Xiao-Guang Ye

Effect of in-plane easy axis orientation in narrow track recording

The effect of crystalline easy axis orientation in longitudinal thin-film recording media is studied via micromagnetic modeling. The focus is on bicrystal films, in which the crystalline easy axes are oriented in two specified orthogonal directions. Hysteresis properties, side writing phenomena, and on-track noise behavior of the films are studied. Comparisons were made with oriented and planar isotropic films. It has been found that in the bicrystal film the effective magnetic easy axes are along the diagonal directions in between the crystalline easy axes in the film plane. Along the effective easy axes, the hysteresis loop is similar to that of the oriented film with high remanent and coercive squarenesses. Unlike oriented films, the side-written band in the bicrystal film acts like an erase band containing no edge transitions. The on-track noise of the bicrystal film is low at all recording densities, while the noise in oriented films sharply increases at high recording densities. It is concluded that the bicrystal film is more suitable for narrow track recording than either the oriented film or the planar isotropic film. >

Modeling of thin‐film media with advanced microstructure for ultrahigh density recording

Magnetic thin films with subgrain structure, or nanocrytalline films are studied via micromagnetic modeling. The subgrain nanocrystallites form a bicrystal structure inside a normal grain, due to proper epitaxial growth. It is shown that a relatively small but nonzero exchange coupling through normal grain boundaries is critical for obtaining high coercive squareness. Maintaining low exchange coupling between subgrains in a normal grain is important for achieving low transition noise. The calculation shows that films with this microstructure can have S*≊0.95 and a signal‐to‐noise ratio of 36 dB at 385 KFCI with 1‐μm track width. Such film microstructure is practically achievable in principle and is suited for future high recording density applications.

Nonlinear partial erasure and its correlation with transition noise in longitudinal thin‐film media

Nonlinear partial erasure and the supralinear noise enhancement at high recording densities are the two critical factors limiting linear recording density in longitudinal thin‐film disk media. In this paper, via spin stand measurements, nonlinear partial erasure was studied in terms of medium magnetic parameters, such as MrT, Hc and orientation ratio, and recording conditions, such as fly height and write current optimization. It is found that the nonlinear partial erasure and the supralinear noise increase always occur at the same recording density, independent of media magnetic properties and recording condition.

Impact of medium noise correlation on various partial response channels

This work investigates the impact of correlated medium noise at high recording densities to various advanced recording channels. Based on spin-stand measurements, noise spatial correlations of dibit transitions at small bit intervals is characterized as two statistically independent correlation modes: the coherent amplitude fluctuation of the dipulse and shift-in-unison of the entire dipulse waveform. In the density region where supralinear noise enhancement is relatively pronounced, the amplitude fluctuation mode dominates the noise spatial correlation. It is found that the correlated medium noise can degrade the performance of a (0, k) coded PR4-ML channel by 5-6 dB over white Gaussian noise. However, (1, k) coded EPR4 and EEPR4 channels are much more immune to the coherent amplitude fluctuation mode noise. It is concluded that in the supralinear noise region, (1, k) coded EPR4 and EEPR4 channels could have much improved performance over (0, k) coded PR4 channel in the medium noise dominated environment.

Micromagnetic study of magnetization process in bicrystal thin film

A computer simulation study of the magnetization process in bicrystal thin films is presented. Nearly unity squareness is obtained in these films with field applied along the diagonal directions between the two crystalline easy axes, which indicates that the diagonal directions are the effective easy axes. When the field is applied at angles between the effective easy and hard axes, it is found that a magnetization reversal always initiates by forming a transverse domain with magnetization virtually perpendicular to that at the saturation remanent state. The reversal proceeds by forming completely reversed domains within the transverse domains and expanding through the entire film. >

Track edge overwrite and easy axis orientation in narrow track recording

Micromagnetic modeling is utilized to study the track edge overwrite characteristics and correlations with grain easy axes orientation in longitudinal thin film recording media. The authors compare well-oriented films with planar isotropic films. It is found that for well-oriented films, the side-written band contains signal-related magnetic pole density. The pole density patterns exhibit strong dependence on previously written transitions at track edges. For planar isotropic media, edge overwrite simulation shows a clear side erase band. Read-back voltage waveforms are analyzed. A planar isotropic film shows much better off-track properties than an oriented film. >

MFM study of edge overwrite in perpendicular thin film recording media

This paper presents a magnetic force microscopy (MFM) study on recording and edge overwrite in a keepered perpendicular thin film medium with a pole-tip head which is in-contact with the disk surface during the recording. The MFM image of a single recorded track shows that the edge of the recorded track is well defined by transition overshoot at track edges. The images of edge overwrite patterns show that at track edges, there is no separation between the newly recorded and residual previous tracks. It is argued that the sharp track edges and lack of edge erase band in perpendicular single pole head recording can be advantageous as well as disadvantageous depending on system and track serving designs. >

Micromagnetics of dual spin‐valve GMR heads

Micromagnetic simulations were performed to study the biasing characteristics and playback performance of actual dual spin‐valve GMR heads. Both symmetric and asymmetric dual spin valve heads were analyzed. The study shows that the symmetric dual spin‐valve head is not properly biased in its patterned form, without any additional transverse biasing schemes. On the other hand, the asymmetric dual spin valve head is self‐biased since in this form, the free layer is free from the demagnetizing fields arising from the pinned layers. However, opposite GMR effects with similar magnitudes would be required for the two spin valve structures in the same multilayer film.

Narrow track recording in perpendicular thin film media

Narrow track recording in a double-layer perpendicular film medium is studied via micromagnetic modeling. A 3-D probe head field with a track width W=1.5 mu m is used for recording simulations. Recordings of multiple consecutive transitions are simulated. Edge transition patterns and edge overwrite properties are characterized. Simulated transition magnetization pattern at track edges turns towards to the head motion direction and extends to the next transition front. Edge overwrite is studied by simulating recording of a new track over a previously recorded track. It is found that the edges of the new track exhibit no separations with the overwritten track and the intertrack edge of newly recorded transitions is strongly modified by the previously written transitions. >

Experimental and micromagnetic study of track edge noise reduction effect in multilayer thin film media

A time domain spin-stand tester noise measurement and a micromagnetic modeling study are conducted to investigate the track edge noise reduction effect in multilayer thin film media. Both experimental and modeling studies show that track edge noise is concentrated in the side written band of a reversed bit-cell, in which the magnetization is opposite to the previous dc erased state. The track edge noise level is substantially suppressed for films laminated with a very thin non-magnetic interlayer. Thus the multilayer thin film media is more suitable for future narrow track recording. >