalapplications, is tested by writing and read ing dat a as itis wound.8

The thickness of a magnetic medium plays a crucialrole in determining the maximum recording density forlongitudinal recording. It is shown in a straightfowardargument by Bate (1966) that the pulse width is somefunction of I , t lH, where t is the thickness of th emedium. Clearly it is not possible to reduce the rem-anence I, too far since this would reduce the flux avail-able at the replay head. Therefore, there isconsiderable advantage in recording o n films of smaller

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Magnetic characterization of recording m edia

Figure 6. A possible recording system utilizing the perpendicularcomponent of magnetization. T he magnetizing field is created bya single-pole head.

thickness. Using ultrathin films and a magnetoresistivehead, workers at IBM have recently reported exper-imental CoPtCr thin film media capable of operationat an areal density of 1Gbit per square inch whichrepresents the cu rrent world leading position (Tsang eta1 1990).

I Although thin film recording media have significantadvantages in recording in the longitudinal mode, anequally large impetus for their development has beenthe possibility of perpendicular magnetic recording.This has been a theoretical possibility for some time,and has the advantage of increased recording densitiesdue to the reduction of the demagnetizing effects onthe transition region between bits. The major break-through in this area was made by Iwasaki and co-workers (1977, 1979) who proposed not only a new(cobalt-chromium alloy thin film) recording medium,but also a novel single pole head which is mo re efficientthan a ring head in the perpendicular mode. The re-cording system is shown in figure 6.The essential requirement for a perpendicular re-cording medium is an intrinsic anisotropy la rge enoughto support a perpendicular component of magnet-ization against the large demagnetizing energy (of theorder of 2 ~ 1 : ~ ) .e can define a figure of merit as theratio of the anisotropy and demagnetization energydensity, i.e. K / 2 x & which must be B l for a mediumcapable of supporting a perpendicular component ofdemagnetization. Th us the basic material requirementmust be for a high anisotropy and a perpendiculartexture, in the case of perpendicular recording. Weshall now briefly review the major techniques for theproduction of thin film media for perpendicular andlongitudinal recording, with emphasis on the relationbetween microstructure and magnetic properties whichremains an important problem in this field.4.3.1. Spu ttered metallic media. RF sputtered thin filmsof cobalt-chromium alloys have be en extensively inves-tigated with a view to applications as perpendicularrecording media. Sputtering is carried ou t in a vacuum

system evacuated to 1O-6 to W7 orr and then filledwith a working gas (e.g. argon) to IO-' to 10-3Torr.The ionized gas atoms are accelerated by means of aDC or RF field and gain sufficient energy to knock offsurface atoms from the target material. This forms aplasma from which deposition onto the substrateoccurs. Under the correct preparation conditions sput-tered CoCr films have an HCP crystal structure w ith alarge intrinsic magnetic anisotropy and a c-axis orien-tation perpendicular to the film, resulting in a highfigure of merit for the perpendicular recording mode.Optimum conditions are obtained by adjustm ent of th emost important parameters of the sputtering processwhich are the RF sputter voltage, the inert gas pressureand the substrate temperature. Important factors inthe characterization of perpendicular media are thosewhich represent the degree of perpendicular texture.Examples of this are the hysteresis loop, torque mag-netometry and the x-ray rocking curve method, all ofwhich will be discussed in deta il later.The structural properties and morphology of thinfilms are of enormous importance. These are inves-tigated by a variety of techniques including x-ray dif-fraction and electron microscopy. This is particularlyrevealing about the microstructure of th e films whichseem to consist of colum ns oriented perpe ndicu lar tothe film and spatially distinct. A micrograph of th estructure of a Co7,Cr2, film (G rund y er a1 1984) is show nin figure 7. Such observations give rise to a descriptionof thin film media as strongly coupled particulatesystems. Magnetic isolation of individual grains isassumed here to be enhanced by migration of Cr tothe grain boundaries. This is in marked contrast tomodels of thin films as continuous media in whichdomain wall motion is the dom inant magnetization pro-cess.4.3.2. Metal evaporated (ME) hin films. The productiontechnique here consists of evaporating a ferromagneticalloy in a vacuum chamber and depositing the atomscontinuou sly on to a moving plastic substra te. A review

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Figure 7. Electron micrograph of a cross section of a CoCrthin film clearly showing t h e columnar microstructure(courtesy of Dr P J Grundy).

related to the microstructure-Tada et a1 (1986) haveshown that small amounts of Pr give rise to finer grainsin CoNi with a corresponding increase in corrosionresistance. Corrosion resistance of the films is oftencharacterized by monitoring the decrease in remanencewith time at an elevated temperature and humidity. Adecrease in M , is often associated (see, for example,Sugaya and Tomago (1983)) with an increase in H,.presumably via a decrease in the average grain size.Clearly corrosion is a problem which needs to be over-come. It is possible to reduce corrosion by overcoatingwith sputtered protective films of Si, although this hasthe disadvantage from the recording viewpoint ofincreasing the head to medium separation and conse-quently the spacing loss.

Currently, metal evaporated tapes are available ascommercial products, principally in Japan. Some ofthese materials consist of a number of layers in whichangle of incidence alternates, producing a rather com-plicated microstructure. These tapes represent animportant development.4.3.3. Electroless deposition. Films of Co/Ni/P with theinclusion of small amounts of rare earth elements canbe prepared by electroless deposition. This is essen-tially an autocatalytic chemical reaction in which a cata-lytic substrate is immersed in a solution of metal salts.Small palladium particles adsorbed onto the substrateare used for catalysis. When the initial deposition hasoccurred the reaction becomes autocatalytic, since COand Ni themselves act as very good catalysts. Elec-troless deposition has one major advantage; it is acontinuous process and therefore much more con-venient than sputtering and evaporation which must becarried out as batch processes in vacuum. It is,however, a complex chemical reaction which is difficultto control, and presents problems with the main-tenance of chemical concentrations during the pro-duction process.

