Kenichi Takano

Micromagnetic-FEM models of a perpendicular writer and reader

We have developed a micromagnetic finite element method (micromagnetic-FEM) to analyze perpendicular magnetic recording (PMR) writers and readers. The head field reversal time is 0.55-0.76 ns including the 0.2-ns current rise, and it is dependent on the current amplitude. The main pole has a faster reversal time, but the yoke and media soft-magnetic underlayer (SUL) show a slower response. In the media recording layer, the trailing region shows a faster reversal due to the longitudinal head field component of the trailing write shield. After writing, the media state affects the remanent field behavior. Since the moving media changes the head-media coupling, the remanent field can be relieved with a long relaxation time within 10 ns. To suppress the remanent field, a good anisotropy control of the main pole and yoke is required. A TMR reader with a flat top shield shows less side-reading, especially when the physical track width is below 0.1 /spl mu/m. The PMR media plays a role in read width reduction over a wide range of physical track widths.

Magnetization dynamics of planar writers

We have developed a micromagnetic finite-element method (FEM) to treat a whole write head structure as a micromagnetic model, and it is utilized to study the magnetization dynamics and head field of a planar writer. As the head has a domain structure, the leakage field differs from what is calculated by a conventional FEM except for the gap region, and it can induce some adjacent track erasure (ATE) problems in the case of high write currents. This leakage field comes from the shoulder of the bottom yoke pedestal and the sidewall of the upper pole. A conventional FEM for electromagnetic eddy-current field does not show much time delay of the head field rise in core sizes of 10 /spl mu/m or below. On the other hand, the head field cannot follow the write current waveform perfectly, and shows time delay by the micromagnetic FEM analysis. This indicates that the delay mainly no longer comes from eddy-current effects but magnetization dynamics of the gyromagnetic precession, damping, and flux conduction. Furthermore, the head field rise time is affected by the damping constant, material, yoke length, and pole-tip dimension.

Write head analysis by using a parallel micromagnetic FEM

We develop a parallel micromagnetic finite-element method (micromagnetic-FEM) to handle a large number of micromagnetic elements and utilize it to analyze full head and media models. When the calculation domain decomposition and the one-one node communication are applied, the program shows four times faster calculation in eight CPU operation, as compared with one CPU calculation. We study head field and media magnetization reversal on the frequency of 714 MHz, using different yoke length heads of 6, 10, and 15/spl mu/m. The shorter yoke head shows faster head field reversal and stronger head field strength, and the recorded media magnetization is also larger. The small damping parameter of the head causes not only smaller head field but also the disorder of the media transition.

Relationship Between Head Design and Media Remanent Magnetization in Perpendicular Magnetic Recording

This paper presents micromagnetic modeling results in a perpendicular magnetic recording, which consists of a single-pole head and double-layered media. To know the inplane head field contribution, the relationship between applied angular field and media remanence is studied, and we propose a perpendicular corresponding field for the head field assessment, which can explain media remanence as well such as the main-pole footprint on the media. The shielded-pole/single-coil head shows the best signal-to-noise ratio but the overwrite performance is somewhat poorer than monopole heads, with a large positive to negative head field, which implies a large field gradient. Although the shielded-pole head has smaller erase bands with straight transitions, it can cause trailing-shield-related erasure by the negative field under the high write current input in the single-coil design

Read sensitivity in abutted-junction type spin-valve head

The sensitivity and the distribution of read heads have become more and more vital as the recording track density has increased. From the view point of the sensitivity distribution, the read performance of abutted-junction type spin-valve (SV) heads are investigated by using Landau-Lifshitz-Gilbert micromagnetic simulation. In this model, the hard magnet film is set up at a slant to be abutted with the SV sensor edge. A long overlap length introduces negative longitudinal field. This inadequate field distribution causes the magnetization array to be disordered and irreversible magnetization changes in the free layer of the head. We also investigated the relationship between the magnetization state and the micro-track profile, as a function of sense current direction and track width. As a result, it is confirmed that the micro-track profile can be explained by the magnetization state of the free layer.

Micromagnetics and Eddy Current Effects in Magnetic Recording Heads

We developed a micromagnetic modeling program for magnetic recording head analysis, taking eddy current effects into account, and studied the 1-GHz high-frequency response. When gyromagnetic precession is dominant with a small damping parameter of 0.01, the eddy current field causes the delay of the response, but the head field intensity increases a lot, compared to the no eddy current case, because the eddy current field cancels the precessional motion and improves the magnetization alignment. Since magnetic soft films have an intrinsic small damping, low-resistivity material is preferable for high-end magnetic recording heads

A Study of Media Dependence of Trailing Shield Perpendicular Write Head

The performance of two types of trailing shield perpendicular writers with either double coil or single coil are investigated. It has been found that signal-to-noise ratio (SNR) performance of different designs strongly depends on the tradeoff of field gradient and writability. Reverse DC noise analysis is used to understand the recording properties of head and media quantitatively. To realize the performance advantages of the single coil design and to suppress the trailing shield erasure, a novel single coil structure with double write gap is proposed.

Write induced instability in spin-valve heads

The output amplitude variation of a spin-valve (SV) head, induced by a write operation, was investigated. The characteristics well depended on the sign of the write current just before a readback process started. It indicated that the write-induced instability should come from a magnetization hysteresis of somewhere magnetic materials. The magnetic core, which consisted of shields and a yoke, was under suspicion as a candidate of the hysteretic origin by sense current, medium and external field excitation tests. Magnetization modeling of a head core revealed a possible mechanism of the instability, which was a shield-related hysteresis.

Relationship between head design, pole-tip magnetization, head field, and media magnetization in longitudinal recording

We investigated the relationship between head design, pole-tip magnetization, head field, and media magnetization in longitudinal recording. At first we estimate the effective switching field of current mobile media with isotropic anisotropy, and introduce longitudinal corresponding field for the head field assessment. The smaller perpendicular component or large perpendicular field gradient of the head field at the upper pole improves the transition quality, but the small perpendicular field degrades the overwrite performance. So the ideal head field is large longitudinal and perpendicular field for good overwrite and fast attenuation with the large perpendicular field gradient for the transition quality. Some head designs make it possible to control the ratio of the perpendicular component to the longitudinal head field, by changing the pole-tip magnetization direction. As a project to control it, we discuss the effect of the bottom and upper poles configuration on the transition, overwrite, and adjacent track erasure in longitudinal recording.

Readback Spatial Sensitivity Function by Reciprocity Principle and Media Readback Flux

The sensitivity function is studied by using conventional reciprocity principle, and it is compared with the readback models one, which is calculated as the read sensor output excited by the media recorded magnetization. These models are constructed by finite elements with finite permeability in a detailed three dimensional geometry. It is found that the sensitivity function of the reciprocity is different from one of the readback model, although it is believed that the sensitivity function should be equal to the read head field distribution. It is caused by the three dimensional structure, complicated media flux gathering of the sensor, and the media top and bottom magnetic charge effect. In addition to the sensitivity function difference, the reciprocity procedure is lack of sense of the media grain information, vector magnetization, and multilayered structure in the reproducing process. On the other hand, the developed readback model takes these features into account automatically.