Magnetic Materials for Digital-Computer Components. I. A Theory of Flux Reversal in Polycrystalline Ferromagnetics

Abstract

It is proposed that magnetization reversal in polycrystalline ferro- and ferrimagnetic materials is primarily due to the nucleation and growth of 180° Bloch walls. The origin of domains of reverse magnetization is discussed. The rate of growth of these domains is determined by a study of the elastic and frictional forces which retard the motion of their 180°-Bloch-wall boundaries. This theoretical model successfully explains the output-voltage wave forms of polycrystalline materials. A figure of merit for the magnetization reversal of magnetic cores is defined as the switching coefficient Sw=(Hm−H0)τ, where τ is the time required to reverse the magnetization, Hm is the applied magnetic field, and H0 is the threshold field value at which the average domain-wall velocity is zero. Sw is composed of an eddy-current contribution Swe and a spin-relaxation contribution Swr. The value of Sw is derived in terms of various fundamental parameters of the material. It is shown that in ferrites and ultra-thin metal tapes, Swe«Swr. Theoretical relationships expressing the contributions of spin-relaxation, eddy-current, and hysteresis effects to energy losses are derived on the basis of this model. A study of the factors which affect the magnetization-reversal time of materials with square hysteresis loops indicates that a better figure of merit will result if the proper hysteresis shape is obtained by grain alignment rather than by the techniques currently employed in ferrites. A number of experiments are presented in support of this model.