Generally the plating process produces films whichare isotropic in the plane and useful as longitudinalrecording media. An interesting de v e k q " t , how-ever is the addition of Mn and 'Re in small quantitiesto a CoNiP alloy (Goto eta[ 1984, Takano and Matsuda1986). This had the effect, as demonstrated by mag-netic measurements and x-ray rocking curves, of pro-ducing a significant Perpendicular anisotropy. As aconsequence, electroless Plating could be a viable tech-nology for the Production of Perpendicular recordingmedia. At present, however, the use of electrolessplating is restricted to a relatively low volume of out-put of rigid disks fo r standard longitudinal digital re-cording.5. The magnetic properties of recording media5.1. Static propertiesthe hysteresis loopBecause of their relative ease ofmeasurement the staticproperties of recording media are an established means

Figure 8. Schematic of the apparatus used fo r theproduction of metal evaporated films.

of the behaviour of M E film for recording has beengiven by Sugaya and Tomago (1983). The process iscritically dependent o n many factors including the sur-face roughness which has a bearing on the radiationeffects on the film, which-must he minimized by effec-tive shielding of the source. This is shown in figure 8which illustrates the apparatus used for the productionof M E films. Another important factor is the angle ofincidence of the atoms, because of a shadowing effectwhich tends to produce a particulate structure orientedat an angle to the film. The coercivity and remanenceare critically dependent on the angle of incidence ofthe atoms. The presence of reactive gases such as OXY-gen also has an effect on the magnetic properties asshown for example by Gau et a l (1 9 8 6 ) for CoNi films.This was attributed to a shape transition in the col-umnar structure and the formation of oxides in andaround the polycrystalline columns, influencing thenucleation and pinning of domain walls.

A problem with thin metallic films is corrosion inair and other reactive atmospheres. This is perhaps10

~-

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Magnetic characterization of recording m ediaby which their behaviou r is characterized. As discussedpreviously the re is a relation between the primary m ag-netic quantities ( H c , 1-S*, etc) and their recordingperforma nce, although this is by no means fully under-stood. In this section we outline the basic static mag-netic properties and the principal parameters whichgovern them , for the m ajor types of recording media.The archetypal demonstrat ion of hysteresis is theM / H loop (with M =4nf the magnetization in Gauss)which is generally measured by means of a vibratingsample magnetometer, or a E-H looper whose tech-nical details will be discussed la ter. T he hysteresis loop(see, for example, figure 3) is a non-equilibriumphenomenon arising from local energy barriers whichcan arise via many mechanisms depe ndent on the typeof recording medium. In particulate media the dom-inant mechanism is the magnetic anisotropy intrinsicto the particles. T his itself can arise from a com binationof magnetocrystalline, shape and strain anisotropies.The particle size in particulate media is small enoughfor the particle to be considered as a single domainwith a magnetic moment p =f,,V, with V th e particlevolume. The origin of the high coercive force in par-ticulate dispersions can be explained by the Stoner-Wohlfarth (1948) theory. For a system of particles withuniaxial anisotropy and easy axes oriented at an angle0 to the field H , the energy has th e form

where K is the anisotropy energy density and q is theangle between the easy axis and the magnetic mom ent.Numerical solution of equation (3 ) with variable Hgives the theore tical M / H curve for a given orientatio n.Particularly illuminating is the case 0 =0 (a perfectlyaligned system) which retains all the essential physicswhile being analytically soluble. It is straightforwardto show that under th ese circumstances there exist twoenergy minima sepa rated by an energy barrier

A E =KV(1 - H / H k ) * (4 )where Hk ( = 2 K / l S b )s the anisotropy field.Clearly, when H =Hk the energy ba rrier vanishesand the magnetic moment can make a transitionbetween the minima. Thus Hk corresponds to thecoercive forc e at absolute zero. In practice t he coerciveforce is generally lower than this value for severalreasons. The first of these is the angular distributionof easy axes or orientational texture. Numerical solu-tion of equation (3) in the original paper by Stoner andWohlfarth (1948) dem onstra tes this effect, as is shownin figure 9 which shows the coercivity of a randomlyoriented system to be just less than half the alignedvalue. The re are, however, m ore important and fun-damental reasons for the relatively small observedvalues of coercivity. The first of these is the effectof thermal agitation which gives rise to transitions fora finite value of the energy barrier with a consequentreduction of H , as was shown by Gaunt (1968). T heeffects of thermal agitation are most apparent in the

-AlignedCase

-h-

15

Figure 9. Calculated magnetization cuw es using Stoner-Wohlfarth theory for systems with random and alignedeasy axes. The beneficial effects of alignment include th eincreased coercivity and loop squareness.

observed time dependence and print-through of re -cording media which will be discussed in detail shortly.Th e temp erature effects are strongly depend ent on theparticle size. As the particle volume is reduced, a criti-cal value V, is reached at which the energy barriers ar en o longer large enough to sustain a remanent mag-netization in the presence of thermal agitation. Theparticles exhibit reversible magnetic behaviour and ar etermed superparamagnetic by Bean and Livingston(1959). Clearly it is crucial for the p articles of recordingmedia to have volumes greater than V,.A second m ajo r factor contributing to the reduction

of H , is the mode of nucleation of the magnetizationreversal. The simplest reversal mechanism is coherentrotation which is dominated by the exchange energyan d in which the spins of individual atoms remain par-allel during the reversal. This mechanism tends to takeplace only in very small particles. In larger particlesthe demagnetizing energy is correspondingly large andincoherent reversal modes in which the surface poledensity is reduced, becom e important. Per hap s themost extensively investigated mod e is the curling modedescribed in ter ms of its micromagnetic basis by Shtrik-man and Treves (1963). This predicts a decrease incoercivity with the square of the particle diameter.Reversal mechanisms in individual particles is verymuch a current problem. Della Torre (1985,1986) car-ried out computer simulations which show that non-uniform demagnetizing effects at the ends of the par-ticles can drive the nucleation process. A similar idealies behind the flipping model of Knowles (1986).Solution of this problem is necessary if realistic modelsof recording media are to be developed. G enera lly, theapproach to this problem is via the numerical solutionof the micromagnetic equations for a particle.Currently, this field of numerical micromagnetics isvery important and a number of advances have beenmade in the understanding of magnetization reversalin both particles and thin films. However, this isbeyond the scope of the current review. For a review

