Superparamagnetic Behavior of MnFe ₂ O ₄ and α‐Fe ₂ O ₃ Precipitated from Silicate Melts

Abstract

Manganese ferrite and α‐Fe 2 O 3 particles were precipitated within silicate melt systems to produce very unusual magnetic properties. Assemblies of particles of both kinds behaved super‐paramagnetically when the particle size was small enough. As the particle size was increased, the magnetic properties of the ferrite system increased, but those of the α‐Fe 2 O 3 system decreased; the latter is expected from Néel's theory of a net spontaneous magnetic moment created by uncompensated magnetic sublattices at very small particle sizes. Liquid‐in‐liquid phase separation was pronounced in the manganese ferrite‐glass systems, which may have influenced the precipitation behavior. Room‐temperature initial mass susceptibilities were as high as 2 × 10 −2 cgs, and specific magnetizations as high as 26 gauss/g were observed. Precipitation of α‐Fe 2 O 3 particles exhibiting super‐paramagnetic behavior was possible only with very low‐viscosity melts. Initial mass susceptibility values changed by as much as a factor of 30 between 296° and 77°K.