Magnetic Properties of Co-Based Metallic Glasses

In: Cobalt: Characteristics, Compounds and Applications ISBN 978-1-61324-103-5 Editor: Lucas J. Vidmar, pp. © 2011 Nova Science Publishers, Inc.

Chapter 5

MAGNETIC PROPERTIES OF CO-BASED METALLIC GLASSES

J. Jaworski 1,2 , E. Dryzek1 and E. Fleury2

1Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland

2Korea Institute of Science and Technology, Seoul 136-791, Korea

ABSTRACT

The chapter is a short compilation of methods of production and magnetic properties of the cobalt-based metallic glasses. Owing to their remarkable magnetic properties and excellent corrosion resistance, most of the Co-based metallic glasses are used for their functional properties essentially as materials for magnetic cores, electronic surveillance articles and electromagnetic wave attenuators. After introducing briefly the most common production methods enabling the formation of amorphous structure in metallic alloys, the magnetic properties like magnetization, magnetic moment and Curie temperature are first presented for binary, ternary and quaternary Co metallic glasses as well as for Co granular alloys containing Gd element. The chapter includes a description of the magnetoresistance (MR) and Hall effects in binary and ternary metallic glasses from room temperature to the temperature of liquid nitrogen. While most of these Co-based metallic glasses contain metalloid elements such as B and Si, the magnetic properties including Hall effect and resistance measurements are also described for two compositions of metalloid-free Co-Zr-V alloy system. The change of the microstructure during the transition from amorphous-to-crystalline structure in these Co-Zr-V alloys is presented based on measurements by means of positron annihilation lifetime spectroscopy (PALS). Beside the magnetoresistance properties, the magneto-impedance effect and its variation owing to ion irradiation are also presented. Finally the recent developments on the formation of Co-based bulk metallic glasses and their possible applications are discussed.

J. Jaworski, E. Dryzek and E. Fleury 2

INTRODUCTION Cobalt, discovered in 1735 by the Swedish chemist Georg Brandt, is a metal of VIIIB

(iron) group of the periodic table of elements with atomic number 27. Its name comes from German mythology and means ‘goblin’, an evil spirit. Co exists in two forms: the hexagonal α-phase below 4170C, and the cubic face-centered β-phase, which is stable above that temperature. Its melting point is 14920C and its boiling point 31000C, and it has only one stable isotope. It is a hard, strong and ductile metal, with a molar weight of 58.93 g, a density of 8.89 g/cm3, a Mohs hardness of 5.5. It is also a good conductor, with an electrical conductivity of 16.3•106 S/m, and it is ferromagnetic with Curie temperature 11210C [1].

Co can produce alloys resistant for high temperatures such as superalloys [2] as well as triballoys that are extremely hard cobalt-chromium stellite alloys designed for wear resistance, and elinvar type cobalt alloys, which are characterized by a very small change of the modulus of elasticity with the temperature. The coefficient of thermal expansion of elinvar type cobalt alloys is also very small among metals and metallic alloys. In a study on several types of Co-elinvar performed in 1950’s, Masumoto et al. [3, 4] reported for an alloy containing 56% of Co a coefficient of linear expansion of 3.94 x 10-6 K-1 in the temperature range from 100 to 500 C, while value for pure cobalt is 13.0 x 10-6 K-1. Co alloys containing Cr and Ni are characterized by high strength, ductility, fatigue life, good mechanical properties and also corrosion resistance in numerous environments elgiloy alloys. These alloys have also a good biocompatibility [5]. Another Co alloy with high resistance to corrosion and good biocompatibility is vitallium [6, 7]. However, since the largest cobalt mines are essentially found in one country, the usage of Co is limited to alloying element in industrial metallic materials.

Beside mechanical properties, cobalt alloys have also interesting magnetic properties. In addition to alloys with polycrystalline structure mentioned above, Co alloys can be produced with an amorphous structure, and are also referred as metallic glasses. The production of

Figure 1. Melt spinning method [8].

Magnetic Properties of Co-Based Metallic Glasses 3

metallic glasses from the melt was discovered in 1959 by an American team of California Institute of Technology led by Dr. Pol Duwez, a Belgium-born scientist. For the production of metallic glasses, the speed of cooling is one of the most important parameters. During the cooling from liquid phase, alloys normally solidify with a crystalline structure. The reorganization of the atom configuration from the liquid phase to the solid phase requires diffusion of the atom, which is dependent on the time, and controlled by the viscosity of the liquid metal. In liquid alloys with high viscosity, the atomic rearrangement is impeded. The disordered positions of the atoms in the liquid are frozen into the solid and the crystallization is thus hindered. Usually such cooling process occurs at a rate 104–106 K/s. The temperature at which the metallic glasses become solid is called the glass transition temperature.

Among the techniques used to form metallic alloys with amorphous structure, the four most popular are the melt-spinning technique, splat quenching, gas condensation and laser glazing. Figure 1 shows a schematic picture of the most common method called melt spinning [8]. An ingot of alloy is melted in a quartz tube by radiofrequency heating coil. An inert gas is injected to push the molten metal through a small hole at the bottom of the tube. The stream of liquid metal falls down on a rotating copper wheel and is thus cooled at a high cooling rate of about one million degree per second depending on the processing conditions. Another method of fast cooling is by splat quenching [9] (figure 2).

Figure 2. Splat quenching method [9].