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R W Chantrell an d K OGradyof micromagnetics as applied to particles the reader isreferred to the work of Schabes (1991).The final major factor determining the magneticproperties of particulate media is the dipolar inter-action between particles which gives rise to a varietyof phenomena, chiefly (as regards the static magneticproperties) a decrease in coercivity with packingdensity. This is well documented experimentally (Mar-tin and Carmora 1968, Umeki et al 1981, Morrish andYu 1955, Corradi and Wohlfarth 1978) and has beenextensively investigated theoretically, most recently bycomputer simulation (Lyberatos and Wohlfarth 1986,Knowles 1985). Interactions present an im porta nt prac-tical problem in the production of recording media,since the degree of dispersion of the particles by them e t h d s oii:lincd picvvouslj; inus: be very carefullycontrolled in order to produce a consistent micro-structure.The static properties of thin film recording mediaare also imperfectly understood. The columnar micro-structure (see figure 7) often observed in films pro-duced by evaporation and sputtering (Weilinga andLodder 1986, Grundy and co-workers 1983, 1984)seems to suggest a description of these media in termsof a system of strongly coupled fine particles. Modelssuch as those of Andra et al(19 84) a nd Victora (1987)are based on this premise, the la tter giving encouragingagreement with experimental data. However, theobserva:iGns of doinain str uctu res in fi:ii~s Okhoshi cial 1983, Schmidt ef al 1985) support the models ofthin film media which take domain nucleation and themotion of domain walls as the central factors in themagnetization process. Hoffmann (1986) reviewed theexperimental situation and concluded that films withlow coercivities are continuous, but that there seemsto be a transition to particulate behaviour a t the highercoercivities which are required for high-density re-cording media. Thus, the basic ideas of fine particlemagnetism a pp ear likely to be applicable t o thin filmsusable as recording media. Successful models of longi-tudinal thin film media viewed as strongly (sometimesexchange) coupled grains have also been developed(Zhu and Bertram 1988, Miles and Middleton 1990).5.2. Remanence curvesAlthough the primary standards for the characteriz-ation of recording media a re derived fr om the M / Hloop, further useful information can be derived fromthe remanence curves. Th e principal reman enc e curvesare of the form shown in figure lO(a). The isothermalremanence I , ( H) is obtained after the application andremoval of a field H with the sample initially de-magnetized. Th e DC demagnetization rem anence I , (H)is obtained from the saturation remanent state by theapplication of increasing demagnetizing fields. Gen-erally, these curves are normalized to the saturationremanence I,(m). The reduced remanence is defined asI = I / I c (m) . By differentiation of the remanence loopsone obtains a detailed representation of the switching12

I, IHI- 0 . 0 1 c I-0.015

0 500 1000 1500 2000 2 5 0 0 3000applied f ie ld I O e I

F i j l i i e :e. (a ) Piiiicipai remanence curves jaiier Sprati eia/ (1988)).The definition of t h e remanence coercivity H,and alternative estimate H: are given. In principle, for anon-interacting system H: =H,. (b) Switching fielddistribution obtained by differentiation of t h e DCdemagnetization remanence curves of figure lO(a).

field distribution, as shown in figure 10(b) which givesan example of th e SFD etermined by this method fromthe demagnetization remanence curve.A further characteristic of the rem anence curves istheir dependence on many-body interaction effects.This was first demo nstrated by H enk el(l9 64 ) who plot-te d &(H) against I , (H ) for a variety of magneticmaterials. In the absence =f m - a r y - b ~ d y ffcc:s i: hasbeen shown (Wohlfarth 1958) that the reduced rem-anence curves are related by

I#) = 1- 2 i , ( ~ ) . (5)Th e experimental data showed a prono unc ed deviationfrom linearity, which was ascribed to many-bodyeffects. Similar effects have also been observed inNdFeB permanent magnets by Pinkerton (1986) andGaunt et al (1986). The effects are also present inrecording media a s demonstrated by th e ex perimentalwork of Spratt et al (1988). Corradi and Wohlfarth(1978) observed a similar effect but represented it interms of a single param eter, t he in teractio n field factorIFF =(H, - H:)100/Hc where H : is an estimate, of theremanence coercivity H , obtained from the r , (H) curveas shown in figure lO(n). The IFF is a measure of th edeviation from linearity and is potentially a useful par-

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Magnetic characte rization of recording media

- 0 . 2 10 200 400 600 800 1000 1200 1400applied field I Oe 1

Flgure 11. Modified Henkel plot for a CO-P thin film (afterKelly et a/ (1989)).ameter characterizing the strength of interactions ina recording medium. Certainly the observation of anincrease in IFF with packing fraction by Corradi andWohlfarth is consistent with this hypothesis.Kelly et ai (1989) used a similar technique to inves-tigate C-P thin films. T he approac h essentially meas-ures the deviation from linearity in the Henkel plotusing the p aram eter 6r=jy -@ where f y s themeasured value of reduced DC demagnetization rem-anence and @< is the value predicted from the IRMcurve using equation (5). A positive Sf indicates thatinteractions have a tendency to stabilize the mag-netized state. The da ta for a C e P f ilm are g iven infigure 11. The low-field region shows an initially posi-tive S f , which distinguishes the behaviour of the filmfrom standard particulate media in which d s gen-erally negative. However, this is followed by a rapidchange to a negative S i during the demagnetizationtransition region. This is indicative of strong coop-erative reversal which has been predicted by Hughes(1983).

