Influence of Particle Size on the Coercive Force of Barium Ferrite Powders

Abstract

The influence of milling on the intrinsic coercive force (H_ci) of different ferrite powders has been studied recently by several investigators. These have ascribed changes of coercive force to dislocations which are introduced through milling and are removed through subsequent annealing. In our own studies of barium ferrite powders we relate the changes of H_ci to changes in particle size. This interpretation is supported by experimental data and electron micrographs. The large particles of barium ferrite material (calcined at 1300°C) change during ball milling from predominantly multidomain size (several microns) to nearly superparamagnetic size (0.02–0.5 microns). Accordingly, coercive force increases at first and then decreases as milling is extended. Annealing of ball milled material between 800° and 1100°C results in slight sintering. Since the sintering reduces the number of nearly superparamagnetic particles, considerable increases in coercive force occur. These interpretations are further supported by the temperature dependence of the coercive force. After short milling, when multidomain particles are still predominant, H_ci decreases as temperature of powder is lowered. After extended milling, however, this effect changes sign which is attributed to thermal fluctuations of the magnetization in nearly superparamagnetic particles.