  The thermal remagnetization (TR), i.e. the reentrance of magnetization upon
heating in a steady-field demagnetized sample, is a common feature to the four
types of polycrystalline permanent magnets, mainly utilized for practical
purposes, i.e. barium ferrites, SmCo5, Sm2Co17 and NdFeB magnets. The effect is
small for pinning controlled and large for nucleation controlled magnets. The
effect is strongly dependent on the demagnetization factor and may reach nearly
100 per cent in SmCo5 samples measured in a closed circuit. The TR is very
sensitive to a small superimposed steady field. The maximum effect and the
position of the peak is dependent on the initial temperature. The direction of
the TR is correlated with the temperature coefficient of the coercivity,
resulting in a inverse TR in barium ferrite. The susceptibility of the
thermally remagnetized samples is increased. Repeated cycles of steady-field
demagnetization followed by heating result in the same TR. The phenomenology of
TR and ITR is explained by means of a model taking into account both the
internal field fluctuations due to grain interactions and the decay of single
domain grains into multi-domain state. By taking the measured temperature
dependencies of the coercivity and the saturation magnetization the theory is
able to reproduce the experiments very well, allowing to determine the width of
the field fluctuations, the width of the switching field distribution and an
internal demagnetization factor as characteristics of the materials by fitting.