The use of Hen kel plots and 6 i i s becoming increas-ingly common in the investigation of interaction effectsin particulate and thin film media. Mayo ef ai (1990)used these techniques to examine the dispersion ofbarium ferrite particles and fou nd a strong correlationbetween the form of the interaction effects as measuredby the techniques and the degree of dispersion charac-terized by the milling time.Clearly Henkel plots are a useful device in thecharacterization of recording media. The theoreticalbasis has been examined by Fearon et ai (1990) usinga Monte Carlo simulation to examine the departurefrom linearity of the Henkel plot for a system of par-ticles dispersed on a lattice. The work predicts behav-iour similar to that observed experimentally andconfirms a strong dependence on the magnetic andphysical microstructure. This is also in agreement withexperimental observations.5.3. Time-de pendent magnetization and print-throughTime dependence of magnetization occurs in all mag-netic systems an d was first observed som e time ago by

Street and W oolley (1949) who used th e term magneticviscosity to de scribe the effect. T he phen omen on orig-inat es in thermally activated transitions over localenergy barriers, provided in the case of particulatesystems by the intrinsic anisotropy. In the presence ofthermal agitation a particle bas a characteristic relax-ation time for magnetic reversal given by the Ar-rhenius-N6el law:

c foexp( -AE/kT) (6)where AE is the energy barrier and fo- 109s- is acharacteristic frequency factor. From equation (6) wecan firstly estimate the critical volume for super-paramagnetic behaviour mentioned previously. In zerofield the particle energy barrier is KV and on setting requal to the t ime of measurement f we can solve for Vto give the criterion:

Vp =(In tfo)kT/K. (7)Taking f - 00 s for quasi-static m easure men ts we haveV P- 25 kT /K . This simple criterion enab les a mini-mum volume to he estimated fo r particles to exhibitferromagnetic behaviour and hence to be usable asrecording media. In fact it is possible to refine thisestimate somewhat by consideration of the print-through phenomenon as will be shown shortly.

Consideration of the magnetization dynamics ofindividual particles leads to the expectation of anexponential time decay of the magnetization with acharacteristic time given by equation (6). In fact thisbehaviou r is never observed exp erimentally; in practiceit is found that the decay is more closely logarithmic,characterized by the time dependence coefficient S =dM/d In f. This is true for a wide variety of materials,specifically weakly interacting particulate dispersions(OGrady et ai 1981) and recording media (Sharrockand McKinney 1981, OGrady and Chantrell 1986,Oseroff ef ai 1985). The origin of this behaviour lies inthe distribution of energy barriers which exists in allsystems and which consequently gives rise to a spreadof relaxation times through equation (6). It is the sum-mation of the exponential decays which gives rise toan approximately logarithmic relation.In order to illustrate this, consider a particulatedispersion at th e remanent state afte r previous satu-ration. The system is assumed to have a distribution ofvolumes j ( V ) dV representing the fraction of the totalmagnetic volume having particle volumes between Vand V +dV. The situation is shown schematically infigure 12. The remanent magnetization arises fromtho se thermally stable particles with V > V,. However,from equation (7) it is clear that the critical volumeincreases linearly with In t and this-results in a decreasein the rema nent magnetization 0~ f(V,) In f.

The energy barrier appropriate to the deter-mination of the relaxation time is generally dependentupon the applied field, the simplest example being afully aligned system for which AE has the form givenby equation (4). Using this expression it is possible to13

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7 II

>P V

Figure 12. The distribution of the particle volum es showingthe origin of magnetic viscosity in terms of t h e timedependence of the critical volume V,generalize the calculation of V , o include an externalfield and thereby define a critical volume V , (H ) givenhv-,

The corresponding expression for the time depen-dence is (OGrady et al 1981)

where S is the maximum time dependence and U isthe s tandard deviation of the particle size distribution(assumed lognormal).Discussion of time dependence effects is generallybased on the empirical relationM =constant - S n f

which is often used to represent ex perim ental data.Use of equation (9) has been crit icized by Aharoni(1985) on the grounds that it must fail in the regionsof small and large f. However, equation (9) remains auseful relation over time intervals as large as 2-3decades. Clearly S =S ( H , T ) . At a constant tem-perature the form of the variation wiih field is similarto that shown in figure 13, having a maximum valueS,,, which occurs at a field close to the coercivity.Recently Oseroff et al (1985) measured the variationof S,,, with temperature, which can be explained interms of a model (Charap 1988) which takes intoaccount the distribution of anisotropy fields.In th e context of the characterization of recordingmedia it is interesting to relate the time dependencephenomenon to two important practical problems,namely print-through and the variation of the coer-civity with measurement frequency. The term print-through refers to the unwanted transfer of a signalfrom one layer of tape to another due to the strayfield. Print-through tends to increase with time andtemperature which suggests a relation with the time-dependence phenomenon. On this basis print-throughis simply the long-term response of the medium to a14

