Local relaxation mechanisms and collective stochastic dynamics

Damping and thermal fluctuations have been introduced to collective normal modes of a magnetic system in recent modeling of dynamic thermal magnetization processes. The connection between this collective stochastic dynamics and physical local relaxation processes is investigated here. A system of two coupled magnetic grains embedded in two separate oscillating thermal baths is analyzed with no a priori assumptions. It is shown explicitly that by eliminating the oscillating thermal bath variables and using a Markovian approximation, stochastic dynamics occur in the normal modes of the magnetic system in the form of independent damped harmonic oscillators driven by thermal fields. The grain interactions cause the local coupling to the thermal bath to be felt by the collective system and the resulting dynamic damping to reflect the system symmetry. This form of stochastic dynamics is in contrast to a common phenomenological approach where a thermal field is added independently to the dynamic equations of each discretized cell or interacting grain. The dependence of this collective stochastic dynamics on the coupling strength of the magnetic grains and the relative local coupling to thermal bath is discussed.