THERMALLY ACTIVATED MAGNETIC VISCOSITY IN NATURAL MULTIDOMAIN TITANOMAGNETITE

Magnetic viscosity and hysteresis versus temperature and field have be en studied between room and Curie temperatures on two selected coarse- grained submarine basalts carrying multidomain (MD) low Curie temperat ure titanomagnetite. These samples exhibit typical characteristics of magnetic viscosity in rocks, including imperfect linearity of the magn etization change with logarithm of time and a large increase in viscos ity with temperature, followed by a decrease occurring some 30-degrees -40-degrees below the average Curie point of the rock sample. The fiel d dependence of magnetic viscosity exhibits a maximum located slightly above the sample coercive force. Other experiments, reported elsewher e, show that diffusion plays a negligible role in these rocks. Thus, m agnetic 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 pr esence of a large component of high coercivity of PSD or SD origin. Th e remanence of the other sample seems to be wholly of MD origin. The t hermal dependences of magnetic viscosity and irreversible susceptibili ty are only approximately similar, and the field dependences are quite different. This indicates that the proportionality of irreversible su sceptibility with magnetic viscosity predicted by the Neel and the Str eet 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 dat a are compatible with the fundamentals of the theory of thermal fluctu ations if we assume that the pinning of Bloch walls in natural titanom agnetite is due to several kinds of energy barriers with a broad distr ibution of both activation volumes v(a) and critical fields h(c). Thus , the remanent magnetization acquired in a low field (1 Oe) applied fo r 1 d at room temperature is found to be pinned by two categories of d efects with (1) v(a) = 7 x 10(-15) cm3 and h(c) = 3 Oe and (2) v(a) = 1 X 10(-15) cm3 and h(c) = 15 Oe. Using a (v(a), h(c)) diagram to repr esent each barrier, we show that the 'blocking' (or 'unblocking') curv es are quite dissimilar for field and time effects, just as for SD par ticles. This indicates that the proportionality between magnetic visco sity and irreversible susceptibility requires particular distributions of energy barriers in the (v(a), h(c)) plane and cannot be considered as a general characteristic of thermallly activated magnetic viscosit y.