(9)

small applied field. Th e print-through phen om enon hasrecently been investigated in detail by Flanders andSharrock (1987) who give an explanation involving atime-depende nt coercive force which isa natural conse-quence of the t ime decay of the m agnetization. I n thiswork th e print-through w as found to corr elate well withthe time dependence of the magnetization, although inthis case the behaviour was characterized by a plot ofM against (In t). n =1 and n =1 both gave closelylinear relations, n =4being used by Flanders and S har-rock to effect a comparison with the print-throughmeasurements.From the previous explanation it is clear that theparticles involved in the print-through process arethose with volumes just above the crit ical volume forsuper- paramagnetism whose relaxation times will hestrongly influenced by small external fields. This sug-gests that the small-field tim e-de pen den ce would be auseful parameter characterizing recording media,although such measurements a re very rarely mad e. Inorder to minimize the print-through it is necessary toensure that the volume distribution is narrow, conse-quently ensuring a minimal number of particles in thesize rang e just above V,. Essentially this mean s kee pin gthe switching field distribution as narrow as possibleis intimately related to the distribution of particlevolume.Measurements of t ime-dependen t effects are tech-nically somew hat d ifficult, so it is interestin g to t ake therelation with the SF D one stage further. Early studiesof time dependence proposed the following empiricalrelation

since the SF D

S=XirrHf (10)where xicr s the irreversible susceptibility and H , =kT/VI$,, is a fluctuation field Characterizing the ther-mal agitation.For a fully aligned system of uniaxial particles ithas been shown by Chantrell el al (1986a) that H f canhe calculated analytically, giving

where in this case xicr s defined as the differentialof either of the principal remanence curves. This isparticularly useful since xirc s a readily accessibleexperimental .mea surem ent. Although equ ation (11)refers to a special case it has been s ho w n to give th ecorrect form of the variation of S with H fo r a partiallyaligned tape (Uren er al 1988) although the mag nitudeof S s not correctly predicted. More recently, a moredetailed investigation by el-Hilo et al (1990) has shownthat taking into account a distribution of anisotropyfields (which to some extent also represents the angulardispersion) gives a much better agreement with exper-imental data. This is a fruitful are a for furth er researchas regards the characterization of recording media sincei t would appear that relatively simple measurementsof I , (H) can potentially provide information on print-through via equation (11). Since xir, s formally ident-ical to the switching field distribution this makes the

S=xs( l -H / H , ) H , / 5 0 (11)

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-0 .5 - 1 . 0 -1 .5 -2.0H I k Oe 1Figure 13. The variation of the time-dependent behaviour with field andtemperature. There is a characteristic variation with field having a maximumclose to the coercivity H, (data for CrOZ ape after Oseroff et a / (1985)).

SF D even m ore a central para me ter characteristic of thebehaviour of recording media.Coverdale ef a1 (1990) examined theoretically thevariation of the fluctuation field with applied field forfine particle systems where the anisotropy axe s are notnecessarily aligned parallel to the applied field andfound a complex variation having a minimum in H ,close to the coercivity. In a separate study d e Witte eral (1990) examined experimentally the variation of Htand the related activation volume parameter VACdetermined by H , =kT/(VAcIsb) for systems of finecobalt particles. Reasonable agreement is obtainedwith the calculations of Coverdale et al, confirming thevalidity of the activation volume approach. Furtherwork o n recording media .particles indicates that VACis sensitive to the reversal mechanism, a fact which ispotentially important in the study of all recordingmedia since the magnetization reversal mechanism is acentral problem. The value of the activation volume issignificantly smaller than the particle volume, con-sistent with the work of Flanders and Sh arroc k. Thisis presumably due to the incoherent reversal modes,which do not involve simultaneous switching of thewhole particle.

A fur the r consequ ence of the relaxational behav-iour of recording media is the variation of coercivitywith frequency which has been observed by Sharrockand McKinney (1981), Sharrock (1984) and Corradi efal (1987). This is important since measurements of H ,are normally made quasi-statically in vibrating samplemagnetometers, yet the magnetization process duringdigital recording now ta kes place over timescales of theorder of a microsecond. Thus the static charac-terization of recording media must be used carefully

since their actual dynamic behaviour when used in re-cording systems can differ widely from these measure-ments. Much work still needs to be done in order toestablish the relationship between static and dynamicproperties more firmly. This is possibly a long-termaim since the relationship is likely to be governed bya combination of intrinsic particle prope rties a nd many-body effects, which make a rather complex con-tribution to the dynamic behaviour of recording media .Clearly significant progress has been made recentlyin the understanding of time-dependent phenomena.However, furthe r study in this are a is important sinceit is the long-term coercivity which will determine thelower limit of particle size usable in recording media(Sharrock 1990). Also it would appear that it is thebehaviour of subunits of the particles that must beconsidered since they determine the value of S andultimately the archival stability of the stored dat a. Thispresents a very important a rea for furth er research.6. Dynamic proper t iesIn this section we concentrate on those propertieswhich are most relevant to the performance of ana-logue recording. The response of the medium is deter-mined by its anhysteretic behaviour which is analternating field process to be described in detailshortly. W e shall also consider th e noise of the me diumwhich is a limiting factor in its performance .6.1. Anhysteretic behaviourThe anhysteretic remanent magnetization (A R M) of asystem is obtained by firstly applying a large AC field

