MICROSTRUCTURE OF MAGNETIC PROPERTIES OF MANGANESE-ZINC-FERRITE

Grain boundaries in MnZn-ferrites have been characterized and their effects on the magnetic and electrical properties have been investigated. In addition, controlled atmosphere annealing has been performed on high permeability ferrite to improve the electrical resistivity of the bulk materials.;Partial dissolution of Ca ions into the matrix results in a higher magnetic anisotropy energy in the regions near grain boundaries and the amorphous grain boundary phases act as non-magnetic barriers; both of them retard magnetic domain wall motion. The grain boundary phases, on the other hand, also act as insulating layers for electrical conduction. The space charge polarization of the materials, however, renders the blocking effect totally invalid in the high frequency regime. It is concluded from these observations that the Ca addition in these materials does not produce beneficial effects on the electrical properties but only leads to a detrimental influence on the magnetic properties.;Other microstructural features such as secondary phases and stacking faults will also affect, besides grain boundaries, the domain wall dynamics. They not only retard the domain wall motion but can also act as nucleation sites for domains of reverse magnetization.;The controlled atmosphere annealing improves drastically the apparent resistivity of the sintered MnZn-ferrites through the reduction of ferrous ions content. The magnetic permeability is, unfortunately, degraded as a result of annealing. This is ascribed to the inhomogeneity of the oxidation in the polycrystalline specimens. Therefore, a stringent control of the oxygen partial pressure at an earlier stage of processing rather than post fabrication annealing is called for, in order to raise the intrinsic electrical resistivity of the bulk materials by reducing ferrous ion concentration but not to affect the magnetic permeability detrimentally.;The addition of a small amount of CaO into MnZn-ferrite materials is observed to lead to the formation of secondary phases along the grain boundaries. These grain boundary phases which consist of a thin layer of CaO/MnZn-ferrite intermediate compound, exist as liquid phases at sintering temperature and as amorphous phases when cooled to room temperature.