Jim Giusti

Giant magnetoresistance properties of patterned IrMn exchange biased spin valves

Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve stripes. As the stripe height is decreased both the pinning field and free layer bias field increase while the GMR ratio stays almost constant. The free layer coercivity becomes zero for the shortest stripe while the pinned layer coercivity drops slightly at high aspect ratio. The pinned layer switching field is enhanced in the patterned films, indicating a strong magnetostatic coupling between the free and pinned layer that was further simulated micromagnetically.

Micro-magnetic and electric analysis on MR head baseline popping and instabilities

A general explanation for four types of instabilities in magneto-resistive (MR) recording heads is given. A micro-magnetic analysis of the root causes of the instabilities shows that baseline popping (BLP), permanent magnet reversal instability (PMRI), spiking noise and writer induced instability have the same root cause as random telegraph noise (RTN) which was reported previously. The details and differences in the formation of each individual phenomenon by the same cause have been investigated in this paper. An MR reader and pre-amplifier electric circuit simulation confirms that baseline popping is due to random telegraph baseline shift passing through a pre-amp DC blocking capacitor which is discharged and recharged at the two levels of the baseline.

Telegraph noise mechanism and LLG noise model

A preliminary study on random telegraph noise (RTN) is made. RTN is defined as the magnetization states switching by thermal fluctuations. State jumping resulting in baseline shift from the media field could also be RTN-associated multi-states switching. Our analysis shows that the root cause of such magnetization multi-states could be due to: (1) free layer edge states switching caused by a weak PM field; (2) surface and/or interface roughness, especially the lapped air bearing surface (ABS); and (3) random defects in the stack layers. A thermal noise model with Landau-Lifshitz-Gilbert equation (LLG) was developed and was applied to RTN demonstration.

Head field angle dependent writing in longitudinal recording

Summary form only given. The writing process in magnetic recording is the switching of media grains by the head magnetic field. As the head moves across the media, the grains in the recording layer experience the writing fields that vary both in magnitude and direction. Fundamentally, in the longitudinal recording, the writing field perpendicular component provides a path for the media magnetization gyromagnetic motion. Therefore it is likely to reduce the required longitudinal field for the grain reversal. The traditional Willium-Comstock model includes only the longitudinal components of the head field. It has predicted that the required deep gap field should be roughly 3 times the media (writing) coercivity. This requirement has been contradicted by the recently published experimental result of Liu et al. (see IEEE Trans. Magn., vol. 37, p. 613 et seq., 2001). Using our micro-magnetic recording model, which combines the writing and reading process at head-media level, the impact of the full 3D writing field on the recording process was examined. The present work focused especially on the effect of field angular dependence. A modified Willium-Comstock model was also developed to include the effects of the perpendicular field component. The corrected calculations predicted an optimum writing field that was reduced and agreed reasonably well with the results obtained from experiment and micromagnetic simulations.

Off-track bit error rate modeling

Traditionally, channel generated bit error rates (BERs) at a certain signal to noise ratio (SNR) have been modeled by a Gaussian distribution function. This approach has been suitable for a peak detection channel as the analysis is performed in only one bit. In the case of a partial response maximum likelihood (PRML) channel, one data bit is determined by multiple code bits. There is a burst of errors detected if any one of the involved code bits gives an error. The chi-square distribution function, which is a sum of several independent Gaussian-squared distributions, is another method to model BERs of a PRML channel. Particularly, different PRML codes could be modeled by a chi-square distribution with different degrees of freedom. In this paper, the relationship of SNR to BER is generated using the chi-square method. The measured on-track rms SNR, read sensitivity function, and media magnetization are the input parameters. Old data and adjacent track interference are considered as random variables. At sp...

Microtrack base line instability of spin valve heads

The baseline shift in a micro-track profile of spin valve heads was investigated. Experimental and micro-magnetic modeling results were compared and found in close agreement. It was confirmed that the nonuniformity of pinning field and variation of anisotropy were among the potential sources of the formation of multi-states and the baseline instability. It was shown that the resulting instability could be suppressed by applying a strong permanent magnet stabilization field.

Finite element study of the effect of permeable media on longitudinal writer field calculations

The effect of permeable media on writer field calculations was examined in this study via a finite element analysis in which the nonlinear characteristics of both the writer and the permeable media were represented. The applied field of the writer was calculated for the two cases: one without a permeable media and the other with a permeable media adjacent to the gap of the writer. No significant difference was found between the applied fields of the two cases, indicating that the demagnetizing fields present in typical longitudinal media are not large enough to perturb the writer pole magnetization, and thus the writer field during writing. The results validated the common practice of using applied fields directly from writer simulations for write process analysis in longitudinal recording.

Design and performance of a recessed thin film inductive transducer

An improved inductive transducer is achieved by placing the coil and the majority of the transducer body into a recessed alumina basecoat. Process advantages of a recessed over planar structure includes lower top pole topography, improved pole trim capability, and improved top pole composition uniformity. Coil and photoresist processing in a recessed cavity allows for multiple layers with lower resistance without increasing transducer Permalloy path length. Recessed trimmed design performance has increased efficiency with higher amplitude and a narrower pulse width along with improved overwrite compared to planar untrimmed transducers. The recessed basecoat design is explained, and compared to planar transducers using modeling and performance results.

Micromagnetic simulation of pseudorandom sequences for equalized SNR estimations in longitudinal recording

Summary form only given. The determination of signal to noise (SNR) and signal to distortion (SDR) ratios is important in estimating bit error rate in recording systems. The effects of signed equalization also modify the relative contributions of SNR and SDR. In this paper, we simulate the recording of a 31 bit pseudorandom sequence (PRS) of data by writing with head fields derived from a finite element model (FEM) with an exponential risetime onto a longitudinal recording media. The media is modeled by an array of uniformly magnetized cubes and brought to equilibrium after discrete downtrack translations of the write head until the PRS is completed. The process is repeated 46 times with different easy axis distributions of the cubes (grains).

Thermal micromagnetic model with a pseudo-three-dimensional junction for magnetoresistive heads

We developed a pseudo-three-dimensional (3D) thermal model in conjunction with a thermal micromagnetic model to study the performance of magnetoresistive (MR) readers. With decreasing sensor dimensions, the junction plays an important role in the overall sensor response. With an image method, we converted the 3D thermal problem to a set of coupled nonlinear differential-integral equations in three regions: sensor stack region, junction region, and lead region. Analytic methods for each region allow a speedy solution for rapid development in MR head design and performance analysis, such as numerical design of experiments and regression for design optimization, Monte Carlo analysis, and statistical analysis for product yield.