A drop of liquid metal is falling down, passing through a light beam and a photosensor, which initiates the motion of a hammer crushing the drop onto an anvil. This method cools the metal on both sides in opposition to the first method in which the metal is cooled down only from one side. The third method is the condensation from the gas phase (figure 3) [9]. In a vacuum chamber, the vapour of a metal is cooled onto a cold substrate. In this method the liquid phase is skipped.

The method provides the largest cooling rates among all rapid solidification processing,

and it is used when the other methods do not work. The last method among the most popular methods used to produce metallic glasses is laser glazing [9]. The technique is used to produce materials in the bulk form. A short impulse of laser beam is focusing on a small spot of the surface of a crystal. The absorption of the laser beam occurs only in a very thin layer of metal with thickness around 10 nm. The energy of the laser beam melts the surface of the crystal and next the surface is cooled with a cooling rate around 1010 - 1012 K/s.

In an ideal metallic glass, because of its disordered and uniform structure, the properties are isotropic, i.e., there should be no specific direction favouring mechanical, electrical or other properties. It is known that some direction in crystalline materials could be easier magnetized and in another direction one – harder. Ideal magnetic glasses are isotropic in contrary to crystalline alloys. But real metallic glasses present a slight directional dependence of the magnetization because of the existence of local atomic short-range orders and/or internal stresses. The energy needed for the magnetic saturation and the coercive field of metallic glasses are reduced and the permeability is increased in comparison to crystalline materials. For example the coercive field of the Co58Ni10Fe4Si11B16 metallic glass [7] is in a range 0-8 A/m and the maximum its relative permeability is 9 x 105, which are, respectively, among the lowest and the highest values reported for these properties among ferromagnetic materials. These magnetic properties and high resistivity, resulting from the disordered

Figure 3. Condensation form gas phase [9].


atomic structure, which damps the eddy currents and thus lowers the energy loss, make these Co-based amorphous alloys excellent material for cores of electric transformers.

Owing to their remarkable magnetic properties and excellent corrosion resistance, most of the Co-based metallic glasses are used for their functional properties. However recent progress in this field has led to the development of bulk Co-based alloys with high strength and composites with good malleability at ambient temperature [8, 9], that presage alternative applications in the near future.

The chapter will essentially described the magnetic, magnetostrictive, magnetoresistance and magnetoimpedance properties of binary, ternary and multi-elemental metallic glasses as well as granular alloys rich in Co produced by several processing techniques. The results of an investigation on the glass-to-crystal transition by positron annihilation in metalloid-free Co-based metallic glasses are also presented.

CO-BINARY ALLOYS The first research studies on the saturation magnetization of Fe-Co binary alloys were

made by Weiss and Forrer in 1920’s [10]. In 1960’s Bardos [11] investigated the magnetic moments in BCC Fe-Co alloys containing cobalt in a range from 5 to 70 at. % annealed during 1 week at temperature 11000C for homogenizing and next cooled or quenched from 8500C to 150C at various rates using five ways: slowly cooling – 1 week, air cooling ∼ 5 min., water quenching ∼ 2 sec., brine quenching ∼ 1 sec., agitated brine quenching ∼ 1 sec. The results of these experiments show independence of the saturation magnetization on the cooling rates for alloys containing Co atoms less than 30 at. %. Differences were found for cobalt concentration larger than 40 at.%; the magnetization decreased for alloys treated with a fast quenching and the faster the cooling rate, the smaller the magnetization. The mean magnetic moment per atom for slowly cooled and rapidly quenched alloys increased with increasing of the Co content until a concentration of 30 at.%. In alloys treated with a high cooling rate, the value of the mean magnetic moment per atom decreased for alloys containing more than 30 at.% Co. In contrast the value was constant between 30 to 40 at.% in slowly cooled alloys and it decreased rapidly for the further increase of Co concentration. Finally, for composition with 70 at.% of cobalt, the magnetic moments reached a constant value independently of the treatment. An investigation on the magnetic structure of the alloys by neutron scattering technique indicated that the magnetic moment of cobalt was constant with increasing amount of cobalt in iron, but the iron moment for stoichiometric CsCl-type structure increased with the increase in the cobalt concentration. It is connected with the increase in the number of Co around Fe atoms. Saturation of the effect comes at a concentration of Co near 50 at.% when all Fe atoms have only Co neighbours. For disordered alloys the moment is much lower than that of the ordered one. An interesting point worth mentioning is that in the off-stoichiometric ordered alloys containing between 30 and 50 at.% of Co with an excess of iron atoms the iron moment decreases toward the value found in disordered alloys of the same composition. Apparently the magnetic moment on an iron atom depends on the number of cobalt nearest-neighbour atoms.


CO-TERNARY ALLOYS

1. Co-TM-B Amorphous Alloy

1.A. Magneto-Resistivity

At the end of 1960’s, Duwez produced the first ferromagnetic amorphous alloy in the Fe-P-C alloy system [12], which led to tremendous research investigation on amorphous alloys based on 3d transition metal-metalloid systems in the 1970 and 1980’s. A few research groups investigated the anisotropic magneto-resistance [AMR] in these systems. Japanese scientists [13] particularly considered the AMR in binary and ternary (Fe, Co, Ni)-B amorphous ribbons prepared by rapid quenching from the melt using metallic rollers. They reported that the magneto-resistance decreased for these alloys as the temperature increased. Also an increase of the amount of boron in both binary and ternary alloys leads generally to a reduction of the AMR effect. The only exception is the Fe100-xBx alloy for which the magneto-resistance measured at room temperature increased as the boron content was increased until 17 at.% of B the, stabilized until 18 at.% and then decreased beyond. The behaviour in both binary and ternary alloys was explained by the reduction of the Curie temperature and a stronger temperature dependence of the AMR in boron-poor Fe-B alloys. Generally Co-B binary alloys present a lower AMR effect and a much higher resistivity than pure Co in contrast to Fe-B alloys, which have a larger magneto-resistance than pure Fe. The resistivity of amorphous Fe-B alloys is also, like for Co alloys, higher than for pure iron.

