Yasutaro Uesaka

Direct Solution of the Landau-Lifshitz-Gilbert Equation for Micromagnetics

A mathematical framework is presented for solving the Landau-Lifshitz-Gilbert equation expressed in Cartesian components of magnetization according to the backward difference method without conflicting with the constraint of constant magnetization. Test calculation shows that the method allows the use of a large time step almost independent of spatial mesh size and damping constant. The derived program is used to calculate the magnetization structure of a crosstie wall in a Permalloy film yielding calculated structures which closely resemble the electron-holography image of an actual cross-tie wall. It is also used to investigate magnetization reversal mechanisms in fine ferromagnetic particles by pursuing time dependent changes in magnetization structures. The paper gives detailed descriptions of the reversal mechanisms which differ depending on the size of the particle.

Monte Carlo simulation of thermal fluctuation in magnetization of longitudinal and perpendicular magnetic recording media

Abstract Thermal fluctuation in magnetization of longitudinal and perpendicular magnetic recording media was calculated using a Monte Carlo simulation method. Perpendicular recording is superior to longitudinal recording when the thickness is larger than the grain size. Longitudinal recording is superior to perpendicular recording when the grain size is larger than the thickness. Distribution of grain size increases thermal fluctuation of output signal of longitudinal and perpendicular recording media. In longitudinal recording, thermal fluctuation of the output signal increases with increasing recording density, but in perpendicular recording, it decreases with increasing recording density. In longitudinal recording, the reversal of magnetic moments begins near the transition region and then spreads to the other region, but in perpendicular recording the reversal of magnetic moments begins from the saturation region. In longitudinal recording media with well-oriented easy direction or in perpendicular recording media, the residual ratio of the output signal increases with increasing exchange interaction between adjacent grains. In longitudinal recording media in which the easy directions are in the plane of the media, but not well oriented, the residual ratio of the output signal decreases with increasing exchange interaction between adjacent grains when the interaction is not large ( A −6 erg/cm), and increases with increase in the exchange interaction when it is large ( A > 0.6 × 10 −6 erg/cm).

Noise characteristics of barium ferrite particulate rigid disks

This paper discusses the relationship between the noise characteristics and magnetic properties of longitudinal barium ferrite (Ba‐F) rigid disks with different switching field distributions (SFD). The magnetomotive force dependencies of reverse dc‐erase (RDC) noise are measured and compared with SFD values. Coated disks with acicular magnetic particles have dips and thin‐film disks peaks in the RDC. In Ba‐F disks, both cases are observed depending on the SFD values, though the depths or heights of the RDC noise are much smaller than those of coated disks with acicular particles or thin‐film disks. Disks with small SFD values have peaks, and disks with large SFD values have dips. In order to find the relationship between noise properties and magnetic properties, interparticle interactions in Ba‐F disks are investigated. Reverse dc remanence Id(H) and ac‐demagnetized isothermal remanence Ir(H) are measured. Both are normalized by the saturation remanence. The deviation from the noninteracting system, ΔM = ...

Noise from underlayer of perpendicular magnetic recording medium

Noise from the underlayer is observed with a Mn‐Zn ferrite head and a Co‐Mo‐Zr low coercivity amorphous underlayer. Three kinds of noise are observed, spike noise (s), medium noise (m), and low noise (l). s noise is generated through the domain walls of an underlayer. m noise originates from the interaction between an underlayer and the reproducing head’s residual magnetization. Three effective methods are tested for reducing the noise: (1) increase of coercivity of an underlayer, (2) use of an ac‐erased head, and (3) use of a thin‐film head.

Studies of spacing loss in longitudinal and perpendicular recording

The effect of head‐to‐medium spacing on longitudinal and perpendicular magnetic recording with ring heads has been clarified. Media tested are a γ‐Fe2O3 particulate disk and a Co–Ni–P plated disk for longitudinal recording, and single and double layer of Co–Cr sputtered disks for perpendicular recording. Experiments are performed utilizing Mn–Zn ferrite ring heads with 0.30, 0.56, and 0.78‐μm gap lengths. In addition, self‐consistent calculations by finite element method, which takes into consideration head‐to‐medium interaction, are applied to clarify the mechanism of read spacing loss.

