Influence of Crystallite Size on the Magnetic Properties of Acicular γ-Fe2O3 Particles

Abstract

Magnetic properties have been measured for a number of samples of acicular γ-Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x-ray line broadening, ranged from 50 to 700 Å. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 Å wide. Room-temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size. The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite diameter of order 400 Å may be inferred. Below this crystallite size, superparamagnetic behavior is observed; above this size, noncoherent magnetization reversals in the crystallites are suggested. This model is quite consistent with the ``fanning'' mode of reversal based on a ``chain-of-spheres'' model which has been considered by several authors to be most compatible with their data on γ-Fe2O3 particle assemblies.