Some statistical aspects of magnetization processes in ferromagnetic bodies. Motion of a single 180° Bloch wall in an imperfect crystalline medium

The ideally simple situation of a single 180° Bloch wall in a single crystal already involves rather complex physical mechanisms. For a given variation of the magnetic field, two types of irreversible motion of the wall can be observed, depending on the velocity imposed to the wall and the temperature. It can move like a rigid plane, then its irreversible displacements are always larger than its thikness. It can also move by localized, small jumps thanks to its internal degrees of freedom. The features of the Barkhausen noise are different in these two kinds of motion. The transition between them occurs either at a constant temperature by increasing the velocity of the wall, or at a constant velocity by lowering the temperature. The wall can also move under the effect of thermal fluctuation, which should not be regarded as some little perturbation, but rather as an essential physical parameter which has to be taken into account in any realistic model of wall motion, and hence of “Barkhausen noise”. The random character of these phenomena allowed us to employ simple, but very useful, statistical methods. Besides, this study suggests some specific problems of “signal processing”.