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 S_w=(H_m−H₀)τ, where τ is the time required to reverse the magnetization, H_m is the applied magnetic field, and H₀ is the threshold field value at which the average domain‐wall velocity is zero. S_w is composed of an eddy‐current contribution S_w^e and a spin‐relaxation contribution S_w^r. The value of S_w is derived in terms of various fundamental parameters of the material. It is shown that in ferrites and ultra‐thin metal tapes, S_w^e«S_w^r. 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.