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R W Chantrell and K OGradyHAC nd small DC field Hoc to the sample. T h e AC fieldis then reduced to zero after which the DC field isremoved. For relatively small values of Hoc, the AR Mis directly proportional to Hoc and the response ischaracterized by the anhysteretic susceptibility x , ~dARM/dHoC. The highly linear response is ideal forrecording analogue information in contrast to the DCresponse (characterized by I , ( H ) ) which is very non-linear. The technique of AC bias recording is rathermore complex than the anhysteretic process describedhere. However, anhysteresis remains the closest re-lated physical measurement. A useful description ofAC and DC bias recording is given by Rossing (1981).Funda mentally anh ysteresis is a fascinating process.Essentially the AC field provides the energy to switchthe magnetic moments over the energy barriers andmust he larg e enou gh to melt the system magnetically(in the sense that all the moments follow the AC fieldexcursions) at the beginning of the process. In thisrespect it is very similar to the the rmo rema nent mag-netization (TRM)which produces an equilibrium mag-netic configuration. For a system of non-interactingideal Stoner-W ohlfarth particles, x. is infinite. Earlytreatments of anhysteresis concentrated o n the intro-duction of interactions (Wohlfarth 1957, Nee1 1943) inorder to explain the finite measured values. A laterseries of papers by Jaep (1969, 1971a, b) introducedthe dynamics of the process and showed tha t x , ~s finiteat a finite temperature although up to two orders otmagn itude larger th;n obse rved values. A mean fieldapproach was used to introduce interactions in orderto reconcile theory and experiment.Clearly anhysteresis is dominated by the inter-actions between particles, a problem which is onlyreally amen able to modern com putation al techniques.Th e first use of the se was by Ber tram (1971) who essen-tially treated anhysteresis as a growth process duringwhich the magnetic configuration and hen ce th e inter-particle interactions varied. A more recen t model hasbeen developed by Lyberatos et al(1985) an d Chantrel let al (1986b) which takes account of the dynamics ofthe process. Th e interactions between particles are pre -dicted to have an important bearing on th e dynamicsof the process via fluctuations in the local interactionfield. These tend to act as an effective temperaturewhich helps the system to escape from local energyminima and thereby to approach th e equilibrium statewhich is characte ristic of anhysteresis. Th is was dem on-strated by increasing the rate of reduction of the ACfield which essentially dam ped ou t the fluctuations an dled to the production of non-equilibrium states.In an actual recording system the tape moves at aconstant speed past the head and responds to a fieldconsisting of the signal field (analogous t o IfDc)nd ahigher frequency AC bias field. A s the t ap e moves outof the recording region both the AC a n d DC fieldsreduce t o zero at the same ra te, in con trast to the idealsituation so far considered. The practical process isreferred to as modified anhysteresis, and can result ina phenomenon known as overbiasing. This arises due16

to the application of too large an AC field. Un de r thiscondition by the time that the AC field is low enoughto allow the particles to freeze magnetically th e signalfield is too small to give rise to a significant magneticresponse. This situation is demonstrated by cal-culations (Lyberatos 1986, Chantrell et al 1986b) theresults of which are illustrated in figure 14. Since theanhysteretic susceptibility xi. represents the responseof the recording medium there is clearly an optimumbias point as can be seen in figure 14. The optimumbias point is characteristic of the tape material used,being dependent on the coercivity of the medium.Because of the rather complex nature of the exper-iment anhysteresis is not suitable for the day to daycharacterization of recording media. This tend s t o relyon the use of static measurements. Although the linkbetween static and dynamic properties is not fullyunderstood they are undoubtedly coupled. Thus anychange in the dynamic properties might be expectedto be reflected in variations in the static propertiesrepresented by H c , squareness, (1- S*), tc, which o nthis basis remain perfectly good production monitoringand control parame ters. B ecause of the wealth of infor-mation contained in anhysteretic m easure me nts, how-ever, these remain an excellent tool for research anddevelopment purposes.6.2:Medinm nniseTh e noise from a recording m edium arises from severalmajor factors, and is usually much greater than noisearising from other parts of the recording channel .Large scale variations in th e thickness of the recordingmedium and head to medium spacing give rise tomodulation noise which has been stu died by Cou tellierand Bertram (1987). A second contrib ution arises fromthe discrete amou nt of flux contributed by an individualparticle at the read head. This gives rise to statisticalfluctuations in the readback voltage, and consequentlynoise. Particulate noise depends critically on the mag-netic state of the system and on the local magneticstructure which is strongly influenced by dipolar inter-actions. Essentially thrsr rrsuii in iocai nragueiic cur-relations, often involving closed loop configurationswhich tend to reduce the noise. Noise measurementsare often used for the characterization of recordingmedia, generally by comparison with the results forstandard tapes which exist for all the common types ofrecording media. For exa mp le, bias noise, which refersto the noise of a medium having been exposed to ahigh-frequency A C field as the tap e passes a record headis often measured as a function of the bias current .Often the total noise power is measured, ratherthan the actual spectrum, which nonetheless carriesconsiderable information about the medium. Here weshall discuss briefly the origin of the noise power fluxspectrum which with modern instrumentation andpowerful methods of on-line statistical analysis is itselfpotentially useful for th e characterization of recordingmedia. According to Thurlings (1982) the problem of

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1.0-

0.5-

o WEAL ANHYSTERETIC PROCESSMOOlFIEO ANHYSTERETIC P ROCESS

0 0 0

8

HA C I HK

I 0

Chantrell et ai (1986)). -

noise in particulate media was first investigated byMann (1957) and Daniel (1960) who produced ex-pressions fo r random noise. Noise theories assumingcompletely uncorrelated particle moments are in pooragreement with experiments. Most successful theoriestake into account the microscopic magnetic structureinduced by interparticle interactions. For example,Arratia and Bertram (1984) investigated theoreticallythe noise of AC erased media (which are essentiallysubjected to a n ARM process with zero static field) usingthe earlier model of Bertram (1971) to carry out theerasure. Mo re recently Fea ron et a1 (1987) have shownthat the dynamics of the AC erase process have asignificant effect on the local correlations. This is apossible explanation for the observation (Ragle andSmaller, 1965) that bias noise is a few d B higher thanA C noise. Although these m easurements are made afterthe medium is subjected to similar processes, a dif-fere nce in th e dynam ics could result in such small dif-ferences in the noise.Th e noise originates in statistical fluctuations in th ereadback voltage and is strongly dependent on themicroscopic magnetic configuration. There is direct evi-dence for this in that AC erased noise differs vastlyfrom DC erased noise in which the system is subjectedto a large applied field to cause magnetic saturation.Using a cross correlation technique Thurlings (1985)demonstrated the differences in noise between the dif-ferent magnetic states including the demagnetized

