Magnetoresistive Heads Research Articles

5 Gb/in/sup 2/ recording with dual stripe AMR heads and low noise thin film disks

We have demonstrated magnetic recording at an areal density of 5 Gb/in/sup 2/ using dual stripe magnetoresistive heads (DSMR) on low noise and low-glide height quaternary CoCrTa-based alloy thin film disks. The data rate is 120 Mb/sec with a PR4 channel, and the on-track error rate achieved is around 10/sup -9/. The demonstration was accomplished in two cases using two different types of heads. In the first case, it was done with a DSMR head having shield-shield spacing of 0.165 /spl mu/m and 1 /spl mu/m stripe height flying at 25 nm, resulting in a linear density of 321.7 KBPI, and a track density of 15.6 KTPI. The second case was performed with DSMR heads having shield-shield spacing of 0.18 /spl mu/m and 8 /spl mu/m stripe height flying at proximity height of 12 nm, achieving a 358.75 KBPI and 13.8 KTPI. This is the highest linear density published so far for either GMR or AMR heads. It is the direct consequence of high signal/noise of DSMR heads and proximity recording over low noise, low-glide-height media. To explore the large dynamic range of DSMR heads, this study was also done with a variety of low noise disks with Mrt ranging broadly from 0.45 to 1.0 memu/cm/sup 2/ and coercivity of 3000 Oe.

Williams-Comstock model with finite-length transition functions

The theory of the third-order-polynomial (TOP) and fifth-order-polynomial (FOP) magnetization transitions is presented. These transitions have a finite length, rather than an asymptotic approach to /spl plusmn/M/sub r/., which is the case with some widely-used transition functions. The Williams-Comstock model is used to obtain the transition parameters, which are equal to half the transition lengths, resulting in quadratic equations and simple expressions. In this analysis, the write-field gradient is maximized with respect to the deep-gap field, as well as with respect to the distance of the transition from gap center, which results in a higher gradient than is obtained with the original Williams-Comstock approach, at the expense of a higher write current. Analytic expressions are obtained for the read pulses of inductive and shielded magnetoresistive heads, and equations for nonlinear transition shift are derived for the arctangent and the TOP transitions. The results are compared with those obtained using arctangent and tanh transitions and with experiment. In addition, certain aspects of the write-process Q function and the optimum deep-gap field are discussed.

Dynamic in-situ measurements of head-to-disk spacing

The read signal from a magnetoresistive (MR) head is used to measure the dynamic head-to-disk spacing (flying height) inside of an operating disk drive. Changes in flying height modulate both the envelope of the read signal and its mean value. While the envelope is related to the magnetic spacing, the mean value is a function of the thermal spacing. The thermal spacing signal is the result of Joule heating whereby some of the thermal energy generated in the MR head is dissipated into the air film between the head and disk. Changes in head flying height alter the heat transfer and hence the temperature and resistance of the head. The thermal and magnetic signals are separable and give complimentary information about the flying height. To accurately measure flying height changes using the thermal signal when the head is inside a disk drive requires that the signal distortion caused by the head preamplifier and high-pass filter be corrected. The thermal signal, unlike the magnetic signal, is not sensitive to track misregistration making it more accurate for dynamic flying height measurements. In-situ dynamic flying height measurement results using both thermal and magnetic methods are discussed.

Equalization and detection for nonlinear recording channels with correlated noise

At high recording densities, signal nonlinearity due to both the medium and the magnetoresistive head and correlated noise due to the medium and the equalization process, substantially degrade the error-rate performance of the Viterbi algorithm (VA). This paper describes a method for dealing with both problems by simultaneously adapting the equalizer, its target and the VA branch metrics. A monic constraint is employed to locate a target that limits noise correlation at the input to the VA. Pattern-dependent offsets, rather than actual VA ideal values, are adaptively determined to account for channel nonlinearity in the VA. The effectiveness of these techniques is demonstrated on very high density data collected from a spin stand.

Analytic solutions for double-element shielded magnetoresistive heads

New analytic solutions are presented for idealized mathematical models of double-element shielded magnetoresistive (MR) magnetic recording heads. Two types of heads are considered, represented by poles of either equal or opposite magnetic potentials, each in the presence of an infinitely permeable underlayer. The solutions for heads without underlayers are special cases of the more general results. Typical field plots and spectral response functions are given and comparisons are made with published approximate solutions.

Effect of stress induced anisotropy on SAL-biased MR head performance

The effect of magnetostriction (MS) on the device characteristics of soft-adjacent-layer (SAL) biased magnetoresistive (MR) head was investigated. As MS changes from -7/spl times/10/sup -7/ to 1.77/spl times/10/sup -6/, the magnetic track width increases from 1.26 to 1.83 /spl mu/m. Asymmetry of bias point, output sensitivity, and output amplitude are also affected by the stress induced anisotropy. The variation of magnetostriction (/spl Delta//spl lambda/=2/spl times/10/sup -6/) induces the change in bias field by 60 Oe associated with the estimated stress of +800 MPa on the MR stripe.

