IRREVERSIBLE PROCESSES IN BLOCH WALL MOTION

We report experimental results on the motion of a 180° Bloch wall in a Fe-Si single-crystal frame. Below a critical velocity ẋ1, the irreversible wall motion takes place by a large number of small, independant, thermally activated jumps. The onset of large jumps at the critical velocity has the character of an instability. At intermediate velocities, the wall moves by large, reproducible jumps. At a second threshold velocity ẋ2, those jumps begin to break up into smaller and smaller jumps. The evolution of the Barkhausen noise and its power spectrum suggests an analogy with the onset of a turbulent flow. Those phenomena are discussed in terms of thermally activated magnetization processes.