Transcript 幻灯片 1

Magnetic Data Storages
(1) Magnetic recording
(a) Generalation (why SNR∝ N1/2, 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 magnetic
recording write process.
The recording medium
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 ?
Transition width
Hx(x,0)≈4Mrδ/x
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)
y
Schematic representation of field above a longitudinal
recording medium.
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 width
at half maximum as indicated in the fig. High SFD implies a spatially
sharp and requires a narrow magnetisation transition particle size
distribution.
Material Requirements
For recording media
(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 Ms2/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.
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 medium
and head material.
For write head:
For read head:
µ >>1, Ms large and Br=0;
µ >>1 , Hc = 0
Thin film recording head
Film thickness
2-3 micrometer;
Gap 200 nm.
Thin film recording head. Left, layout of pole pieces
and windings; right, enlarged, cross-sectional view
of magnetic pole pieces
High frequency 109Hz;
A weak uniaxial anisotropy;
High electrical resistivity
Permeability versus frequency for
four thin films.
Magnetoresistive head
Field dependence of magnetoresistivity 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% Ni81Fe19
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 .
M2
M1
Experimental transfer curve for a 2 µ m high spin
valve sensor for +5mA (solid) and -5mA (dashed)
sense current.
Longitudinal and perpendicular
recording
Comparision of recorded bits in longitudinal (a) and
perpendicular (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 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:
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
Longitudinal
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 Mr δ .
In the case of two layers AFC media
Mrt = Mr t1 – Mr t2
KuV1<KuVeff < (Ku V1+KuV2)
KuV/KBT ≌ 50 - 70
Magnetic hysteresis loop for a
single layer media (a) and an
AFC media (b). Jex=0.06 erg/cm2,
Hex~800 Oe.
Fitted by Eq.(1)
(1)
(a) Room temperature HcR vs Mrt for
single layer media and AFC media
(b) Thermal decay.
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
Interlayer antiferromagnetic
Coupled two grains
Magnetic loop as a function of
Ru thickness
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
Low noise,
high density
Scanning electron microscopy image of a square array
of electodeposited Ni pillars of high 300nm and period.
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 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*
Huth’s equation (1974 IBM J..Res.Dev. 10 100-9)
Where r is the domain radius, Hd the demagnetizing
Field, Hext the applied external field and σW the
wall energy density of the magnetic medium.
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 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/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