TEMPERATURE-DEPENDENCE OF DOMAIN-STRUCTURE IN NATURAL MAGNETITE AND ITS SIGNIFICANCE FOR MULTIDOMAIN TRM MODELS

Magnetic domain structures of natural magnetite grains were observed as a function of temperature with the Bitter method. The domain walls were completely rearranged during heating from 20 to 90-degrees-C. Parallel 180-degrees domain walls at 20-degrees-C changed to complicated curved domain walls of the stress-controlled type when heated in zero field to 90-degrees-C. These changes in domain structure could be explained by the faster decrease of the magnetocrystalline anisotropy energy with increasing temperature compared with the slower decrease in the magnetoelastic energy term. Temperature-dependent domain wall changes in magnetite were postulated previously by measurements of partial thermoremanent magnetization. A synthesis of microscopic domain observations, macroscopic magnetization measurements and multidomain theories is attempted. Published data on the acquisition and decay of partial thermal remanent magnetization are interpreted in terms of the continuous rearrangement of domain structure with changing temperature.