Manoj K. Bhattacharyya

Dual stripe magnetoresistive heads for high density recording

The design and recording performance of dual stripe magnetoresistive read/inductive write heads with read widths of 4 /spl mu/m and write widths of 4.5 /spl mu/m are described. A linear density of 75 kfci (D50) was measured in heads with shield-to-shield spacing of 420 nm and 70 nm of dielectric separating the two magnetoresistive stripes. Large signal amplitude, linear cross-track profile, and good second harmonic suppression are observed in accordance with theoretical expectations. Readback waveforms contain little baseline shift and the ratio of positive to negative peak amplitudes is very close to unity. Stable signals are seen for heads with and without exchange stabilization. Conductor topography in the read head is replicated in the write head and can adversely affect cross-track behavior. Non-planarity of the write head must be considered in the design of shared pole magnetoresistive heads. >

Impact of new magnetoresistive materials on magnetic recording heads (invited)

Advances in magnetoresistive materials have recently enabled magnetic recording heads to achieve higher levels of performance. This article describes why higher signal outputs are necessary for improvements to be made in areal density. The requirements for recording at an areal density of 16 Mb/mm2 (10 Gb/in.2) are discussed with regards to both the channel and the head design. Increased output from new multilayer magnetoresistive materials is required to counteract the decrease in output due to the reduction in the size of the head geometry. An areal density of 16 Mb/mm2 is shown to be feasible with spin valve recording heads using materials with magnetoresistance ratios of 10%. Fabrication issues relating to the manufacturing of these materials are shown to be more stringent than previously required.

Thermal variations in switching fields for sub-micron MRAM cells

Thermal effects in switching of sub-micron tunnel junctions are investigated. The switching field is shown to he inversely dependent on temperature, and switching field jitter is shown to be strong function of temperature. Micromagnetic modeling is used to understand thermal effects. In some instances a stability factor (defined as KV/kT) of 100 or more may be required for acceptable switching field jitter, while with proper optimization of FM layers, stability factors of 50 or 60 may suffice.

Bias scheme analysis of shielded MR heads for rigid disk recording

Biasing schemes for MR heads for use at high areal densities, O(108 bits/in.2), longitudinal recording, are reviewed. By use of the finite element analysis, a number of important design parameters are examined including the transfer function and the peak shift for each biasing scheme. We show that the roll‐off peak shift and linearity characteristics are dependent on the type of bias scheme.

Determination of overwrite specification in thin-film head/disk systems

A self-consistent write analysis using the Preisach model is presented and used to calculate overwrite in thin-film head/disk systems. Two overwrite characterization procedures are discussed, and for each of the procedures the calculated values are compared with measurements. Through a correlation of overwrite, nonlinear peak shifts, and readback output voltage the authors determine the overwrite requirement. They show that whereas -30 dB of overwrite will ensure a satisfactory overall performance of the recording system, a much lower overwrite, say -20 dB, can work in certain situations. An analytical implementation of the self-consistent model is presented and verified with measurements. The analytical approach can be used to determine the overwrite at any applied field for any head/disk combination. The authors introduce a normalized effective field h/sub n/, which depends on M/sub r/ delta , H/sub c/, and other head/disk parameters. It is shown that a value of h/sub n/ greater than 0.8 is needed for overwrite values of better than 25 dB. The importance of various head/disk parameters in optimizing the overwrite is also discussed. >

Head field measurements in three dimensions

A method is described for obtaining all three components of the field produced by a magnetic recording head. The perpendicular component of the field, H/sub y/, is measured in the plane of the air-bearing surface, and from this the three components, H/sub x/, H/sub y/, and H/sub z/, are calculated in the region below the surface. An example is presented of the field distribution for a thin-film head, and a comparison is made with both analytic and three-dimensional finite-element calculations. >

3D analysis of MR readback on perpendicular medium

A 3-D analysis of magnetoresistive (MR) read on perpendicular media using the reciprocity method is presented. It is shown that the conduction of magnetic flux, obtained from an equivalent reciprocity field, away from the active area is an important phenomenon for this structure, and may introduce error if 2-D analysis is used. The authors examine the effect of shield width and show that when the shield width is equal to the width of the active area of the MR head, 3-D analysis closely resembles the 2-D results. This calculation is extended to a differentially connected dual element MR head for perpendicular recording. >

Pulsed current switching of submicrometer MRAM cell

Thermally assisted switching of submicrometer magnetic tunnel junctions is investigated. It is found that writing can be done with pulses of the order of 1 ns. Switching current and switching current jitter are seen as strong functions of temperature, whereas both are only weakly dependent on current pulse duration. Micromagnetic modeling using a stochastic Landau-Lifschitz equation is used to understand thermal effects. The simulation predicts the observed switching current but does not explain the magnitude of the switching current jitter.

MR head manufacturing sensitivity analysis using transmission line model

Transmission line based equations are developed to describe the operation of MR heads. An analytical solution for magnetic flux in MR films is derived for linear operation. This transmission line method is used to analyze the design center of shielded dual stripe, soft adjacent layer and barberpole MR heads. Error propagation analysis is used to analyze the manufacturing tolerance sensitivity of these various heads. Finally, a Monte Carlo simulation is used to investigate the expected distribution in performance of manufactured MR heads. >

Shielded MR gradiometer read pulse shape for perpendicular medium

Isolated output pulse shapes are calculated for magnetoresistive gradiometer read heads having various geometric and magnetic parameters. The two‐dimensional finite‐element and reciprocity methods are used. The ferromagnetic shield has a big effect on the pulse shape, especially for closely spaces shields. High shield reluctance causes pulse broadening and large positive tails. Addition of a medium underlayer decreases the pulse undershoots and increases pulse amplitude. A reluctance model is given that explains most of the observed effects on pulse shapes and illustrates the importance of reluctance balance to achieve optimum pulse shape.