The AMR effect for ternary alloys Fe-Co with small amount of boron is less sensitive to the temperature than Fe-B alloys containing the same or a larger amount of boron. For (Co1-

xNix)100-yBy alloys, the AMR effect declines with temperature the faster the higher amount of Ni atoms is in the alloy. For ternary alloys For (Co1-xNix)100-yBy where y is constant, the magneto-resistance is dependent on the Co/Ni atomic ratio and is found to reach a maximum value for specific atomic ratio. The maximum is higher and shifted more toward Ni rich alloys for ternary alloys with a large amount of boron.

In crystalline Fe-Ni and Co-Ni alloys, the maximum of magneto-resistance occurs in alloys possessing a magnetic moment of ~0.9 µB. In that case, it was found that the linear saturation of the magneto-striction λS and spontaneous Hall coefficient change their signs, too, which was explained by the spin-orbit interaction [14,15,16]. The application of the split band model [17,18] to ternary amorphous transition metal alloys with boron Fe-Ni-B indicated that the charge transfer from boron to transition metal causes a shift of the composition with a saturated value of the magneto-striction equals to zero λS = 0 and a larger value for the Hall coefficient toward Ni rich alloys. The magneto-striction and AMR effect are positively correlated in these alloys. Experiments with (Co1-xNix)100-yBy reported that a value of magnetic moment for maximum AMR effect is near 0.55 µB, which is smaller than 0.9 µB presented in other works for crystalline alloys. Also the magneto-striction of (Co1-

xNix)77B23 is negative for these alloys and its absolute value decreases rapidly beyond 40 at.% of Ni, while the maximum for magneto-resistance effect in these alloys is associated with the minimum of the magneto-striction. In contrast the Fe-Ni-B alloys have large AMR associated with big magneto-striction.


The magnetic moment and Curie temperature of Co-TM-B (TM = Fe, Mn, Cr, V) ternary metallic glasses were considered by O’Handley [19]. In this group of transition metals Fe is ferromagnetic, Cr is antiferromagnetic below 380C, and Mn and V are paramagnetic. Experiments for Co80-xMxB20 (where x is from 0 to 16) showed that as the atomic percent of Fe increases the specific saturation magnetization MS increases linearly. For manganese in the concentration range 0 - 12 at.%, the MS value varies parabolically: it rises, reaches a maximum and then decreases however the changes are not significant. For Cr and V the specific saturation magnetization decreases almost linearly as the amount of Cr or V in the amorphous alloy is increased. The Curie temperature for alloys doped by V or Cr declines rapidly and almost linearly. For V in a range of atomic concentration between 6 and 14 at.%, and between 8 and 16 at.% for Cr, the Curie temperature drops from around 8000C to 2000C. The dependence of the mean magnetic moment per transition metal atom µ, reflects the behaviour of specific saturation magnetization. It behaves exactly in the same way.

In his interpretation, O’Handley [19] assumed the rigid-band-approximation, which should be satisfied only for small concentration of M element that weakly perturbs the periodic potential of the Co-B matrix. According to the Friedel’s virtual bond state model, a fivefold degenerated 3d virtual band state with electrons “up” (figure 4) is lifted out of the

Figure 4. Schematic representation of virtual bound states in cobalt-rich glasses with a) small concentration of Mn (less than 6%)b)V impurities. Mn concentration bigger than 6% or Cr doping shifts the states between a) and b) with the 3d↑ virtual bound state passing through the Fermi level. (based on [18]).


band near the impurity relative to its repulsive potential ((figure 4a). If the majority-spin 3d virtual bound state is over the Fermi level, the M admixture possesses the electronic properties because of the difference in the number of its 3d “down” electrons and the number of electrons in the matrix. If the potential of the doped metal is repulsive enough to move the majority spin virtual bound state over the Fermi level (figure 4 b), then five 3d electrons will be shifted to 3d “down” states, reducing the average magnetic moment by ten µB. The potential, which is a base of the virtual bound state, is clearly localized at the M atoms. Holes in the virtual bound state give rise to a magnetic moment, which is identified with the M atom place, but not necessarily localized there. The charge displaced from the virtual bound state could increase the M atom moment at the expense of the moments in the surrounding matrix.

