Monte Carlo simulations of inter- and intra-grain spin structure of Ising and Heisenberg models

LeBlanc, Martin (2010) Monte Carlo simulations of inter- and intra-grain spin structure of Ising and Heisenberg models. Masters thesis, Memorial University of Newfoundland.

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Abstract

In order to keep supplying computer hard disk drives with more and more storage space, it is essential to have smaller bits. With smaller bits, superparamagnetism, the spontaneous flipping of the magnetic moments in a bit caused by thermal fluctuations, becomes increasingly important and impacts the stability of stored data. Recording media is composed of magnetic grains (usually made of CoCrPt alloys) roughly 10nm in size from which bits are composed. Most modeling efforts that study magnetic recording media treat the grains as weakly interacting uniformly magnetized objects. In this work, the spin structure internal to a grain is examined along with the impact of varying the relative strengths of intra-grain and inter-grain exchange interactions. The interplay between these two effects needs to be examined for a greater understanding of spuerparamagnetism as well as for the applications of the proposed Heat Assisted Magnetic Recording (HAMR) technology where thermal fluctuations facilitate head-field induced bit reversal in high anisotropy media. -- Simulations using the Monte Carlo method (with cluster-flipping algorithms) are performed on a 2D single-layer and multilayer Ising model with a strong intra-grain exchange interaction J as well as a weak inter-grain exchange J’. A strong deviation from traditional behavior is found when J’/J is significant. M-H hysteresis loops are also calculated and the coercivity, Hc, is estimated. A large value represents a strong resilience to the superparamagnetic effect. It is found that taking into account the internal degrees of freedom has a significant effect on Hc. As the Ising model serves only as an approximation, preliminary simulations are also reported on a mre realistic Heisenberg model with u;niaxial anisotropy.

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