Theoretical analysis of thermomagnetic properties, low-temperature hysteresis and domain structure of titanomagnetites

Magnetic hysteresis of coarse-grained titanomagnetites at room temperature is characterised by low coercive force, low relative remanence, and a high ratio of coercivity of remanence to coercive force. These properties are generally interpreted in terms of multidomain structure. At low temperatures, however, ulvöspinel-rich compositions exhibit hysteresis properties similar to those of single-domain assemblages, and on this basis Radhakrishnamurty and Deutsch have proposed an alternative interpretation of the domain structure of titanomagnetites having x ≥ 0.3 in terms of a mixture of single-domain and superparamagnetic particles. Low apparent Curie temperatures are attributed to the effects of thermal agitation above the blocking temperature. We have examined theoretically the effects of thermal agitation on the low- and high-field thermomagnetic curves and find that observed Curie temperatures in general represent an intrinsic property of the magnetic mineral present, rather than reflecting thermal agitation. The high coercive force and relative remanence at low temperatures for titanomagnetites having x > 0.5 can be explained on the basis of the interaction of domain walls with crystal defects when the large increases in magnetocrystalline anisotropy and magnetostriction with decreasing temperature are taken into account. We discuss the evidence for the existence of domain walls in coarse-grained ulvöspinel-rich titanomagnetites and conclude that multidomain structure is well established. It is also shown that fine titanomagnetite grains may have more than one blocking temperature. In any temperature interval for which superparamagnetic grains are present they will disproportionately influence susceptibility and low-field hysteresis.