1.B. Magneto-Striction The magneto-striction is a very important property of material. Small magneto-striction

or even zero magneto-striction is needed for transformer core materials. The core of transformers looses a lot of energy, which is transformed into heat as a result of magneto-striction. The aim of a few research studies was to identify parameters influencing the dependencies of the magneto-striction. Three main parameters affecting the magneto-striction have been determined as: the linear saturation magneto-striction - λS, the spontaneous volume magneto-striction - ωS, and the magnetic saturation polarization – JS. In many Co-metalloid type amorphous alloys, λS varies as the square of JS [20], by changing the ratio of glass-matrix constituent elements. For Co-Fe based alloys, λS depends linearly on JS. O’Handley [19] investigated (CoxTM100-x)B20 amorphous alloys in which TM was Fe or Ni. He discovered that the presence of Ni atoms in these alloys decreases the Curie temperature below the crystallization temperature in contrast to Fe doping. Investigations on λS showed that the binary Co80B20 alloy and compositions doped with Ni present negative magneto-striction in contrast to the Fe addition that exhibits a positive λS. The smallest λS = +0.6*10-6 was found for composition Co74Fe6B20. The measurements of the magnetization and magneto-striction as a function of the temperature showed, for (Co,Ni)80B20 alloys, a systematic decline of these parameters as the temperature rises. For binary Co80B20 and iron-containing Co-TM-B ternary alloys, the magnetization decreased also as the temperature increased while the magneto-striction behaved differently. For Fe doped Co-TM-B alloys, the magneto-striction is positive. With temperature magnetostriction grows and for a composition Co70Fe10B20 the growth of magneto-striction decreases insignificantly but for the Co74Fe6B20 increases a little. For binary Co80B20, the magneto-striction is negative at room temperature and increases with the temperature to become positive at high temperature. The theoretical compensation point where λS = 0 is found at around 6600C.

Further research studies were undertaken by Czech researchers [20] who showed that, for Co80-xFexB20 metallic glasses with a Fe content lower than 5.7 at.%, λS is negative. It is equal to zero for 5.7 at.% Fe and then becomes positive for high Fe concentration. The variation is almost linear. For Co80-xNixB20, λS changes insignificantly but parabolically reaching a minimum value around 7 at.% Ni. The spontaneous volume magneto-striction ωS of Co80-

xFexB20 metallic glass decreases with increase of the Fe amount but the fall is not linear. The curve of the dependence has two inflection points. As in the λS case, the spontaneous volume magneto-striction reaches a zero value at around 5.7 at.% Fe, but in contrary to λS, the value


changes from positive to negative. The JS of Co80-xNixB20 alloy decreases as x increased according to a relation λS=c(x)JS

2(x), where c is linear function of the Ni concentration. This behaviour can also be explained by the quantum mechanical split-band model

rough-hewn mentioned above, which describes a one-ion magneto-striction. The two-ion magneto-striction model [16] has also been applied to describe successfully the behavior of Co-rich metallic glasses.

1.c. Metalloid-free Co-Zr-V Amorphous Alloys

Another type of metallic glasses presented in this chapter is the metalloid-free Co-Zr-V amorphous alloy system. The formation of amorphous structure and properties in that alloy system were first reported by Nose et al. in 1981 [21], and the magnetic properties were further investigated by Iskhakov et al. [22]. For a detailed investigation, Dryzek et al. [23] selected two compositions in this alloy system: the Co80Zr10V10, which is magnetic and Co65Zr10V25, which is non-magnetic. At room temperature, no magneto-resistance effect could be detected for both alloys. The Co80Zr10V10 magnetic composition presented magnetoresistance effect on a level of about 0.2% at the temperature of liquid nitrogen. However, the magneto-resistance effects in these alloys are negligible for practical applications.

The Hall effect was found to be negligible in the non-magnetic alloy composition but not for the magnetic one. Figure 5 presents the dependence of Hall voltage UH on the external magnetic field B0.The variation of the Hall voltage in that graph consists of a line with two different slopes. The first one is a fast growing line connected with spontaneous Hall

coefficient RS and magnetization of metal M dependent on B0. The second group of lines – saturated lines, almost parallel to the B0 axis, are connected with ordinary Hall coefficient R0.

The Hall coefficient was calculated based on the equation of the Hall resistance ρH: ρH = R0 B0 + RSM(B0) (1)

Figure 5. Hall voltage dependence on external magnetic field.


The first component of the equation is related to Lorentz force on electrons and is responsible for slowly growing part of the ρH = f(B0) curve above saturation magnetization. The second component characterizes the ferromagnetic state of the metallic glass and is represented by initial part of the ρH = f(B0) curve. The component is the result of phenomena such as: side jump mechanism, spin-dependent scattering, skew scattering and also transition from the low field regime to the high field regime. The two factors of the equations determine the shape of the resistance - magnetization curve below and above the saturation of magnetization. The spontaneous Hall coefficient RS could be calculated from the initial, linear part using the method of linear regression defined as a tangent of an angle between the graph line and the B0 axis:

Rs = (∂ρH /∂B0)Bo→0 (2) Values of the Hall coefficients, R0 and RS, for this metalloid-free Co-based metallic glass

were found to be, respectively, about 10-9 and 10-8 m3/(A·s), which are similar values to those reported for as-spun Co72Fe2B17Si5Mn4 amorphous ribbon [24]. As explained by Lozovan et al. [24], the Hall coefficient for Co-containing metallic glasses is an order of magnitude higher than that for crystalline alloys and it is situated between values for crystalline metals and semiconductors and could find application in Hall sensors when semiconductor elements cannot be applied.

The transition from metallic glass to crystalline structure during annealing of the metalloid-free Co-Zr-V amorphous alloys was investigated using positron annihilation spectroscopy. This method is a sensitive tool for the detection of vacancy-type defects in solids. Positrons penetrating into a metallic glass localize in regions where the electron density is lower than average,(e.g. vacancies, vacancy clusters, dislocations) and then they annihilate with electrons. The differences in the electron density are reflected in positron lifetime, τ. Lower electron densities in sites where positrons are localized produce longer positron lifetimes.

