Magnetic hysteresis properties and rotational hysteresis losses of synthetic stress-controlled titanomagnetite (Fe2.4Ti0.6O₄) particles-II. Rotational hysteresis losses

Rotational hysteresis losses WRH of a ferro- or ferrimagnet (determined by torque measurements) give information on anisotropies related to irreversible magnetization processes. We measured WRH as a function of magnetic field H for milled and etched synthetic titanomagnetite Fe2.4 Ti0.6 O4 particles of 2.4, 12.5 and 165μm grain size, which are assumed to show stress-controlled magnetic properties, between room temperature and the Curie point. For uniaxial single-domain (SD) particles, typical parameters that can be determined are in the low-field range a critical field for the onset of WRH , a peak WRH and a further typical field in the high-field region where WRH tends to zero; all these parameters are associated with the anisotropy constant. For 2.4μm particles, assumed to be SD, the large experimental WRH at all fields in the whole temperature range is ascribed predominantly to the action of high internal microstresses arising from a large density of dislocations and other lattice defects introduced by the milling process. The reduced magnitude of WRH for pseudo-single-domain 12.5μm particles and the still smaller WRH for multidomain (MD) 165μm particles is thought to be in the first place the result of much lower defect densities in conjunction with irreversible domain rotations. For each particle size, a rise of some WRH parameters is observed for temperatures >120°C, which coincides for MD 165μm particles with an increase of magnetic hysteresis parameters. The non-zero WRH at the maximum applied field of H = 1200kAm−1 ≈ 15kOe for all particles in the whole temperature region analysed is attributed to some kind of exchange anisotropy, caused by centres or areas in a particle related to a very high anisotropy, due probably to Fe2+ in the neighbourhood of lattice defects.