Magnetoresistive read/write channel models

We studied the magnetic read/write channel with a magnetoresistive (MR) read head at a normalized channel density range of 2.5-3.0, observing and modeling partial erasure and nonlinear transition shift. The asymmetry of the MR head had a significant effect on the channel. We applied the transition-width-reduction model, the partial-erasure-plus-transition-shift model, and the Volterra model to the MR read/write channel, and evaluated the accuracy of these models on the basis of parameters obtained from experimental data. In order to consider the asymmetry of the MR head, we introduced a nonlinear MR head model into the read/write channel. These modified models achieved higher accuracy. Of all the models considered, the Volterra model provides the largest improvement over the linear model.

Transmission electron microscopy of cross-sections of magnetoresistive read heads

Abstract Cross-sectional images of the multilayered structures of magnetoresistive (MR) read heads showing the nanostructure of individual layers are desirable for process evaluation and failure analysis. We have successfully prepared thin cross-sections of MR read heads with soft-adjacent-layer transverse-biased sensor and pattern exchange longitudinal biasing for study by transmission electron microscopy (TEM). The perfection and defects of individual structural layers (about 10–20 nm thick) of the MR head are clearly visible in cross-sectional TEM images. Two very thin defect layers due to residual photoresist materials have been observed at the MR sensor-exchange layer interface and exchange-conducting layer interface. The origin of the defect layers is the incomplete removal of residual photoresist materials by ion–beam etching during processing. Deeper etching by ion-beam milling reduces the size of the defect layers. The dummy resistance of the MR sensor is substantially reduced after deeper etchin...

MR reader sensitivity and SAL magnetic anisotropy

Two kinds of abutted junction magnetoresistive recording heads with different SAL effective magnetic anisotropy were manufactured. Experimental results show that heads with near zero magnetostriction have stable performance. However, heads with very positive (10/sup -6/) magnetostriction show multi-domain state, bumpy microtracks. The amplitude of this head with highly magnetostrictive SAL depends on the initial SAL magnetic state with respect to the bias current. All these abnormalities are mainly caused by the effective magnetic anisotropy of the SAL being perpendicular instead of parallel to the ABS of the SAL layer.

Micromagnetic simulation of 20 Gb/in/sup 2/ spin valve heads with heating effects

A micromagnetic model is described and used to study the behavior of spin valve recording heads. The finite difference method is used to solve for the magnetostatic potential, the electrostatic potential and the temperature. With this formulation we can investigate the effect of structure shapes, current distributions, and temperature on the magnetic performance of magnetoresistive read heads. Details of device response degradation due to increased resistivity is shown.

Transmission electron microscopy of cross-sections of magnetoresistive read heads

Transmission electron microscopy of cross-sections of magnetoresistive read heads

Magnetoresistive read nonlinearity correction by a frequency-domain approach

The readback process of a magnetoresistive (MR) head is intrinsically nonlinear. To correct the nonlinearity, a new technique based on a frequency-domain measurement is proposed. In this technique, square-wave patterns of certain frequencies are written as probing signals. The harmonics of the readback signals are then measured and used to derive the coefficients of a correction polynomial. Experimental results show that the correction method significantly reduces the effect of MR saturation on other parametric measurement such as nonlinear transition shift.

Magnetic properties of ion beam deposited CoPt and CoCrPt films for hard bias application in high density magnetoresistive heads

We have observed a 30% of enhancement in coercivity, H/sub c/, or a 40% increase in remnant magnetization in ion beam deposited (IBD) Cr/CoPt films as compared to IBD Cr/CoCrPt films. In addition, the magnetic properties of these hard bias films exhibit a strong dependence of on substrate and underlayer. Higher coercivity values associated with enhanced hcp Co (101~0) crystallographic orientation were measured in films grown on substrates in the preference order of glass, Si/Al/sub 2/O/sub 3/ and Si. CoPt films grown on CrV underlayer show lower H/sub c/ values than films deposited on Cr underlayer. H/sub c/ exhibits a maximum with increasing Cr underlayer thickness for both Cr/CoPt and Cr/CoCrPt films. This Cr thickness dependence of H/sub c/ is correlated well with that of Co (101~0)/(0002) X-ray diffraction peak intensity ratio, indicating the role of crystallographic texture in control of coercivity of IBD hard bias films.

