Nucleation and Growth of Magnesioferrite in MgO Containing 0.9% Fe 3+

The initial stage of precipitation of magnesioferrite from a supersaturated solid solution of 0.9 cation % Fe 3+ in MgO at 500 °C was studied by fitting the magnetization curves to 7000 Oe at low temperatures with the Brillouin function. The averase umber of Fe 3+ ions in a precipitate particle, increased monotonically with aging time from 9 in the as‐quenched condition to 88 after 16 h. The average spin quantum number per Fe 3+ ion decreased to near its final value at an aging time corresponding to = 50. Thus the volume fraction of precipitate is near the final value when the average particle size is only about three unit cells of magnesioferrite.