  In order to study the dependence of the coercive force of sintered magnets on
temperature, nucleation and domain wall propagation at the grain boundary are
studied as rate-determining processes of the magnetization reversal phenomena
in magnets consisting of bulk hard magnetic grains contacting via grain
boundaries of a soft magnetic material. These systems have been studied
analytically for a continuum model at zero temperature (A. Sakuma, et al. J.
Mag. Mag. Mat. {\bf 84} 52 (1990)). In the present study, the temperature
dependence is studied by making use of the stochastic Landau-Lifshitz-Gilbert
equation at finite temperatures. In particular, the threshold fields for
nucleation and domain wall propagation are obtained as functions of ratios of
magnetic interactions and anisotropies of the soft and hard magnets for various
temperatures. It was found that the threshold field for domain wall propagation
is robust against thermal fluctuations, while that for nucleation is fragile.
The microscopic mechanisms of the observed temperature dependence are
discussed.