A single lifetime component observed in metallic glasses is interpreted in terms of complete trapping of positrons into the large number of free volume sites exhibiting a size distribution and representing irregular array of potential wells with different positron binding energy. The annihilation characteristics are regarded as statistically averaged quantities over the annihilation sites.

Figure 6 presents the dependence of the main positron lifetime on the annealing temperature. It is well known that the positron lifetime in as-received amorphous alloys is longer than in well annealed constituent pure metals and shorter than in monovacancies existing in alloys. The values of the positron lifetime for both samples are similar and do not change significantly after annealing at temperatures up to 260°C. Both metallic glasses contain the same amount of Zr for which the values of positron lifetime in free state and in vacancy-trapped state are higher than for Co or V. It is believed that the difference in the Co and V concentrations, which have similar positron lifetimes, do not influence the positron lifetime in both studied metallic glasses.

For temperatures between 300°C and 420°C, the positron lifetime for both ribbons is slightly reduced, i.e., 156 ps. This decrease starts at 300°C for Co65Zr10V25 and at 340°C for Co80Zr10V10. The decrease of the average positron lifetime upon annealing can be attributed to


irreversible structural relaxation phenomena and it is widely observed for rapidly quenched conventional metallic glasses. Structural relaxation involves atomic rearrangements leading to more stable structure though it is still metastable. These structural changes result from annealing out of excess volume at temperatures even far below crystallization temperature and are reflected in density changes.

At temperatures 460°C for Co80Zr10V10 and 480°C for Co65Zr10V25 the positron lifetime rises to values which are higher than that for as quenched samples. For Co65Zr10V25, the increase continues as the temperature is further increased. For Co80Zr10V10 the positron lifetime remains in the range 161 – 164 ps up to 580°C which is higher than crystallization temperature, TX for this metallic glass. The increase in the positron lifetime connected with crystallization is caused by positron trapping in vacancies and grain boundaries not present in the glassy alloy.

The difference in the variation of the positron lifetime for the two metallic glasses was explained by the differences in the positron trapping sites arising during crystallization, which can be connected with the density and chemical composition of the grain boundary regions.

Figure 6. Positron lifetime as a function of the annealing temperature. Vertical lines mark glass transformation temperatures Tg (DSC at 20 K/min) and crystallization temperatures Tc for corresponding alloys (the black solid lines for Co65Zr10V25 and the gray dashed lines for Co80Zr10 V10) [23]. (Image courtesy of Materials Science Forum).


2. Granular Co-Cu-Gd Alloys An interesting group of ternary alloys presenting unusual magnetoresistance properties is

the group of alloy containing rare earth. The representing system of this group is the Co-Cu-Gd crystalline alloy system [25]. Investigations on binary Co-Cu indicated that the largest magnetoresistance effect was obtained for thin ribbons (∼30 µm thick) of composition Co20Cu80 prepared by melt-spinning technique. For ternary alloys obtained from the addition of Gd, the best result in term of magneto-resistance was achieved for the Gd0.62Co19.88Cu79.5 alloy composition. The electron probe analysis (EPMA) taken from the cross-section of the ribbon Gd0.62Co19.88Cu79.5 perpendicular to the ribbon length is shown in figure 7 [26]. Elongated Co-rich grains in Cu-rich matrix are visible and gadolinium is found to be distributed in the Cu-rich phase. During the production of the ribbons, the grains solidified with a preferential orientation along the direction of rotation of the Cu wheel, which gives them an elongated shape along the length and width but with a reduced thickness of only a few nanometers. Because of the geometry of the grains, the rapidly quenched ribbons could be treated as rolled granular alloys and as some kind of multilayer system.

Figure 7. The electron probe analysis (EPMA) picture of cross-section of the ribbon Gd0.62Co19.88Cu79.5 cut in direction perpendicular to tape feed during production. There are visible long Co-rich grains in Cu-rich matrix. The width of Co-rich grains is relatively large compared to their thickness. Gadolinium is distributed in Cu-rich phase [26].


The MR effect of Cu-Co ribbons was found to increase almost linearly as the temperature

is reduced as it is shown in figure 8 for the CoCu4 ribbon. This result is in good agreement with that of Co-Cu multilayer systems described by Parkin [27]. For Gd0.62Co19.88Cu79 ribbon, the magnetoresistance increases as the temperature decreases in the same way as the MR of CoCu4 ribbon until the temperature reaches the level of -700C (203.15K). Between -70 and -1000C, there is observed jump of the magnetoresistance. At the temperature of liquid nitrogen, the value of the MR for Gd0.62Co19.88Cu79 is almost twice that of the CoCu4 ribbon. The “jump” of magnetoresistance for the alloy containing Gd could be explained by the fact that the testing temperature reached the Curie point of the Cu-rich phase containing Gd [28, 29]. As shown by several authors [29-32] the coupling between gadolinium and transition metals (TM) is anti-ferromagnetic. Theoretical works by Camley [31] has indicated that the magnetization of Gd in a zero field is set antiparallel to the magnetization of TM such as Co. So in Cu-rich phase Co and Gd of Gd-Co-Cu alloy atoms probably are coupled antiparallel. The coupling exists at temperature above the Curie temperature of Gd, which for gadolinium thin films is lower than for a bulk [35] and strongly depends on the thickness. There is unknown dependency of the Curie point upon the temperature for Cu-rich Gd-TM-Cu alloys.

