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

Schematic representation of longitudinal, digital magnetic recording 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 h x for a longitudinal transition of zero width (a=0) and for a = 0.5 at y = 0, 0.5( Eq.1), respectively. (1) H x (x,0)≈4M r δ/x Transition width

Schematic representation of field above a longitudinal recording medium. y

Transition width

The coecivity squareness parameter S * is defined as S * = 1 – M r / x o H c, x o = [∂M/ ∂H] Hc S* varies from 0 to 1 The switching field is defined as SFD=ΔH/H c,ΔH is the full width at 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 M s 2 /K 1/2, the solutions: decreasing M r t and increasing K. (3) The signal is proportional to the number of measured events or particles per bit, N. Hence SNR ~ N 1/2. (4) The heads must approach to the hard disc surface. Material Requirements For recording media

CoCrPtB films

Write head : having a sufficient high M s so that the fringe field exceeds the H c of the medium ( Oe); an adequate magnetic permeability (easy saturated). Read head: low H c, 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 medium and head material. For write head: µ >>1, M s large and B r =0; For read head: µ >>1, H c = 0

Thin film recording head. Left, layout of pole pieces and windings; right, enlarged, cross-sectional view of magnetic pole pieces Film thickness 2-3 micrometer; Gap 200 nm. Thin film recording head

Permeability versus frequency for four thin films. High frequency 10 9 Hz; A weak uniaxial anisotropy; High electrical resistivity

Field dependence of magneto- resistivity for uniform response to a uniform field. Geometry of magnetoresistive sensor showing sense current, anisotropy field, and external or fringe field of medium, and their effect on magnetization. h=1-2 µm, w=2-4 µm t=10-20 nm Δρ/ρ =2.0% Ni 81 Fe 19 Magnetoresistive head

Spin-Valve Read head Structure of a simple spin valve; the device dimention are approximately h=2-6 µ m and w=10 µ m.

Experimental transfer curve for a 2 µ m high spin valve sensor for +5mA (solid) and -5mA (dashed) sense current. M2M2 M1M1

Longitudinal and perpendicular recording Comparision of recorded bits in longitudinal (a) and perpendicular (b) media. Demagnetization factor for a recorded bit : (a) proportional to M r t/ λ; and (b) to M r λ/ t. Linear bit density: (a) 10 5 bit per inch (λ=0.5 µ ); and (b) x10 5 bpi

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

Thermal Stability In the physics of magnetic recording there are two key factors 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 with the ambient thermal energy.

The signal to media noise is approximately by the number of magnetic grains (or switching units) per bit: SNR media ~ Wbt / v g Where, wbt (bit volume, read-width x bit-length x thickness) v g (the grain volum) In order to avoid thermal instability, a minimal stability ratio of stored magnetic energy, K u V, to the thermal energy, K B T, K u V/K B T ≌

Interlayer antiferromagnetic coupling media Schematic illustration of (a) a two layered AFC media, (b) LAC media with high J and (c) advanced three layers LAC media for much lower M r δ. Longitudinal

M r t = M r t 1 – M r t 2 K u V 1 <K u V eff < (K u V 1 +K u V 2) K u V/K B T ≌ In the case of two layers AFC media Magnetic hysteresis loop for a single layer media (a) and an AFC media (b). J ex =0.06 erg/cm 2, H ex ~800 Oe.

(a)Room temperature H cR vs M r t for single layer media and AFC media (b) Thermal decay. (1) Fitted by Eq.(1) Where t p is about 1 s and f o ~10 9 Hz; from the fit, we obtain H o =8.6 KOe, K u V/K B T=75 for the single layer; H o =8.3 KOe, K u V/K B T=100 for AFC one.

Interlayer antiferromagnetic coupling media Perpendicular Magnetic loop as a function of Ru thickness Interlayer antiferromagnetic Coupled two grains

Correlation between exchange field, H ex, coecivity field, H c, and nucleation field, H n.

Normalized effective energy barries, KV eff /KV 1, as a function of the apparent exchange coupling J app.

hybrid recording (Solid immersion lens) ZnS:SiO 2 NA ~1.1 Media: Co x Tb Ag x, x=

Patterned Media Scanning electron microscopy image of a square array of electodeposited Ni pillars of high 300nm and period. Low noise, high density

High K u Materials Approach to 100 Gbits/in 2 Smaller, thermally stable media grains Prominent candidates are RE-TM Co 5 Sm and L1 o phases FePt (H c >1T), CoPtY….. 3 times smaller grain diameters d and potential 10 fold areal density increase ( ∝ 1/d 2 ) Write field KOe K u V/kT>40-60 D.Weller et al., IEEE Trans on Mag., 36(2000)10

Magneto-optical Recording Principle of thermomagnetic recording (Curie point writing): (a) before, (b) during and (c) after the writing.

Temperature dependence of the magnetization for a 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, H d the demagnetizing Field, H ext the applied external field and σ W the wall energy density of the magnetic medium. Huth’s equation (1974 IBM J..Res.Dev ) The spot size and signal to noise d=λ/(2NA); S/N ~ θ K R 1/2 * Bobeck IEEE Tran. Mag., MAG-5(1969)554.

From Oppeneer Magneto-optical Kerr spectra in Handerbook of magnetic Materials, 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 and M-H, ΔR/R characteristics of the PSV.

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

Table: composition and dimensions of the principle layers in a current representative 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/in 2. 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