THERMALLY ACTIVATED MOTION OF A 180-DEGREES BLOCH WALL IN A FE-SI SINGLE-CRYSTAL

The activated motion of a 180° Bloch wall is investigated in a frame-shaped Fe-Si single crystal. Two kinds of behaviour can be observed. In one of them, the irreversible motion of the wall takes place in a small scale by localized, independent jumps. When a constant magnetic field is applied, the jumps can be thermally activated and the induction increases. The temperature dependence of the change of induction is consistent with a spectrum of energy barriers and can be interpreted by the activation model of Street and Woolley. The order of magnitude of the energy barriers is 1000 K. The magnetic aftereffect recorded on a small-amplitude hysteresis loop is proportional to the irreversible susceptibility. The wall motion can also take place by large, collective jumps. The aftereffect then exhibits an exponential time dependence which reflects a single energy barrier (∼ 7000 K). When the sample is cycled, the losses per cycle are frequency dependent; a logarithmic deviation from the linear law is observed at low frequencies, showing that thermal fluctuation assists wall motion. A good agreement is found with experiments previously reported by Sasaki on commercial Fe-Si. © 1978 IEEE