Magnetic Data Storages

(1) Magnetic recording (a) Generalation (why SNR N∝ 1/2, Mr samll ) (b) Longitudinal and Perpendicular (c) Thermal stability Antiferromagnetic coupling media Hybrid recording Pattern media High Ku medium(2) Magneto-optical recording(3) MRAM

Schematic representation of longitudinal, digital magneticrecording write process.

When there are fewer particles per bit, the transition between domains becomes less sharp and pickup signal decreases.

Why not make each recorder region a single domain particle or grain ?

The recording medium

Horizontal fringe field hx for a longitudinal transition of zero width(a=0) and for a = 0.5 at y = 0, 0.5( Eq.1), respectively.

(1)

Hx(x,0)≈4Mrδ/x

Transition width

Schematic representation of field above a longitudinalrecording medium.

y

Transition width

The coecivity squareness parameter S* is defined as

S* = 1 – Mr / xo Hc , xo = [∂M/ ∂H]Hc S* varies from 0 to 1

The switching field is defined as SFD=ΔH/Hc ,ΔH is the full widthat half maximum as indicated in the fig. High SFD implies a spatially sharp and requires a narrow magnetisation transition particle size distribution.

(1) When there are few particles per bit, the transition

between domains becomes less sharp and pickup signal

decreases. About 1000 isolated particles.(2) The more irregular transition are referred to as zigzag or sawtooth transitions. Noise is due primarily to the formation of zigzag transition between bits. the sawtooth pattern scales roughly as Ms

2/K1/2, the solutions: decreasing Mrt and increasing K.(3) The signal is proportional to the number of measured events or particles per bit, N. Hence SNR ~ N1/2. (4) The heads must approach to the hard disc surface.

Material Requirements

For recording media

CoCrPtB films

Write head : having a sufficient high Ms so that the fringe field exceeds the Hc of the medium (500-3000Oe); an adequate magnetic permeability (easy saturated).

Read head: low Hc, low noise and extremely high permeability in order to respond with a substantial change in flux to the weak fringe field above the medium

Schematic M-H loop for ideal magnetic recording mediumand head material.

For write head: µ >>1, Ms large and Br=0;For read head: µ >>1 , Hc = 0

Thin film recording head. Left, layout of pole pieces and windings; right, enlarged, cross-sectional view of magnetic pole pieces

Film thickness2-3 micrometer;Gap 200 nm.

Thin film recording head

Permeability versus frequency for

four thin films.

High frequency 109Hz;A weak uniaxial anisotropy;High electrical resistivity

Field dependence of magneto-resistivity for uniform response toa uniform field.

Geometry of magnetoresistivesensor showing sense current,anisotropy field, and external orfringe field of medium, and theireffect on magnetization.

h=1-2 µm, w=2-4 µmt=10-20 nmΔρ/ρ =2.0% Ni81Fe19

Magnetoresistive head

Spin-Valve Read head

Structure of a simple spin valve; the device dimentionare approximately h=2-6 µ m and w=10 µ m .

Experimental transfer curve for a 2 µ m high spinvalve sensor for +5mA (solid) and -5mA (dashed)sense current.

M2

M1

Longitudinal and perpendicular recording

Comparision of recorded bits in longitudinal (a) andperpendicular (b) media.

Demagnetization factor for a recorded bit : (a) proportional to Mrt/ λ; and (b) to Mr λ/ t.Linear bit density: (a) 105 bit per inch (λ=0.5 µ ); and (b) 105 -5x105 bpi

Perpendicular recording using flux closure layerbeneath the medium (Iwasaki et al., IEEE Trans.MAG-15, 1456(1979)).

Thermal Stability

In the physics of magnetic recording there are two keyfactors in achieving very high areal density:

(1)The superparamagnetic effect (thermal stability);

(2)The finite sensitity of the readback head.

In both cases, the limitations arise because the signal energy becomes so amall as to be comparable withthe ambient thermal energy.

