Download Fundamentals of Magnetism

HEADS I
T. Stobiecki
Katedra Elektroniki AGH
4 wykład 25.10.2004
History of HDD
• 1956 – HDD of IBM, random access method of accounting and control
(RAMAC)
• 1980 – induction thin film head
• 1990 – write induction coil, read AMR sensor
• 1996 – GMR sensor
The writting process
The magnetoresistive head depend on the written magnetization. In order to obtain
the maximum output voltage, it must match properly the written magnetization
transmission in the recording medium.
Schematic representation of a longitudinal recording
process
Magnetic force micrograph (MFM)of recorded bit
patterns. Track width is 350 nm recorded in
antiferromagnetic coupled layers (AFC media)
Disc drive
The slider carrying the
magnetic write/read head.
The slider is mounted on
the end of head gimbal
assembly (HGA)
The magnetic disks (up to 10)
in diameter 1 – 5.25 inches.
5.400 – 15.000 RPM it is
related to about 100 km/h
The air-bearing surface
(ABS) allowing the head
to fly at a distance above
the medium about 10 nm
Inductive write head
The yoke consists of structured Ni81Fe19 (permalloy)
films P1 and P2.These films are all deposited on the
top of substrate which consists of insulators (Al2O3
and TiC). The gap width is defined by the thickness
of Al2O3 insulation layer between P1 and P2 hich is
below 100 nm.
Micrograph of the write/read head taken by
SEM from the ABS side.
Write field calculation
Electric circuit
Magnetic circuit
Current (I)
Flux (=B•A)
Voltage (V)
Magnetic potential (nI)
Resistance (R)
Reluctance (R=l/ 0A)
Conductivity ()
Permeability ( 0)
g=c (1)
by assumption that had is infinite in z direction and
the core is much wider than the gap (g) in x direction
Hg 
lc= length of the ring head core
Ac= core cross-section
Ag= gap cross-section
Rg
Rg  Rc
nI

g
nI

l 
 Ag
g 
 c 0  
Ac 



From Eq.(2) Hg= nI/g if permability . Had has
an ideal efficiency.
(2)
Write field calculation
The Hx component:
Hx 
Hg


 xg


2
  arctan 

 y



where

 xg


2
  arctan  y









(3)
For calculation of the write field within the magnetic medium y can be taken as: y=d+/2
Write head field
Ferrite heads the core is usually made of NiZn or MnZn.
Insulators can be operated at frequency > 10MHz
Thin film heads yoke (core): permalloy (81Ni19Fe) or aluminium - iron - silicon alloy (AlFeSi) typically 2- to 4 µm thicknesses.
Write head field
H0 
0.4NI
gw
In high-density recording, the deep gap
field required is:
H 0  3H C
where HC is the coercivity of the recording
medium
where BS is the saturation flux density
of the pole of yoke material
H 0  0.6 BS


H0
yg w
HX 
tan1 
2
g
 2

2
x y  w
4

Plots of the horizontal
component Hx vs. distance x






Note that the trajectory
closer to the head (A-B)
has both a higher maximum
field and higher field
gradient dHx/dx.
Written magnetization transition
• When the written current is held constant, the magnetization
written in the recording medium is at one of the remanent levels
MR. When the write current is suddenly changed from one
polarity to the other, the written magnetization undergoes a
transition from one polarity of remanent magnetization to the
other.
M ( x) 
x
M Rtan1  

f
2
Written magnetization transition
The write current is adjusted so that horizontal
component of Hx on the midplane of the recording meets
a specific criterion.
HC
3
 dH x 

 
2 (d   / 2)
 dx  max
Written magnetization transition
 4M R
 dH d 

 
2
dx
f

 max
dM dM d H head  H demag 

dx
dH
dx
 2 HC

f  2
 d   / 2
 3 MR


1
2
Written magnetization transition
The writting problem is now completly solved because f is but the single
parameter required to define fully the magnetization transition of
equation:
x
M ( x)  M Rtan  

f
2
1
Possible ways to reduce the transition width, by reducing f, are to use higher
coercivities, lower remanences, smaller flying heights, and thinner media.
With the exception of lowering the remanence, all have been exploited in the
past. When inductive reading heads are used, reducing the remanent
magnetization is not an acceptable strategy, however, because it always
reduces the signal and signal-noise ratio of the recorder.
Equation for transition slope parameter f is also used in the simplified design
of the shielded magnetoresistive head.
 2 HC

f  2
 d   / 2
 3 MR


1
2
Coil write current
Recording medium coercivity Hc = 200 kA/m (2500 Oe)
Write field H0= 2.5Hc= 500 kA/m (6250 Oe)
Medium thickness = 10 nm
The head medium spacing d=20 nm
The parametr y=d+/2= 25 nm
For x=0 the required minimum field in the head gap Hg of width g=100 nm and
y/g=0.25 can be determined from Eq.(3) Hg=710 kA/m (8875 Oe)
If the write head coil consists of n=10 turns, the coil current I=7 mA.
Write head materials
In order to to achieve high data rates, high areal densities as well as reliable
performance suitable magnetic materials for inductive write heads have to fulfill a list of requirements:
•High saturation magnetization (Ms) is necessary because it defines the maximum Hg. High field (Hg) is
necessary to write magnetic media with high coercivity (sitable for for high density storage).
•The soft magnetic materials must have large permeability () over wide frequency range to achieve
sufficient head efficiency at high data rates. A permeability loss at higher frequencies due to eddy currents
can be suppressed by highly resistive materials or laminated materials with insulating layers. Laminated
structures require dry deposition process as sputtering.
•The yoke materials must be magnetically soft in order to minimize hysteresis losses.
•The head materials shoud be high temperature, mechanically and chemically resistant and stable also
during operation within the HDD.
The satisfy the requirements to large extend NiFe alloys, iron nitrides (Fe97Al3)75N25 and laminated
multilayers FeAlN/Al2O3.