Magnetic viscosity and microstructure: Particle size dependence of the activation volume

Abstract

Ba hexaferrite samples having average particle sizes in the range from the tens up to the thousands of nm have been prepared by using conventional ceramic techniques and the exothermic reaction between Ba and Fe nitrates and oxalic dihydrized acid. The magnetic characterization of these samples included the measurement of the temperature dependence of the high field magnetization, the coercive force, and the evaluation, from thermally activated demagnetization measurements, of the activation volume at different temperatures. From the results of this characterization it was possible to reach conclusions about the occurrence of remarkable differences between the average particle size dependence of the coercive force and that corresponding to the activation volume: whereas the coercive force value increased monotonically with the decrease of the particle size, an activation volume minimum was observed for particle diameters of the order of hundreds of nm. We suggest that this minimum reflects a change of the mechanism ruling the magnetization reversal of the particles forming our samples.