Performance of dual-stripe giant magnetoresistive heads on tape

Dual-stripe giant magnetoresistive (DSGMR) devices with an insulating spacer were fabricated and tested on tape media. The head design consists of two mutually biased, unshielded GMR elements based on [Cu 19 /spl Aring//NiFe 13 /spl Aring//CoFe 4/spl Aring/]/sub /spl times/10/ antiferromagnetically coupled multilayers, separated by a 600-/spl Aring/-thick SiO/sub 2/ spacer. The sensor is patterned into a U shape, with leads connected away from the active region to avoid topography. Simultaneous etching of both GMR elements avoids misalignment problems along or across the track width. A DSGMR tape head operating in differential mode (track width of 2 /spl mu/m and height of 0.8 /spl mu/m) was tested versus high density dibits (0.05<bit<4 /spl mu/m) recorded on tape media, using a current bias of J=2.8/spl times/10/sup 7/ A/cm/sup 2/ on each GMR. The DSGMR responds to the dibit with a bipolar pulse independently of bit separation even when the individual GMR elements are partially saturated. Peak-to-peak output is 1.7 mV//spl mu/m, for bit sizes /spl ges/0.4 /spl mu/m. Experimental pulse amplitude and shape are in agreement with a two-dimensional micromagnetic simulation. The head response to a dibit signal loses half its maximum amplitude for a recorded bit size of 0.2 pm. Pulse broadening (read bit cell is 0.8 /spl mu/m larger than recorded bit, for bit sizes <0.4 /spl mu/m) is correlated with sensor geometry.

Field-induced breakdown ESD damage of magnetoresistive recording heads

In this paper a new ESD failure mechanism for magnetoresistive (MR) recording heads is reported and studied. Experimental results show that an MR head can be severely damaged by electric field alone. SEM photographs show pitting and melting damage between the closely spaced conductors that are exposed on the “air bearing surface” of the recording head. The damage is explained by breakdown of the air gaps due to large induced potential differences when the MR head is in an electric field. Construction of a simple “spark generator” is described and used to study the field-induced breakdown behavior of an air gap.

Three-dimensional transient simulation of magnetoresistive head temperature during an ESD event

Magnetoresistive (MR) heads for disc drives are susceptible to very low levels of ESD in the manufacturing and assembly process. The cost of failed heads and the ESD controls in the factories are high. One mode of failure is overheating of the MR sensor due to the ESD current. Knowledge of the head design parameters which affect the peak ESD temperature are desirable to guide the head design. The MR sensor temperature during an ESD event is modeled in three dimensions using finite elements. Results are given for the relative effect of ESD with a range of thermal and electrical properties, dimensions, and discharge time constants. The key parameters are the material properties of the sensor and insulator, the sensor height, and the insulator thickness. Lateral heat conduction reduces the temperature at the edges of the sensor.

High-density recording using MR heads in helical-scan tape systems

This paper presents the application of magneto-resistive (MR) heads to the helical scanning tape system. After we discuss some problems including the contact between the MR head and the Co-O evaporated (ME) tape, we report a feasibility of a high-density recording in the helical scanning tape system using MR heads.

Thermal aftereffects in thin film magnetic recording media

Thermal aftereffects in written bits in CoCrPt thin film magnetic recording media are examined using a magnetoresistive (MR) head and a magnetic force microscope (MFM). Decays in signal output over time at room temperature in media with very thin magnetic layers of about 14-nm thickness or less were observed by using an MR head. The decay in signal output was accompanied by an increase in medium noise. Even with a precise examination of cross-track profile using an MFM, no change in track width caused by thermal aftereffects can be detected. Thermal decay of the signal output in a 14-nm thick magnetic layer with small remanence thickness product M/sub r/t/sub mag/ of about 40 G-/spl mu/m was successfully reduced by increasing the platinum content in the CoCrPt magnetic layer or by adding boron to the CrTi underlayer without increasing the medium noise. The reduction of thermal decay is considered to be caused by the increase of magneto crystalline anisotropy and increase in magnetic-crystal-grain size. These results suggest that it is possible to reduce the medium noise without reducing the crystal grain size and without much degradation of thermal stability even for media with small M/sub r/t/sub mag/ which is needed to achieve a recording density of 10 Gb/in/sup 2/.

Crossfeed response of DMR vs. SAL multi-track heads

A new concept of Dual Magnetoresistive reproductive mode is applied to improve the design and operation of high-performance multi-track Read-While-Write (RWW) heads for tape and/or potential disk applications. RWW operation exists in some magnetic recording units in which verifying what was recorded is necessary. A new multi-track Inductive/Dual Magnetoresistive (DMR) head design is found to reduce the crossfeed noise radiation by about a factor of 20, compared with the conventional Soft-Adjacent-Layer (SAL) Magnetoresistive Heads design, which is the most favoured approach for achieving simultaneous read and write operations, especially in a multi-track head with ultra-narrow trackwidth. To verify these analysis results, measurements with 2.9 /spl mu/m track width inductive/MR and inductive/DMR heads were done.

Estimation of track misregistration by using dual-stripe magnetoresistive heads

We propose here a new concept of servoing with dual-stripe magnetoresistive (DSMR) heads. This paper examines the use of its asymmetric microtrack profile for estimating the data and providing servo information. The misregistration estimation algorithm is addressed for a class 4 partial response (PR4) equalized data channel. This requires a low-pass filter to reach an accurate estimation after enough samples are used. Simulation results for a specific head configuration are given which verify the estimating performance. The estimator estimates track misregistration precisely and quickly enough for practical purposes in a real storage system.