MAGNETIC-PROPERTIES AND ROTATIONAL HYSTERESIS OF A BASALT WITH HOMOGENEOUS TI-RICH TITANOMAGNETITE GRAINS 10-20 MU-M IN DIAMETER

Magnetic hysteresis properties and rotational hysteresis data are reported for basalt samples with homogeneous Ti-rich titanomagnetite grains 10-20 mu m in diameter between approximate to 100 K and the Curie temperature of approximate to 400 K. From the remanence behaviour it is concluded that at low temperatures a pseudo-single domain state exists. Above ambient temperature this character appears to remain up to the Curie point. Magnetic domain-wall observations with the colloid Bitter technique at room temperature reveal wavy patterns that form as a consequence of stress effects. Rotational hysteresis losses occur at high magnetic fields where only reversible magnetization processes are inferred from hysteresis data. This result is interpreted in terms of centres or of internal surfaces with high magnetic anisotropy. The sources are either a very low amount of segregated anti-ferromagnetic phases, leading to exchange anisotropy, or Fe2+ in the environment of lattice defects carrying an angular orbital momentum due to the disturbed crystal field; in both cases, the mechanism leading to high field rotational hysteresis losses may be the same. The assumed high anisotropy Fe2+ and the neighbouring grain regions in pure titanomagnetite particles may be of importance in the formation of a thermoremanence since they may act like single domain particles.