virgin state of an as-prepared tape and an AC dem-agnetized tape. The variation of noise with magneticstate is well illustrated by the calculations ofFearon ef a1 (1987). Here different magnetic micro-structures were achieved by AC demagnetization withdifferent rates of reduction of the AC field which areshown in figure 15 . Highly correlated states with lownoise were observed with small AC decrements, whichresulted also in complete AC demagnetization. As thera te of reduction is increased by increasing the A C fielddecrement the sample achieves a non-equilibrium statecharacterized by finite remanent magnetization at theend of the process. Thus, essentially the system movescontinuously from the AC erased state (small A C dec-rement) to the DC erased state (large AC decrement).The corresponding increase in the noise is very pro-nounced and is in accord with experimental data.Essentially the rapid noise increase from the cal-culations follows the equally rapid change of mag-netization in the non-equilibrium region.I n order to fully unde rstan d noise in recording mediafurth er physical measurementsarepossible. O n eo f th eseis th e technique of neutron de polarization which is sen-sitive to magnetic correlations and also to density vari-ations which can be an important source of noise,Magnetic correlations have been studied using neutrondepolarization in CrO, tapes by Rosm an eta1 (1988) andin alumite by Kraan and Re kveldt (1990). Tape measure-men ts revealed the presence oforiented superdomains.

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i -25 30 tA

A

A

A

A

4 0 1-1 -2 -1 U1 I drrrnnmt 1109 10 1Flgure 15. Calculated noise power flux spectra lor systemsAC demagnetized at varying ra tes (afte r Fearon et a /(1987)).

The da ta were in qualitative agreement with th e fact thatthe bulk erased noise was about 6 d B lower than the DCerased noise, Although interpretation of neutro n depo-larization data is rather difficult it is an intere sting pr obeof correlations and inhomogeneities in magneticmaterials which could provide useful information forstudies of the fundam ental noise mechanisms.A furthe r impo rtant form of noise is transition noisewhich occurs in digital recording. It is related to theimperfect structure of th e transition region between therecorded bits (Belketa11985) and is apotentiallim itationontheachievable bit density. Experim ental work (Arnol-dussen and T ong 1986) has shown that t h e noise can berelated to the characteristiczig-zag structure arising fromnon-uniformities across the track w idth. Microm agneticcalculations (Hughe s 1983, Zhu and B erta m 1988, Milesand Middleton 1990) have dem onstrated th e existence ofthis type of structure. Much work rem ains to be don e inthe understanding of the transition noise. In this contextit is interesting to note the work on remanence curvesdescribed earlier which found evidence fo r cooperativereversal, a possible contribution to transition noise, inthe modified Henkel plots in both COP thin films andbarium ferrite media. Clearly such a relationshipbetween the macroscopic magnetic properties and thenoise is of im portan ce as regards the fun dam ent al under-standing of the materials properties. I n addition itemphasizes the potential importance of remanencecurves for the ch aracterization of practica l med ia.This brief introdu ction to noise in recording media isintended to dem onstrate that the noise pow er flux spec-trum contains considerable information a bou t the mag-netic and physical microstructure of th e recordingmedium. These are fundamental properties of the

mediumitself an d are thuscharacteristicof its behaviour.Although the relationship between noise, staticmagn eticproperties and ev entual recording ch aracter istics is as yetnot fully understood, the present sophistication ofmeasurement techniques m akes this an interesting possi-bility for the characteriza tion of recording m edia . Co n-siderable furt her work is required in ord er to gain thefundamental insight required in ord er to fully realize th epractical use of noise in this context.7. Experimental techniques7.1. The vibrating sample magnetometer (VSM)The V S M , originally due to Foner (1959) is one of th eof the magnetic properties of recording media andother magnetic materials. T he basic design of th e VSMis shown in figure 16 . The sample is caused t o vibrateat a frequency between 25 and 100 Hz by either amotor and cam arrangement or more commonly by aloudspeaker drive system. The moment induced in thesample is detected as an AC signal in th e detec tion coilswhose amplitude is proportional to that moment. Theamplitude of the A C signal is then determined eithervia a lock-in amplifier or via digital analysis to give aDC output proportional to the induced moment. Thus,when the mag netometer is calibrated with a sam ple ofknown mnm.en! it is capable nf giving mea.si~rem~n!sover a wide range of moments (10F EM U to IO2 E M U ) ;and over a wide rang e of applied fields (0 to 150 kOe) .The field is usually measured via a Hall pr ob e of knowncalibration, or via calibration of a superconductingsolenoid for high -field systems.T he reasons for the common usage of th e VS M areprincipally conc erned with its versatility an d reliab ility.Temperature variation is simple, its principle of opera-tion and operating procedures are easy to understand,but the instrument does suffer from some disad-vantages. Considerable care is necessary when using aVS M which all too often is not taken. Areas wherespecial care is needed are described b elow.

(1) The m easurements ar e madc ii ; ai; opci; mag-netic circuit so sample shape demagnetizing effectsmust be taken into consideration. These effects aresmall when measuring with the applied field parallel tothe plane of a tape or thin film but are of great import-ance when measuring dispersions of particles or per-pendicular to the plane of thin film coatings.( 2 ) High-purity nickel metal at saturation is oftenused as the calibration standard but this is rarelyadequate. The saturation moment of nickel is notknown precisely and, as with other ferromagneticmetals, the field required to saturate the sample willdepend on its mechanical history. Nickel calibrationsamples need to be carefully and frequently annealedand cleaned and the field required to saturate themchecked. In reality the calibration sample used shouldbe of a similar moment and have similar dimensions tothe sample to be measured. This is most readily

most commonly used ins!rumen?s for the meas"remen!

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Magnetic characterization of recording mediatransducer assembly

oscil latormoveable plates

A C signal subject t o var iat i ins referencewith changes in vibration amplitudeand freq uenc y. time O C voltage proportional to moment

signalconstan and independent o f changer invibration amplitude and frequency.

t o outputdisplay circuits

/i f ferentialamplifier synchronousdetectoramplifier

A C difference signal which is independento f changes in vibration amplitude andfrequency.

but subject t o variations with changes invibration amblitude and frequency.

sample

magnet pole piecesFigure 16. The basic design and operating principle of th e vibrating sample magnetometer.

achieved by using a paramagnetic calibration samplesuch as palladium, whose moment is well known andwhich, by varying the field can be used to give a widerange of moments , if the field calibration is accuratelyknnwn and t he sample tempe rature controlled.

