R. H. Victora

Composite media for perpendicular magnetic recording

A composite perpendicular recording media consisting of magnetically hard and soft regions within each grain is proposed. Application of applied field initially causes the magnetization of the soft region to rotate and, thus, change the angle of the effective field applied to the hard region. This important change in the effective field is enabled by an exchange layer that moderates the interaction between the two regions. Energy arguments show that the resulting performance (as measured by the ratio of energy barrier to switching field) is similar to the previously proposed tilted media, while avoiding some of the difficulties. In particular, fabrication of the proposed composite media appears to be significantly easier than that of tilted media.

Composite media (dynamic tilted media) for magnetic recording

We designed and fabricated a composite magnetic recording medium with exchange decoupled magnetic grains that consist of two vertically exchange-coupled magnetic regions (one is magnetically soft and one is magnetically hard) as an approach to alleviate the writing field limitation of perpendicular magnetic recording heads. A nonmagnetic layer with different thickness was put between the hard and soft layer to tune the exchange coupling. With proper coupling, significant drop of the coercivity field was observed for this composite medium while still maintaining good thermal stability. Better recording performance was obtained for such medium compared to perpendicular and longitudinal medium. The results have proved the possibility of fabricating a writable recording medium having an ultrahigh magnetic anisotropy constant (Ku) value.

Areal density limits for perpendicular magnetic recording

Summary form only given. It is becoming clear that conventional longitudinal magnetic recording is approaching the superparamagnetic limit and that further density increases must be achieved using an alternate approach. Perpendicular recording with a soft underlayer provides one such approach. Relative to longitudinal recording, it is commonly claimed to possess three major advantages, each exhibiting an apparent flaw that we argue can be solved: 1. The record field is approximately twice that obtainable in longitudinal recording owing to the image field generated by the soft underlayer. However, the coercivity of a perfectly oriented perpendicular grain is also nearly twice that of the unoriented longitudinal grain with equal anisotropy field. Solutions to this problem include a head field that generates some longitudinal components or an incompletely aligned perpendicular media. 2. The demagnetization fields of perpendicularly oriented media favor thicker films that can better resist thermal fluctuations. However, at high densities, the enhanced distance between head and soft underlayer will weaken the head fields and gradients. The solution to this problem rests in the observation that even the recording layer has non-zero susceptibility. 3. Magnetostatic fields do not destabilize transitions in perpendicular recording, unlike longitudinal recording. Unfortunately, they do destabilize the middle of the bit. However, this problem can be overcome by introducing a uniform exchange coupling within the media.

Electronic structure of Ni2MnIn for use in spin injection

Data and Das [S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)] proposed an electronic analog to the optoelectronic modulator which would employ spin injection from a ferromagnet into a semiconductor. We investigate the ferromagnet–semiconductor system consisting of the Heusler alloy Ni2MnIn and the semiconductor InAs. Using a full-potential electronic structure code with a basis set of Slater-type orbitals, we have calculated the band structure of Ni2MnIn. We calculate the spin polarization for each atom. It resides primarily on the Mn atom (3.51), with a small moment (0.31) on the Ni. Interestingly, In has a very small moment (−0.04), which is antiferromagnetically coupled to the other atoms. Using a simple model, we estimate the transmittance of minority spins in three high-symmetry directions.

Micromagnetic predictions for barium ferrite particles (invited)

The quantitative application of micromagnetic theory to barium ferrite particulate media is reported. The theory includes magnetostatic, anisotropic crystalline, and exchange interactions. It is found that neither packing, stacking, thermal fluctuations, nor curling can account for the discrepancy between the predictions of noninteracting coherent rotation theory and experimental observations. Induced domain nucleation can reduce the coercivity sufficiently: however, ignorance of the nucleation mechanism prevents first principles predictions. Important practical consequences of the calculation include a demonstration that stacked particles switch together, which implies increased recording noise, and quantitative predictions for the effect of thermal fluctuations on barium ferrite coercivity.

Perpendicular magnetic recording thin film media using Co/Pd superlattice on ultrathin indium–tin–oxide seed layers

Co/Pd superlattices (13 bilayers) have been fabricated on ultrathin (2 nm) indium–tin–oxide (ITO) seed layers for use as perpendicular magnetic recording media. The ITO seed layers have significantly increased the coercivity from 1 to 5 kOe as well as the crystalline orientation texture. It was also found that excellent magnetic properties could be obtained by using Ar instead of Kr as sputtering gas. The sheared hysteresis loop and high squareness suggest weak intergranular coupling and adequate perpendicular magnetic anisotropy which make Co/Pd supelattice a promising candidate for ultrahigh recording densities with thermal stability.

Micromagnetic predictions for magnetization reversal in CoNi films

A dynamic, micromagnetic theory which includes domain nucleation, incoherent rotation of magnetization within grains, and complete magnetostatic interactions is described in detail. All significant input is derived from experimental data; the precise values of several other input variables, such as exchange constant and damping parameter, are found to be largely irrelevant. Preliminary calculations, which may be compared to measured hysteresis loops, are used to establish the considerable quantitative accuracy of the theory. The remaining calculations make predictions for unmeasured properties such as crystalline anisotropy and domain‐wall motion. A substantial prediction of the paper is that it is possible for obliquely deposited CoNi films to have no zig‐zag domain walls; however, a finite transition width owing to the incline of the CoNi grains is predicted to occur.

Advances in Magnetic Data Storage Technologies

This special issue covers advances that have spurred real density growth of magnetic recording technologies, and future technologies that are expected including new architectures of storage systems.

Calculated magnetic and electronic properties of Co/Pd superlattices

The electronic and magnetic properties of several Co/Pd superlattices are predicted using the layer Korringa‐Kohn‐Rostoker method. The superlattice polarizations, per Co atom, are found to exhibit a strong oscillatory dependence on Pd layer thickness; this result is in agreement with previous experimental observation. The effect is mostly caused by the induced Pd polarization which is found to depend strongly on superlattice and distance from the Co layer. Particularly surprising is the behavior of the Pd layer most distant from the Co: in Co/3Pd and Co/4Pd it has enhanced polarization relative to its neighboring Pd layers, in Co/5Pd the polarization is negative. In contrast, spin polarizations on Co atoms are predicted to attain a fairly uniform value of 2.00 electrons regardless of Pd thickness.

Simulation of Heat-Assisted Magnetic Recording Using Renormalized Media Cells

A new scheme for the simulation of heat-assisted magnetic recording (HAMR) that systematically includes fluctuating material properties above a predefined length scale, while retaining magnetostatic interactions, is introduced. Renormalized media parameters Ms, Ku, Aex, and αdamp, suitable for useful length scales, are found numerically. These renormalized parameters are then used to model the Voronoi-cell-composed medium in the HAMR simulation. Transition jitters are obtained under various conditions. The results show that moderate maximum temperature of the heat spot, intergranular exchange coupling, media thickness of at least 10 nm, nonzero canting angle of the head field, relatively low head velocity, and large head-field strength are helpful for a successful recording.