Dynamics of single-domain magnetic particles at elevated temperatures

A stochastic differential equation that describes the dynamics of single-domain magnetic particles at any temperature is derived using a classical formalism. The deterministic terms recover existing theory and the stochastic process takes the form of a mean-reverting random walk. In the ferromagnetic state diffusion is predominantly angular and the relevant diffusion coefficient increases linearly with temperature before saturating at the Curie point ($T_c$). Diffusion in the macrospin magnitude, while vanishingly small at room temperature, increases sharply as the system approaches $T_c$. Beyond $T_c$, in the paramagnetic state, diffusion becomes isotropic and independent of temperature. The stochastic macrospin model agrees well with atomistic simulations.

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