Anomalous magnetic aftereffect and thermal remagnetization in melt-spun Nd4Fe77B19 permanent magnets

Usually measurements of the magnetic aftereffect in permanent magnet materials are performed on the major demagnetization curve. In this investigation, however, we measured the time dependence of magnetization of the spring magnet Nd4Fe77B19 for different magnetic prehistories. The measurements were done with SQUID magnetometers. Depending on the magnetic prehistory the magnetic viscosity S can be positive as well as negative, even for the same "coordinate", (J,H), i.e. the system does not always move directly towards the thermal equilibrium state. In particular the samples spontaneously remagnetize after being field demagnetized. The driving force for this effect may be similar as that for thermal remagnetization. For certain magnetic prehistories the magnetization J(t) depends non-monotonically on time t, i.e. it can not always be described by a formula S ln(1 + t/t(o)) Similar effects observed for alpha-Fe, many years ago, have been called anomalous aftereffect. These observations were attributed to the reversible aftereffect associated with the diffusion of C-atoms in alpha-Fe. However, the reversible aftereffect is not typical for permanent magnet materials. The anomalous aftereffect found in our investigation as well as the effects of spontaneous remagnetization will be explained in terms of magnetic interactions in the material.