-200 -175 -150 -125 -100 -75 -50 -25 0 250,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

T [0C]

∆R/R

CoCu4

TapeBmin= 11 mT, B

max= 350 mT

GdCo32Cu

128

It could be only assumed that an analogue situation takes place in these alloys and the

Curie point dependencies with the Gd concentration in Cu-rich phase of the alloy. Burzo [29] showed that the ferromagnetic alloys of gadolinium couple antiferromagnetically. In Gd-poor Gd-Co-Cu alloys, the ternary Co7.53Cu79.46 phase forms, which composition is similar to the

Figure 8. Temperature dependence of the magneto-resistance for CoCu4 and GdCo32Cu128 (Gd0.62Co19.88Cu79) melt-spun ribbons [25]. (Image courtesy of Journal of Alloys and Compounds – Elsevier).


ferromagnetic Cu7Gd intermetallic compound with a Curie point below – 700C [34]. Accordingly it was proposed that the Gd13.01Co7.53Cu79.46 phase plays an important role in the GMR effect at low temperature by anti-ferromagnetic coupling with Co nanograins in the Gd-Co-Cu rapidly quenched ribbons. Theoretical explanation of magnetoresistance of metallic glasses was given by Paja and Morgan [35].

CO- QUATERNARY ALLOYS

1. Properties of as-Spun Amorphous Ribbons The next logical step is to describe the properties of alloys containing a larger number of

elements such as the quaternary metallic glasses. Owing to its abundance in the earth crust, its electronic structure and excellent magnetic properties achieved in Fe-Si alloys, Si was added to Co-(Ni,Fe)-B alloys. Narita [36] and Goto [37] studied the quadruple Co-Fe-Si-B metallic alloys in which the summation of B + Si atoms did not exceeded 30 at.%. For Cox-ySiyB100-x alloys, where x was 88 and 90 at.%, the magnetic moment decreased linearly as the amount of Si is increased and Curie temperature increased on beginning, reached maximum in certain point characteristic for each composition (y = 23.5 and 24at.% of metalloid content for Co 90-y SiyB10 and Co88-ySiyB12) and after it decreased with further increased in the Si concentration. For Co-Fe-Si-B alloys, two research groups [36, 37] demonstrated the decrease of the magnetic moment per atom with an increase in the Si concentration. Authors researched also the Curie temperature dependence on a ratio of a concentration of Co and Fe atoms with constant ratio of Si and B atoms in alloys. The Curie temperature decreased or varied parabolically with the atomic ratio Co/Fe growth. That behaviour was confirmed by Shen [38].

The stabilizing effect of an annealing treatment on the domain walls was studied by Yamasaki et al. [39] in non-magnetostrictive (Co0.94Fe0.06)79Si2B19 metallic glasses. After annealing in a demagnetized state at 3000C during 30 minutes and 1 hour, the wall coercive force at fixed magnetization level increased significantly and the annealing was at the bottom of an introduction of Brakhausen jump. The wall coercive force depends parabolically on the Co atom concentration in the alloy. With an increase in the Co amount, wall coercive force increases until reaching a maximum at around 60 at.% but then drops rapidly beyond that value of Co concentration.

2. Properties of Irradiated Amorphous Ribbons The influence of the irradiation by Ar, N and Xe ions energy of 70 keV on giant

magnetoimpedance (GMI) of metallic glass ribbon Co66Fe4B15Si15 was investigated by Park et al. [40]. The GMI effect is a large variation of the impedance of a magnetic conductor with an AC current under the influence of a DC magnetic field. Co-based alloy ribbons favor a transversely oriented domain structure (Figure 9). This transverse domain structure is favorable for the GMI effect and can be improved by annealing under a transverse magnetic field [41]. In that work, the measurements of GMI were made with frequency of AC current


form 0.1 to 10 MHz. Tests were performed in an intermediate frequency regime [42], where GMI originates mainly from the variation of the skin depth due to strong changes of the effective magnetic permeability caused by the applied DC magnetic field. It noted here that depending on sample geometry, the GMI profile can reach its peak in the intermediate frequency range (e.g., 100 kHz to 10 MHz), as a consequence of the contribution of the permeability from both domain wall motion and magnetization rotation to GMI. The reduction in the GMI at higher frequencies is related to the domain walls becoming strongly damped by the eddy currents and only the magnetisation rotation contributes to GMI.

The result of irradiation [42] on the GMI was significant for irradiated samples in comparison to non-irradiated samples. A particularly marked effect was observed in the case of argon ion irradiation. In addition, the hysteresis loops for irradiated samples changed their shapes in comparison to the non-irradiated one and became wider.

The defects induced by irradiation can create a domain in the antiferromagnetic (AF) layer and act as pinning points of domain wall motion. Therefore, the increase of the GMI ratio in the low frequency region was attributed to the creation of defects in the interface boundary. The penetration of the Ar ion constituted an AF layer between the ferromagnetic substrate. The number of domain walls in the AF layer is related to the total number of defects throughout the whole AF layer, and these defects increase the exchange bias field. Since this was a volume effect, the total number of defects in the AF layer is greater than that in the interface boundary. The increase of GMI ratio by the Ar ion in the high frequency region was a result of the interaction between the rotational magnetization and the exchange bias field in the AF layer.

