Superparamagnetic Behavior of Solids at Liquid Helium Temperatures

摘要

It is shown in this work that the individual particles of fine powders of ferromagnetic materials represent aggregates of subdomain volumes which are magnetically coupled by means of magnetostatic, magnetoelastic and exchange forces. These forces are responsible for the establishment of an inhomogeneous magnetization within these particles. The magnetization process within the subdomains is assumed to be caused by growth of magnetic nuclei and by thermal fluctuations which are characteristic of the superparamagnetic state. The mechanism of this process is responsible for the slow magnetization relaxation phenomena observed in fine powders. As found these phenomena become very pronounced at liquid helium temperatures. The magnetization process in granular multilayer films behaves in a way which is similar to that of fine powders when the thicknesses of the individual layers are very small so that superparamagnetic properties can be exhibited. Cold rolled ferromagnetic metal with large roll reduction in thickness show a similar magnetic behavior because of their laminated structure. The inhomogeneous magnetization within the different laminations of the multiple films can be investigated using the magnetoresistance effect as an experimental tool. (Author)