The Origin of Ferromagnetism in ε-Fe2O3 and ε-GaxFe2-xO3 Nanomagnets

Up to date, the origin of ferromagnetism in epsilon-Fe2O3 and epsilon-GaxFe2-xO3 has been ambiguous. In this work, the origin can be well explained by using a molecular-field (M-F) model with four nonequivalent Fe sites (A-D sites). The MF calculation demonstrates that epsilon-Fe2O3 is a Neel P-type ferrimagnet, which consists of positive sublattice magnetizations of M-B and M-C and negative ones of M-A and M-D. This magnetic ordering mechanism can be understood by the product of the number of exchange pathways (Z(ij)) and the superexchange interaction constants (J(ij)). The calculated saturation magnetization vs. x plots in epsilon-GaxFe2-xO3 at 300 K have a maximum around x = 0.47, and the T-C value monotonously decreases with increasing x. These magnetic properties are explained by the position of Ga replacement and weakness of the Z(ij)J(ij) values.