Dynamical calculation of magnetization reversal in elongated particles

The Landau–Lifshitz–Gilbert (LLG) equation is directly solved to investigate squareness and time‐dependent magnetization changes of elongated particles. Squareness scarcely changes until the particle size exceeds some critical value. The critical value increases with increasing aspect ratio. It was found that there are three kinds of magnetization reversal mechanism in elongated particles: flower1, flower2, and vortex particles. Some time interval is necessary for the irreversible transition to occur in all cases. In a flower1 particle, the transition occurs from the top and bottom planes. In flower2 and vortex particles, the irreversible transitions occur from vortex states. In a flower2 particle, during the irreversible transition process, all magnetic moments at the top and bottom planes rotate to the same direction; consequently, some magnetic moments rotate to the antiapplied‐field direction and then rotate to the applied‐field direction. In a vortex particle, each magnetic moment at the top and bottom planes rotates to the applied‐field direction.

Perpendicular and longitudinal magnetic properties and crystallographic orientation of Co-Cr film

The magnetic properties and the c axis orientation of Co‐Cr thin films deposited on various substrates with concentrations of 0–20 at. % Cr are investigated. The intermediate layer of C,5 nm thick, between the substrate and Co‐Cr layer is found to bring a great improvement in the c‐axis orientation toward the normal of the film plane, a situation suitable for perpendicular recording. On the other hand, the thicker the underlayer of Cr, the better the c‐axis orientation and a higher coercivity in the film plane is realized; this is suitable for longitudinal recording.

Computer Simulation of Magnetization Reversal Mechanisms of Hexagonal Platelet Particle: Effect of Material Parameters

The effect of material parameters on the switching mechanism and switching field, Hsw, of hexagonal platelet particles was investigated by computer simulation. The field HE was applied perpendicular to the platelet plane which is the easy axis direction of the platelet. The magnetic moments of platelets with a small exchange stiffness constant, A, switch in the normal vortex mode; those of particles with a large A switch in the quasi-coherent rotation mode. The magnetic moments of platelets with a small uniaxial anisotropy constant, Ku, reverse in the normal vortex mode; platelets with a large Ku reverse in the crater mode. The magnetic moments of a thin platelet switch in the quasi-coherent rotation mode; a thick platelet switches in the normal vortex mode.

COMPUTER SIMULATION OF THIN-FILM MEDIA NOISE USING AN AUTOCORRELATION FUNCTION ALONG THE TRACK DIRECTION

The causes of thin‐film media noise are discussed by deriving a formula which shows a direct relationship between the noise spectrum and the magnetization configuration of a thin‐film medium. This formula is derived by Fourier‐transforming the autocorrelation function of jMx‐Mz along the track direction, where Mx and Mz are the longitudinal and vertical components of magnetization, respectively. Noise and signal spectra are computed by means of this formula. The computed noise spectra are the maximum for a relatively long wavelength. Their form and intensity are shown to be strongly affected by the anisotropy constant and the axis orientation of easy magnetization.

Noise and overwrite characteristics for perpendicular magnetic recording on a sputtered Co‐Cr medium

This paper describes noise, overwrite, and pattern peakshift characteristics for a Co‐Cr medium that is used in perpendicular magnetic recording by means of a ring head. Uniform Co‐Cr (20 wt% Cr) films of about 1 μm thickness were formed by rf sputtering on A1 disk substrates, whose saturation magnetization and coercive field were 205 emu/cc and 700 Oe, respectively. Magnetic recording tests were performed for them using a Mn‐Zn ferrite ring head of 0.28‐μm gap length and 100‐μm track width at a relative speed of 2.5 m/s. The maximum D50 recording density was 70 kBPI, with a signal‐to‐noise ratio of 40 dB at 60 kBPI. Overwrite characteristics were measured by writing a 66 kBPI signal over a 35 kBPI signal that had previously been written in. The ratio of the output of the 66 kBPI signal to that of the remaining 35 kBPI signal was 24 dB. The peakshift for the worst pattern in MFM coding was 50% of the detection window at 60 kBPI. From these results, it was deduced that the Co‐Cr medium described above is f...