  A self-consistent mean-field theory explaining the thermal remagnetization
(TR) of polycrystalline permanent magnets is given. The influence of the
environment of a grain is treated by an inclusion approximation, relating the
field inside the grain to the local field outside by means of an internal
demagnetization factor $n$. For the switching fields and the fluctuations of
the local fields around the mean field Gaussian distributions of widths
$\sigmas$ and $\sigmaf$ resp. are assumed. The isothermal hysteresis curve, the
recoil curves, and the TR in dependence on the model parameters $n$, $\sigmas$,
and $\sigmaf$ are calculated. Furthermore, the influence of the initial
temperature and the strong dependence of the TR on the demagnetization factor
of the sample are studied, and it is shown that for reasonable parameter sets
TR effects up to 100 % are possible. The theoretical results correspond well
with the experimental situation.
