MAGNETIC VISCOSITY AND MICROSTRUCTURE - PARTICLE-SIZE DEPENDENCE OF THE ACTIVATION VOLUME

Ba hexaferrite samples having average particle sizes in the range from the tens up to the thousands of nm have been prepared by using conven tional ceramic techniques and the exothermic reaction between Ba and F e nitrates and oxalic dihydrized acid. The magnetic characterization o f these samples included the measurement of the temperature dependence of the high field magnetization, the coercive force, and the evaluati on, from thermally activated demagnetization measurements, of the acti vation volume at different temperatures. From the results of this char acterization it was possible to reach conclusions about the occurrence of remarkable differences between the average particle size dependenc e of the coercive force and that corresponding to the activation volum e: whereas the coercive force value increased monotonically with the d ecrease of the particle size, an activation volume minimum was observe d for particle diameters of the order of hundreds of nm. We suggest th at this minimum reflects a change of the mechanism ruling the magnetiz ation reversal of the particles forming our samples. (C) 1996 American Institute of Physics.