Motion of Individual Domain Walls in a Nickel-Iron Ferrite

The mechanism which contributes most to the permeability of highpermeability magnetic materials is the motion of ferromagnetic domain walls. These walls are thin lamellae in which the direction of the spontaneous magnetization of the material changes from one domain to another. As a result, this mechanism contributes a major part of the energy losses which accompany rapid changes in the direction of the 1023 1024 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 magnetization in such materials. In the ferromagnetic metals, it is well known that these losses ordinarily arise largely from the eddy currents which are induced by the motion of the domain walls. In the ferrites, however, the conductivity is so low that the contribution of eddy currents to the losses is never overwhelming and is often negligible; the losses must therefore in large part arise from other sources not yet understood. It is the purpose of this paper to present some recent studies of these losses and to discuss their relevance to the losses in ferrites generally. In any ordinary sample of a ferromagnetic material, a study of domain wall motion and the associated energy losses is complicated by the fact that the domain pattern is very complex. Any attempt to provide a theoretical explanation of data taken 011 such samples must involve an averaging process over many domain walls of varying area, crystal orientation, etc. This makes it extremely difficult to describe the behavior of such patterns uniquely and quantitatively, although some progress has been made.