A model of multidomain thermoremanent magnetization incorporating temperature‐variable domain structure

Abstract

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 Ntel's theories of TRM. We present experimental results that show (1) that pTRMs are additive at any temperature, (2) that a pTRM acquired in field H between temperatures T1 and T 2 decreases on zero-field cooling below T 2 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.grains are adequate to explain experimental observations, at least qualitatively. However, theories of TRM in multidomain (MD) grains grossly•fail to explain experimental observations in some fundamental areas. Ndel [1955] laid the foundations for the present theories of TRM in iron oxides. He pointed out that remanent magnetization exists because of barriers to change in magnetization.He postulated that in large multidomain grains these barriers prevent the movement of domain walls and that they are proportional to the coercive force H c. In small single domain grains, he suggested that barriers are due to anisotropic magnetic properties. In iron oxides which have uniaxial shape anisotropy, there are two antiparallel low energy favourable states with an energy barrier Kv (K is anisotropy constant; v is volume) which has to be surmounted to change state. A particular SD grain will "block" in its contribution to the total TRM when the thermal fluctuations due to the falling temperature no longer exceed a critical value necessary to change the magnetization direction. We will call this the "thermofiuctuational" model, which has sucessfully predicted...

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

“…Recently, however, questions have been raised about the suitability of archaeological materials for palaeointensity determination. It has been proved theoretically (Shcherbakov et al, 1993;Dunlop and Xu, 1994) that the additivity law of pTRM's, which is a necessary condition for the Thellier-Thellier method (Thellier and Thellier, 1959) holds only for single domain (SD) grains, as predicted by the classic Neel theory (Neel, 1949;1955). The same holds for pseudo-single domain (PSD) grains while the thermal demagnetization of multidomain (MD) grains exhibits "trails" , extending up to Tb of the mineral which carries the TRM (Bolshakov and Shcherbakova, 1979;Xu and Dunlop, 1994).…”

Paleointensity of the Geomagnetic Field in Egypt from 4000 BC to 150 AD Using the Thellier Method.

“…Another explanation for the discrepancy could be the presence of pseudo-single-domain or multidomain magnetite, both of which have unblocking temperatures of TRM far above the blocking temperatures of that pTRM (e.g. Shcherbakov et al, 1993). Table 1), each sample consisting of 2-4 specimens.…”

Cooling behaviour of partial thermoremanences induced in multidomain magnetite

“…During TRM acquisition the net contribution from the statistical ensemble is a magnetised state, whilst on cooling below T ¾ the net magnetisation demagnetises giving rise to the observed behaviour. Shcherbakov et al [20] developed this theory by considering a theoretical micromagnetic approach, i.e., in effect they replaced the statistical ensemble of domain walls used by McClelland & Sugiura [6], with a statistical ensemble of dipole moments. Unfortunately the model of Shcherbakov et al [20] makes some simplifications which rather invalidate the model; most importantly the model did not calculate the inter-cellular interaction effect.…”

“…Incorporating the temperature dependent domain wall nucleation ideas of Moon & Merrill [18,19], kinematic models has been developed to explain thermoremanence acquisition and its behaviour on cooling below T ¾ [6,20]. McClelland & Sugiura [6] assumed that domain walls can be regarded as a statistical ensemble, with each element of the ensemble being in one of two possible states: demagnetised or TRM state.…”