The effect of a Pt impurity layer on the magnetocrystalline anisotropy of hexagonal close-packed Co: a first-principles study

On the basis of the fully relativistic screened Korringa-Kohn-Rostoker method we investigate the variation in the magnetocrystalline anisotropy energy (MAE) of hexagonal close-packed cobalt with the addition of platinum impurities. In particular, we perform calculations on a bulk cobalt system in which one of the atomic layers contains a fractional, substitutional platinum impurity. Our calculations show that at small concentrations of platinum the MAE is reduced, while at larger concentrations the MAE is enhanced. This change in the MAE can be attributed to an interplay between on-site Pt MAE contributions and induced MAE contributions on the Co sites. The latter are subject to pronounced, long-ranged Friedel oscillations that can lead to significant size effects in the experimental determination of the MAE of nanosized samples.     

Multiscale models of hard-soft composite media

The status of models of composite media for ultra-high density recording is reviewed. It is shown that the usual micromagnetic formalism may break down when dealing with interfaces where rapid spatial changes in magnetisation may occur. A multiscale approach is outlined, in which atomistic models are coupled with micromagnetic concepts to provide an improved physical model. The theory is applied to FePt/FeRh bilayers, which have potential as media for heat-assisted magnetic recording.     

Recent studies in thermal activation and spin waves

Some recent results in computational approaches to thermally activated fast reversal in magnetic recording media are reviewed. In particular, recent results reported in the simulation of pulsed-field-induced magnetisation reversal and thermal activation of spin waves are described. The short time scale breakdown of the Arrhenius–Néel law for a single moment is demonstrated and explained in terms of the dynamics of the precessional motion. The variation in response as a function of the damping parameter is found to be an important factor determining the remanent magnetisation for a given pulse width. The effects of interactions between moments are described, including the apparent increase in effective damping. It is shown that interactions between moments can be described in terms of thermally excited spin waves. The spectrum of relaxation times for systems consisting of coupled moments is explained in terms of the thermal excitation of spin waves.     

Characterisation of the dispersion process using transverse susceptibility measurements

We have carried out a programme of investigation of the magnetic behaviour of wet dispersions as a function of milling time. This has included a range of magnetic measurement methods including transverse susceptibility (χ_t). This paper describes χ_t results for a CrO₂ dispersion which is shown to be sensitive to the break up of agglomerates and change in dispersion quality, through the sensitivity of χ_t to particle alignment.     

Reptation effects as a mechanism for localised demagnetisation in video tapes

The decay of video signals due to local demagnetisation effects during replay is considered in terms of magnetostriction and reptation. Calculations indicate that magnetostriction is an unlikely candidate but that the size of the stray fields produced around debris on cassette and recorder surfaces are consistent with measured reptation decay of magnetisation in the media.     

Time-dependent magnetization in fine-particle ferromagnetic systems

A new theory is presented to describe the time-dependent decay of the isothermal remanent magnetization of a fine-particle ferromagnetic system. Analytical expressions are obtained for a model based on the summation of relaxation times over a very narrow range of the particle size distribution. The model predicts the well established ln t decay of magnetization for small t but a t^-1-dependence for large t. The results are compared with precise experimental data taken on a frozen Co fine-particle magnetic fluid. Excellent agreement between theory and experiment is found over a large measurement time-scale.