On the Coercivity of γFe2O3 Particles

Abstract

Over the temperature range −180° to 300°C, measurements have been made of the coercivity of 5:1 acicular γFe₂O₃ particles. Samples of two different lengths, 2000 and 6000 Å, were included in the study. The coercive force is linearly dependent upon two terms, shape (αM) and magnetocrystalline (βK/M). These may be separated by reducing identical particles to Fe₃O₄ and measuring their coercive force at the isotropy point −143°C, yielding the shape factor α. It is found that the shape term accounts for approximately 67% of the observed room‐temperature coercive force. The shape term is almost independent of particle diameter and fits well with the ``chain‐of‐spheres'' reversal model. The remaining 33% of the coercivity has the same temperature dependence as the coercive force of cubic γFe<sub>2</sub>O<sub>3</sub> particles, thus confirming its magnetocrystalline origin. The room‐temperature coercivity of cobalt‐doped cubic particles was measured as a function of particle diameter. Over the diameter range 300–1000 Å the data again fits the linear addition of shape and magnetocrystalline terms. The shape term is diameter‐dependent and fits the micromagnetic ``curling'' reversal model which has been found previously only in metallic whiskers.