Effect of heat treatment on the microstructure and magnetic properties of polycrystalline MnZn ferrites

The effect of heat treatment of commercially available polycrystalline MnZn ferrite slabs was studied for temperatures near typical recording head glassing temperatures. Toroid-shaped specimens were heated in atmospheres of varying levels of oxygen and the deterioration in magnetic properties was measured. The surfaces of the exposed specimens were inspected by means of optical and electron microscopy and the occurrence of ferrite instabilities such as grain growth, segregation of secondary phases etc. were observed. The degradation also appeared to be surface-driven. To investigate this further, some samples were heat treated in highly oxidizing and reducing atmospheres to induce gross ferrite instabilities. The near surface regions of these samples were then characterized by x-ray diffraction, x-ray photoelectron spectroscopy (XPS) and conversion electron and conversion x-ray Mossbauer spectroscopy (CEMS and CXMS). In addition to grain growth and other structural changes, the precipitation of extraneous phases appears to be a major mode of ferrite failure during such annealing