(3) Because of the time-dependent effects de-scribed in section 5.3 the sweep rate of the magneticfield must be carefully controlled or monitored so thatcomparisons between different samples can be mean-ingfully made. Failure to control the sweep rate cangive rise to variations in the measured value of thecoercivity of up t o 15% in extreme cases and the useof a slow sweep rate can result in a variation of th eswitching field distribution when the Koester (1984)meth od is used.7.2. T he B-H loop t racerThe basic arrangement of this instrument is shown infigure 17 . T he o uter coil generates an AC field H whichinduces an AC moment in the sample which is usuallyin the form of a tape . The inner search coil then detectsB (B =H +4 n M ) and electronic analysis of the twosignals enables the M-H curve for the sample to bedisplayed on an oscilloscope.

T h e B-H looper was until recent years the mostcommonly used instrument for the measurement ofmagnetic properties in th e recording industry. It suffersdisadvantages relative to the VSM; i.e. the control oftemperature and its variation are difficult since the

field coil will cause heating. Also the maximum fieldattainable is only of the order of 5 kOe which is barelysufficient to saturate high-coercivity media. Similarlyto the VSM th e B-H looper is subject to calibrationerror particularly with respect to the measurement ofB an d it is also an ope n circuit technique requiring car eto b e taken with regard to demagnetizing effects.The major advantages of this technique are theremoval of sw eep rate p roblems since this is accuratelyknown, although the disparity in the mains frequencybetween Europe and the US can still cause problemsfor an international industry, and the speed ofmeasurement, literally a few seconds compared withseveral minutes for a VSM. This latter point in particularwill ensure that B-H loopers continue to be used forquality control in media manufacture.

It should of course be noted that the B-H loopercan only be readily used t o measure the basic hysteresisloop whereas the VSM is capable of measuring a farwider range of parameters .7.3. Noise measurementsIn general the noise from a recording system is a com-bination of noise from the replay head and associatedelectronics in addition to the required noise spectrum.As suc h, noise measureme nts mu st be carefully madeand interpreted. A widely used experimental con-figuration is to employ a high-quality rec ord er with astandard tape transport scheme using a high-quality

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R W Chantrell and K OGradyR n

A C powersourceFigure 17. Diagram of a B-H loop plotter.

head and amplifier. In all cases, the background noiseof the head and amplifier must b e subtracted from thetotal signal in orde r to a rrive at the no ise produced bythe medium itself. The head used should have a lowvalue of magnetostrictive noise to which so m e attentionhas been given by Thurlings (1982). Noise of this formis very much a complicating fact or since magnetostric-tive effects are dependent on the magnetic field. Bymeasuring noise for tapes in contact with the head andat a small distance away, Thurlings concluded that forhis purposes a sendust hea d was n ot significantly affec-ted by magn etostrictive e ffec ts whiist in contact withthe tape, allowing noise measurements to be made incontact mode.Th e experim ental configuration used by Thurlings(1982) and Luitjens et al (1985) abandons the use ofthe standard tape transport system. The experimentconsists of a turntab le supported in air bearings drivenby a constant-speed crystal-controlled motor. Samplesin the form of glass plates can be held t o the turntableby vacuum an d thin films o n a flexible sub strate can beheld in a 'drumhead' configuration (Luitjen s et a11985). T h e recording head is positioned in c ontact withthe magnetic layer which rotates with respect to thehead at a constant speed. This seems a particularlysuitable technique for investigations of sinall scaleexperimental samples.The noise power flux spectrum contains detailedinformation about the recording medium which isignored in many standard techniques which merelymeasure the total noise power. H ow eve r, by combiningthe use of a spectrum analyser with the power ofpresent microcomputers it is possible to carry out avery detailed analysis on the whole spectrum. Oneparticular advantage is the capability of measuringcross correlation functions between spectra which canhe very revealing about the physical and magneticmicrostructure of the m edium. In ad dition, modulationnoise, which arises from th e variations i n amplitude ofa recorded signal has been shown by Coutellier andBertram (1987) to he principally dete rmi ned by surfaceeffects. Thus noise measurements have the capabilityof analysing surface, bulk and microscopic effects, in20

a way which is only now being realized due to rapidadvances in instrumentation. There is no doubt thatnoise measurements are presently an exciting area,with considerable potential for the future charac-terization of recording media.7.4. Measurements of anisotropy and textureAnisotropy measurements a re not commonly mad e onmagnetic recording media despite the fact that theycan give valuable information concerning the intrinsicproperties of th e materiais. 'Many record ing particiescan be considered as having uniaxial anisotropy withan energy density given by

E =K O+K , s in20+K , s in40 (12)where 8 is the angle between the magnetization andeasy axis. If the magnetization is rotated away fromthe easy axis then the sample exerts a torque on themechanism which suspends it. If K 2 Q K 1 then thetorque L (= - d E / d 0 ) is given by

L = -K, sin(20). (13)Fig. 18 shows the variation of torque with angle. K ,can simply be determined from the to rqu e curve.Torque curves are measured using aii iiistiiimeii:called a torque magnetometer . These are general lyhome made, and the principle of operation is quitesimple. A sample is suspended over a magnet andeither the magnet or the sample is rotated throughsome angle. The resulting torque is measured usingeither a calibrated fibre whose torsion constant isknown or via an electromechanical measurementdevice, for example a force microbalance op erate d ver-tically. For a review of the various designs of torquemagnetometer see Pearson (1979).Some samples of recording media are isotropic inthe plane, and hence exhibit no external torque. Insuch cases an alternative technique, termed rotationalhysteresis can be used, which utilizes the following prin-ciple. When a sample is rotate d

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