A MODEL OF MULTIDOMAIN THERMOREMANENT MAGNETIZATION INCORPORATING TEMPERATURE-VARIABLE DOMAIN-STRUCTURE

There are some fundamental experimental observations of properties of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) magnetite that cannot be explained by Neel's theories of TRM. We present experimental results that show (1) that pTRMs am additive at any temperature, (2) that a pTRM acquired in field H between temperatures T1 and T2 decreases on zero-field cooling below T2 when normalized by M(S) (T), (3) that thermal pre-history has a strong effect on the intensity of a pTRM. These results strongly point to reorganization of domain structure during cooling being the dominant controlling factor in TRM acquisition in MD material. We further develop the approach of McClelland and Sugiura [1987] where TRM and pTRM are considered to be nonequilibrium states, and change in domain structure with changing temperature provides the driving force to allow a pTRM to shift toward the demagnetized state on zero-field cooling, for example. A random element is essential in such a kinetically controlled system; in this paper we consider the physical mechanism providing this random element to be the variation of direction of the easy axis of magnetization throughout the grain due to local crystal defects, or stress effects due to the domains themselves, for example. Thermally driven domain structure changes then cause essentially random local changes of magnetization, which are governed by kinetic equations. Our model is developed by considering the magnetization of discrete cells within a cubic grain chosen to have reasonably uniform magnetic properties within the cell but probably different between cells, and the model satisfactorily explains our experimental observations. The strong effect of thermal prehistory is ascribed to the existence of a spectrum of local energy minima states, and the behavior of an MD grain is likened to that of a spin glass.