Thermally Activated Magnetic Viscosity in Natural Multidomain Titanomagnetite

Magnetic viscosity and hysteresis versus temperature and field have been studied between room and Curie temperatures on two selected coarse-grained submarine basalts carrying multidomain (MD) low Curie temperature titanomagnetite. These samples exhibit typical characteristics of magnetic viscosity in rocks, including imperfect linearity of the magnetization change with logarithm of time and a large increase in viscosity with temperature, followed by a decrease occurring some 30°–40° below the average Curie point of the rock sample. The field dependence of magnetic viscosity exhibits a maximum located slightly above the sample coercive force. Other experiments, reported elsewhere, show that diffusion plays a negligible role in these rocks. Thus, magnetic viscosity is probably entirely of thermal origin. In spite of the MD characteristics of our samples at room temperature, alternating field (AF) demagnetization of remanence shows, in one of them, the presence of a large component of high coercivity of PSD or SD origin. The remanence of the other sample seems to be wholly of MD origin. The thermal dependences of magnetic viscosity and irreversible susceptibility are only approximately similar, and the field dependences are quite different. This indicates that the proportionality of irreversible susceptibility with magnetic viscosity predicted by the Néel and the Street and Wooley theories of thermal fluctuations in MD grains does not hold for natural titanomagnetite like ours, even though it is verified for synthetic ferromagnetic substances. However, our experimental data are compatible with the fundamentals of the theory of thermal fluctuations if we assume that the pinning of Bloch walls in natural titanomagnetite is due to several kinds of energy barriers with a broad distribution of both activation volumes v</it><inf>a</inf> and critical fields h</it><inf>c</inf>. Thus, the remanent magnetization acquired in a low field (1 Oe) applied for 1 d at room temperature is found to be pinned by two categories of defects with (1) v</it><inf>a</inf>= 7×10-15cm3 and h</it><inf>c</inf>= 3Oe and (2) v</it><inf>a</inf>= 1×10-15cm3 and h</it><inf>c</inf>= 15Oe. Using a (v</it><inf>a</inf>, h</it><inf>c</inf>) diagram to represent each barrier, we show that the ‘blocking’(or ‘unblocking’) curves are quite dissimilar for field and time effects, just as for SD particles. This indicates that the proportionality between magnetic viscosity and irreversible susceptibility requires particular distributions of energy barriers in the (v</it><inf>a</inf>, h</it><inf>c</inf>) plane and cannot be considered as a general characteristic of thermally activated magnetic viscosity.