Modeling of long-time thermal magnetization decay in interacting granular magnetic materials

We present a general method to evaluate the long-time magnetization decay in granular magnetic systems. The method is based on Arrhenius-Neel kinetics with the evaluation of the energy barriers in a multidimensional space. To establish a possible reversal mode, we suggest the use of Metropolis Monte Carlo and for the mode statistical sampling-the kinetic Monte Carlo criteria. The examples considered include long-time magnetization decay in CoCrPt low-magnetization longitudinal recording media and in a collection of Co particles with different concentrations

Role of geometrical symmetry in thermally activated processes in clusters of interacting dipolar moments

Thermally activated magnetization decay is studied in ensembles of clusters of interacting dipolar moments by applying the master-equation formalism, as a model of thermal relaxation in systems of interacting single-domain ferromagnetic particles. Solving the associated master-equation reveals a breakdown of the energy barrier picture depending on the geometrical symmetry of structures. Deviations are most pronounced for reduced symmetry and result in a strong interaction dependence of relaxation rates on the memory of system initialization. A simple two-state system description of an ensemble of clusters is developed which accounts for the observed anomalies. These results follow from a semi-analytical treatment, and are fully supported by kinetic Monte-Carlo simulations.Comment: 9 pages, 6 figure

Orientation and temperature dependence of domain wall properties in FePt

An investigation of the orientation and temperature dependence of domain wall properties in FePt is presented. The authors use a microscopic, atomic model for the magnetic interactions within an effective, classical spin Hamiltonian constructed on the basis of spin-density functional calculations. They find a significant dependence of the domain wall width as well as the domain wall energy on the orientation of the wall with respect to the crystal lattice. Investigating the temperature dependence, they demonstrate the existence of elliptical domain walls in FePt at room temperature. The consequences of their findings for a micromagnetic continuum theory are discussed. (c) 2007 American Institute of Physics

Effect of Pt impurities on the magnetocrystalline anisotropy of hcp Co: a first-principles study

In terms 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 of the MAE can be attributed to an interplay between on-site Pt MAE contributions and induced MAE contributions on the Co sites. The latter ones are subject to pronounced, long-ranged Friedel-oscillations that can lead to significant size effects in the experimental determination of the MAE of nano-sized samples.Comment: 16 page

Model of advanced recording media : The angular dependence of the coercivity including the effect of exchange interaction

We use a micromagnetic model based on the kinetic Monte-Carlo approach to investigate theoretically the magnetic properties of advanced recording media. The model is employed to examine the impact of the magnetostatic and exchange interaction between grains of realistic perpendicular recording media on the angular-dependent coercivity since the exchange field between grains is an important factor in recording performance. The micromagnetic model allows to take the easy axis distribution and the exchange interaction between grains into account. The results confirm the importance of exchange interaction since the variation of coercivity with angle between the applied field and the orientation of easy axis which is perpendicular to the film plane, (θ) is seen to broaden with decreasing exchange field. We show that a two-stage fitting procedure involving the separate determination of the exchange field and easy axis dispersion provides a useful tool for the characterization of media for perpendicular recording and heat assisted recording. We find excellent agreement between previous experimental results and the simulations including exchange interactions leading to estimate of the exchange coupling and easy axis dispersion