Study of Bloch wall dynamics and losses in monocrystalline, polycrystalline, and amorphous materials by means of optical techniques (invited)

Abstract

The dynamics of the magnetization process of a ferromagnetic material is in general strongly influenced by interactions between Bloch walls, brought about by internal fields. The resulting stochastic character of the magnetization process has, together with many other effects, an important role in producing the so called excess magnetic loss, found in practically all the ferromagnetic materials undergoing a cyclic magnetization process. Recently an optical technique has been developed which makes use of the Kerr effect, on suitably prepared specimens, to study the Bloch wall dynamics in a wide frequency range (0–100 kHz). Through this technique, and by means of mathematical developments allowing one to evaluate local magnetic losses from the analysis of the optical signal, a rather large amount of information has been obtained on the magnetization dynamics and loss mechanisms in monocrystalline, polycrystalline, and amorphous materials. In particular, in the case of grain-oriented Fe–Si laminations, the correlation existing between Bloch wall interactions and deviations from the Pry and Bean theory of losses, even in well-oriented grains, showing a regular antiparallel domains structure, has been clearly evidenced. In the present paper, this technique, whose main advantage is connected with the possibility of visualizing the domain structure of the region under study, will be described and discussed, together with some of the main results obtained until now.