Substantial modifications in the magnetic properties such as an increase of the coercivity,

a change of the saturation magnetization, and a small change of the exchange bias field suggested the formation of an AF layer. The coercivity was increased with the increasing irradiated ion weights. The coercivity corresponds to the hindrance of wall displacement and was proportional to the distribution of the defects. The increase of the coercivity in the ion irradiated sample was attributed to a domain wall pinning due to the irradiation induced defects.

Figure 9. Schematic of the domain configurations of negative-magnetostrictive amorphous ribbon [41]. The magnetization of domains are perpendicular to length of the ribbon.


3. Dependence of the Resistivity and Hall Resistivity The dependence of the resistivity and Hall resistivity on the temperature and transition

from metallic glass to crystalline structure process in quaternary alloy Co66Ni12Si9B13 has been studied by Jakubczyk [43]. Because of the formation of crystalline grains during annealing, the resistivity of the metallic glass was maintained at a constant level for annealing temperature up to around 600 degrees and then fell down rapidly until 8000C, which corresponds to practically fully crystallized states. Identical behavior was observed for the spontaneous Hall coefficient.

PRACTICAL APPLICATIONS One strong advantage of the Co-(Fe,Ni)-B-Si alloys is that there can be prepared by melt-

spinning technique without requiring high vacuum condition. The main applications of these alloys are essentially magnetic cores, electronic surveillance articles and electromagnetic wave attenuators.

In addition to quaternary components, researchers have investigated in the last years the behavior of multi elemental metallic glasses. Phan et al. [44] checked the valve behavior of GMI in Co70Fe5Si15Nb2.2Cu0.8B7 composite, and reported significant change of the impedance under small magnetic field. Mastrogiacomo et al. [45] tested amorphous alloy [(Fe0.582Co0.418)81Cr10Zr7Ti2]90B10 as a potential material for superconducting solenoids.

Multi-component metallic glasses can also be prepared in the bulk form with diameter of up to a few millimeters [46,47]. In comparison to non-magnetic Fe-based metallic glasses, these alloys are less brittle and can even display plastic deformation when tested under compressive mode [48,49]. These bulk alloys can find applications in devices requiring components with high strength, large plastic deformation and resistance to fatigue and corrosion resistance such as micro-gear and MEMS. Many patents have been issued on Co-based metallic glasses. Inoue [50] was the inventor of a soft magnetic bulk metallic glass Co-Fe-B-Si-M (M = Zr, Nb, Ta, Hf, Mo, Ti, V, Cr, Pd) with extremely sharp and narrow the loop of dependence magnetization on magnetic field, which suggested the absence of remanence. The magnetization was found to jump from positive to negative values in very small magnetic field and immediately reached saturation point. Jin patented soft magnetic cobalt-iron-chromium-nitrogen thin film metallic glass which could be used in electromagnetic devices, e.g. microtransformer cores, inductor cores and magnetic read-write heads [51].

Because of page limitation, we can mention only a few examples among thousands of patents and inventions that take advantage of the remarkable properties of the Co-based metallic alloys however many have already been applied in commercial applications.

ACKNOWLEDGEMENTS

Dr. Jaworski thanks the Korean Federation of Science and Technology for rewarding

with the Brain Pool fellowship. EF acknowledges the financial support from the 21st Frontier Program CNMT (# 2010K000265) of the Korean Ministry of Education Science.


REFERENCES

[1] M. Eagleson, “Concise encyclopedia chemistry”, Walter de Gruyter and Co, Berlin,

1993, p.241 [2] M. Nazmy M., A. Kunzler, M. Staubli, “High-Temperature-Resistant Cobalt-Base

Superalloy”, US Patent Application no. US 2010/0061883 A1, Mar.11, 2010 [3] H. Masumoto, H. Saito, Y. Sugai, T. Kobayashi, J. Japan Inst. Metals, 16 (1952), p.

420-422 [4] H. Masumoto, H. Saito, Y. Sugai, T. Kono, Trans. JIM 3 (1962) p. 4-9 [5] M. Es-Souni, H. Fischer-Brandies, M. Es-Souni, J. Orofac. Orthop. 64 (2003) p. 16-26 [6] L. Wojnar, Materials Characterization 46 (2001) 221 [7] C. Grant, R. McKim, New Scientist, 94 (1982) p. 637-640 [8] P. Chaudhari, B. Giessen, D. Turnbull, Scientific American 242 (1980) p. 98 [9] R. Zallen, “The Physics of Amorphous Solids”, Polish translation, Wydawnictwo

Naukowe PWN, Sp. z o.o., Warszawa, 1994, p. 14-16 [10] P. Weiss, R. Forrer, Ann. Phys. 12 (1929) p. 279-372 [11] D.I. Bardos, J. Appl. Phys. 40 (1969) p. 1371-1372 [12] P. Duwez and S. C. H. Lin, J. Appl. Phys. 38 (1967) p. 4096-4097 [13] J. Yamasaki, H. Fukunaga, K. Narita, J. Appl. Phys. 52 (1981) p. 2202-2204 [14] L. Berger, Physica 30 (1964) 1141 [15] L. Berger, Phys. Rev. 138 (1965) A1083 [16] H. Ashworth, D. Sengupta, G. Schnakenberg, L. Shapiro, L. Berger, Phys Rev. 185

