Thermally activated motion of a 180° 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 after-effect 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 after-effect 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.