The signal to media noise is approximately by the numberof magnetic grains (or switching units) per bit:

SNRmedia ~ Wbt / vg

Where, wbt (bit volume, read-width x bit-length x thickness) vg (the grain volum)

In order to avoid thermal instability, a minimal stability ratio of stored magnetic energy, KuV, to the thermal energy, KBT, KuV/KBT 50 - 70≌

Interlayer antiferromagnetic coupling media

Schematic illustration of (a) a two layered AFC media,(b) LAC media with high J and (c) advanced three layersLAC media for much lower Mr δ .

Longitudinal

Mrt = Mr t1 – Mr t2

KuV1<KuVeff < (Ku V1+KuV2)

KuV/KBT 50 - 70≌

In the case of two layers AFC media

Magnetic hysteresis loop for a single layer media (a) and anAFC media (b). Jex=0.06 erg/cm2,Hex~800 Oe.

(a)Room temperature HcR vs Mrt forsingle layer media and AFC media(b) Thermal decay.

(1)

Fitted by Eq.(1)

Where tp is about 1 s and fo~109 Hz;from the fit, we obtain Ho=8.6 KOe,KuV/KBT=75 for the single layer;Ho=8.3 KOe, KuV/KBT=100 for AFC one.

Interlayer antiferromagnetic coupling media

Perpendicular

Magnetic loop as a function ofRu thickness

Interlayer antiferromagneticCoupled two grains

Correlation between exchange field, Hex, coecivity field,Hc, and nucleation field, Hn.

Normalized effective energy barries, KVeff/KV1, as a function of the apparent exchange coupling Japp.

hybrid recording

(Solid immersion lens)

ZnS:SiO2 NA ~1.1

Media: Co69.48-xTb30.52Agx, x=0-25.68

Patterned Media

Scanning electron microscopy image of a square arrayof electodeposited Ni pillars of high 300nm and period.

Low noise,high density

High Ku Materials Approach to 100 Gbits/in2

• Smaller, thermally stable media grains• Prominent candidates are RE-TM Co5Sm and L1o phases FePt (Hc >1T), CoPtY…..• 3 times smaller grain diameters d and potential 10 fold areal density increase (∝1/d2)•Write field 10-100KOe

KuV/kT>40-60

D.Weller et al., IEEE Trans on Mag., 36(2000)10

Magneto-optical Recording

Principle of thermomagnetic recording (Curie pointwriting): (a) before, (b) during and (c) after the writing.

Temperature dependence of the magnetization fora GdCoMo amorphous alloy films (Chaudhari et al.,APL 42(1973)202).

A schematic representation of a Buble domain stucture

The condition of a written stable bubble domain*

Where r is the domain radius, Hd the demagnetizingField, Hext the applied external field and σW the wall energy density of the magnetic medium.

Huth’s equation (1974 IBM J..Res.Dev. 10 100-9)

The spot size and signal to noise

d=λ/(2NA); S/N ~ θKR1/2

* Bobeck IEEE Tran. Mag., MAG-5(1969)554.

From Oppeneer Magneto-optical Kerr spectra in Handerbook of magneticMaterials, Edited by Buschow (Vol.13)

Experimental pola Kerr ritation an undoped MnBi sample (Di et al. 1992)and Al-doped MnBi (Shang et al., 1997) sample at room temperature.

High-density MRAM(Magnetic random access memories)

Schematically representation of MRAM structure andM-H, ΔR/R characteristics of the PSV.

Schematic of the read and write processes in a PSV random accessmemory.

Table: composition and dimensions of the principle layers in a currentrepresentative MARM device.

Outlook and Fundamental Limits to Recording

• The bit density limit of thin film media is estimated to be approximately of order 100 Gb/in2.

• If bit size, λ/2, is to decreases,the write gap, g must decreases and the write head must be closer to the medium.

• smaller λ demands that the medium is reduced. The fringe field decreases and signal strength drops even more.The read head, then, must be either more sensitive or closer to the medium.

• Thus, all of the relevant dimension of the recording process need to be scaled down together to achieve high recording density.

• Thermal stability