Thermally activated switching at long time scales in exchange-coupled magnetic grains

Abstract

Rate coefficients of the Arrhenius-Néel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of M H hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modelled as two exchange coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers 2

Kinetic Monte Carlo simulations of the effect of the exchange control layer thickness in CoPtCrB/CoPtCrSiO granular media

“…This work presents results based on a hybrid kMC/LLG formalism, that has been applied to single layer PRM [12], to simulate MH loops of dual layer exchange coupled composite (ECC) media [13]. As shown in earlier work the parameters obtained from fitting experimental MH loops for ECC media from simulations using the kMC/LLG formalism at experimental sweep rates differ significantly from those obtained using stochastic LLG that are limited to sweep rates several orders of magnitude greater than the experimental sweep rates [14].…”

“…From this the energy barrier separating the two states and the attempt frequency may be calculated [13].…”

“…When these high frequency fluctuations are detected, minimum energy states that are connected by an energy barrier less than some predetermined threshold are combined into clusters. The kMC is then reformulated in terms of a combination of minimum energy states and clusters [13]. If the fluctuations persist, then the threshold is increased by a factor of 2 until such time as the high frequency fluctuations are suppressed and the time between successive kMC steps is appropriate.…”

“…The threshold is then gradually reduced until the high frequency fluctuations reappear. This adaptive scheme for suppressing the high frequency fluctuations that do not contribute to the long time system dynamics allows us to model complex systems governed by disparate energy barriers both efficiently and accurately [13].…”