Long- and Short-Range Magnetic Order in Titanomagnetite
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- and Short-Range Magnetic Order in Titanomagnetite V. I. Belokona, * and O. I. Dyachenkob, ** a
Department of Theoretical and Nuclear Physics, School of Natural Sciences, Far Eastern Federal University, Vladivostok, 690950 Russia b Department of General and Experimental Physics, School of Natural Sciences, Far Eastern Federal University, Vladivostok, 690950 Russia *e-mail: [email protected] **e-mail: [email protected] Received October 3, 2019; revised November 18, 2019; accepted January 27, 2020
Abstract—It is shown that the method of random exchange-interaction fields is suitable for identifying the temperature interval where the short-range magnetic order is still present while the long-range order is already absent. On this interval, the state of the magnetic system is a set of ferromagnetic clusters whose magnetic moment can vary with time and ensure magnetic viscosity. In the paper, the Curie point and paramagnetic Curie point are determined for a two-sublattice ferrimagnetic with the distribution of iron ions corresponding to titanomagnetite. Keywords: titanomagnetite, Curie point, long-range magnetic order, short-range magnetic order, rock magnetism DOI: 10.1134/S1069351320060014
INTRODUCTION As is known, the magnetic states of rocks are largely determined by the factors that are additional relative to the weak geomagnetic field, e.g., temperature fluctuations, pressure, time, and chemical transformations. One of the widespread ferromagnetic materials is titanomagnetite which is marked with a fairly pronounced effect of magnetic viscosity, i.e., the dependence of magnetization on the duration of application of the external magnetic field. Viscous magnetization of rocks has been studied since a fairly long time (Belokon, 1973; Trukhin, 2006); as the main mechanisms of magnetic viscosity, the thermally activated remagnetization of singledomain grains or the displacement of the domain boundaries are suggested. Also, various modifications of the “diffusion” mechanisms including the emergence of a percolation cluster due to diffusion were considered (Belokon, 2013; 2018). In our opinion, there is another probable mechanism associated with remagnetization of ferromagnetic clusters coupled by exchange interaction at temperatures above the Curie point. The point is that on the interval from the temperature of destruction of long-range order (Curie temperature Tc ) to the temperature of disappearance of the short-range order (paramagnetic Curie temperature θ), the existence of these clusters is possible, and they can change the
direction of the magnetic moment under the influence of thermal fluctuations. The main purpose of this paper is to determine the Curie point and paramagnetic point for a two-sublattice ferrimagnetic with the distribution of iron ions corresponding to titanomagnetites. METHOD OF RANDOM EXCHANGEINTERACTION FIELDS The method of random exchange-interaction fields developed in our works (Belokon, 1993) allows calculation of the distribution function of random exchange-interaction fields W (
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