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 multidoma in (MD) magnetite that cannot be explained by Neel's theories of TRM. We present experimental results that show (1) that pTRMs am additive a t any temperature, (2) that a pTRM acquired in field H between tempera tures T1 and T2 decreases on zero-field cooling below T2 when normaliz ed by M(S) (T), (3) that thermal pre-history has a strong effect on th e intensity of a pTRM. These results strongly point to reorganization of domain structure during cooling being the dominant controlling fact or 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 towar d 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 th roughout the grain due to local crystal defects, or stress effects due to the domains themselves, for example. Thermally driven domain struc ture changes then cause essentially random local changes of magnetizat ion, which are governed by kinetic equations. Our model is developed b y 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 ex plains our experimental observations. The strong effect of thermal pre history is ascribed to the existence of a spectrum of local energy min ima states, and the behavior of an MD grain is likened to that of a sp in glass.