(1969) 792 [17] R. C.O’Handley, Phys. Rev. B 18 (1978) 930 [18] R. C.O’Handley, Phys. Rev. B 18 (1978) 2577 [19] R. C.O’Handley, Sol. St. Com 38 (1981) p. 703-708 [20] G. Vlasak, M. Jergel, P. Duhaj, Mat. Sc. Eng. 99 (1988) 109-112 [21] M. Nose, J. Kanehira, S. Ohnuma, K. Shirakawa, T. Masumoto, J. Applied Physics

Vol.52 (1981), p.1911. [22] R. S. Iskhakov, S. V. Kolmogortsev, Zh. Moroz, E.E. Shalygina, JETP Letters, 12

(2000), p. 603. [23] E. Dryzek, J. Jaworski, E. Fleury, A. Budziak, Mat. Sci. For. 666 (2011) p. 58-61 [24] M. Lozovan, M. Neagu, C. Hison, J. Opt. Adv. Mat., 6 (2004) p. 651-653 [25] J. Jaworski, A. Strzała, O.J. Kwon, E. Fleury, J. Alloys Comp. 492 (2010) 56 [26] J. Jaworski, E. Fleury, "Magnetoresistance effects in Cu, multilayers and granular Cu-

Co alloys and in rapidly quenched Gd-doped Cu–Co alloys”, in "Copper Alloys: Preparation, Properties and Applications", Nova Publishers, 2011.

[27] S.S.P. Parkin. R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152 [28] M. Foldeaki, A. Giguere, B.R. Gopal, R. Chahine, T.K. Bose, X.Y. Liu, J.A. Barclay,

JMMM 174 (1997) 295 [29] E. Burzo, Phys. Rev. B 6 (1972) 2882 [30] R.E. Camley, Phys. Rev. B 35 (1987) 3608 [31] R.E. Camley, Phys. Rev. B 39 (1989) 1231 [32] B. Altuncevahira, A.R. Koymen, J. Appl. Phys. 90 (2001) 2939

http://www.ncbi.nlm.nih.gov/pubmed?term=%22Es-Souni%20M%22%5BAuthor%5D

http://www.ncbi.nlm.nih.gov/pubmed?term=%22Fischer-Brandies%20H%22%5BAuthor%5D

http://www.ncbi.nlm.nih.gov/pubmed?term=%22Es-Souni%20M%22%5BAuthor%5D


[33] J.S. Jiang, D. Davidivic, D.H. Reich, C.L. Chien, Phys. Rev. Lett. 74 (1995) 314 [34] T. Massalski, “Binary Alloy Phase Diagrams”; American Society for Metals, Metals

Park, OH, 1986; Vol.1, [35] A. Paja, G.J. Morgan, Phys. Stat. Sol. (B) 206 (1998) p. 701-711 [36] K. Narita , J. Yamasaki, H. Fukunaga, IEEE Trans. Mag. Mag-14 (1978) p. 1016-1018 [37] M. Goto, H. Tange, T. Tokunaga, Jap. J. Appl. Phys. 17 (1978) p. 1877-1878 [38] B.G. Shen, L. Cao, H.Q Guo, J. Appl. Phys. 73 (1993) p. 5730-5732 [39] J. Yamasaki, K. Mohri, K. Watari, K. Narita, IEEE Trans. Mag., Mag-20 (1984) p.

1855-1857 [40] D.G. Park, C.G. Kim, J.H. Lee, W.W. Kim, J.H. Hong. J. Appl. Phys. 101 (2007) [41] R. Kolano, M. Kuzminski, W. Gawior, N. Wojcik, J. Magn. Magn. Mater. 133 (1994)

p.321–4. [42] M.H. Phan , H.X. Peng, Prog. Mat. Sci. 53 (2008) p. 323-420 [43] E. Jakubczyk, Visnik lviv Univ. 38 (2005) p 354-360 [44] M.H. Phan, H.X. Peng, M.R. Wisnom, S.C. Yu, J. Appl. Phys. 97 (2005) p. 10M108-1

– 10M108-3 [45] G. Mastrogiacomo, J. Loffler, N. R. Dilley, Appl. Phys. Lett. 92 (2008) p. 082501-1 –

082501-3 [46] A. Inoue, B. Shen, N. Nishiyama, in Bulk Metallic Glasses, M. Miller, P. Liaw Eds.,

Springer Science and Business Media, New-York, 2010 [47] A. Inoue, X. M. Wang, W. Zhang, Rev. Adv. Mater. Sci. 18 (2008) p. 1-9 [48] O.J. Kwon, Y.G. Lee, J.J. Lee, Y.C. Kim, E. Fleury, Advanced Materials Research, 26-

28 (2007) 743-746 [49] S.N. Kane, E. Fleury, O.J. Kwon, S.S. Khinchi, A. Gupta, Hyperfine Interactions, 183

(2008) 129–133 [50] A. Inoue, “Soft Magnetic Co-based Metallic glass alloy”, US Patent no. US 7,223,310

B2, May 29, 2007 [51] S. Jin, T. J. Klemmer, Th. H. Tiefel, R. B. Van Dover, W. Zhu, “Article comprising

anisotropic Co-Fe-Cr-N Soft Magnetic Thin Films” US Patent no. 5,998,048, Dec. 7, 1999

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Magnetic Properties of Co-Based Metallic Glasses