HEADS I T. Stobiecki Katedra Elektroniki AGH 4 wykład

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 air-bearing surface (ABS) allowing the head to fly at a distance above the medium about 10 nm The magnetic disks (up to 10) in diameter 1 – 5.25 inches – RPM it is related to about 100 km/h

Inductive write head The yoke consists of structured Ni 81 Fe 19 (permalloy) films P 1 and P 2.These films are all deposited on the top of substrate which consists of insulators (Al 2 O 3 and TiC). The gap width is defined by the thickness of Al 2 O 3 insulation layer between P 1 and P 2 hich is below 100 nm. Micrograph of the write/read head taken by SEM from the ABS side.

Write field calculation Electric circuitMagnetic circuit Current (I) Flux (  =BA) Voltage (V)Magnetic potential (nI) Resistance (R) Reluctance ( R =l/   0 A) Conductivity (  )Permeability (   0 ) l c = length of the ring head core A c = core cross-section A g = gap cross-section  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 From Eq.(2) H g = nI/g if permability . Had has an ideal efficiency. (2)

Write field calculation The H x component: where (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 In high-density recording, the deep gap field required is: where H C is the coercivity of the recording medium where B S is the saturation flux density of the pole of yoke material

Plots of the horizontal component H x vs. distance x Note that the trajectory closer to the head (A-B) has both a higher maximum field and higher field gradient dH x /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 M R. 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.

Written magnetization transition The write current is adjusted so that horizontal component of H x on the midplane of the recording meets a specific criterion.

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: 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.

Coil write current Recording medium coercivity H c = 200 kA/m (2500 Oe) Write field H 0 = 2.5  H c = 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 H g of width g=100 nm and y/g=0.25 can be determined from Eq.(3) H g =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 H g. High field (H g ) 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 (Fe 97 Al 3 ) 75 N 25 and laminated multilayers FeAlN/Al 2 O 3.