MAGNETIZATION AND EXCHANGE IN NONSTOICHIOMETRIC MAGNETITE

Measurements of saturation magnetization and remanence in Fe3(1-delta)O4 single crystals with (0.000 < delta < 0. 006) between 4.2 and 300 K are reported. The exchange constants J(AB), evaluated with spin-wave theory in the harmonic approximation, agree with those derived from low-temperature heat-capacity investigations. A discontinuous change in behavior for delta > 0.0039 is consistent with the change from a first- to second-order Verwey transition, associated with splitting of the octahedral electronic levels. A self-reversal of the remanent moment is observed at T(V), in the second-order regime. Simple nearest-neighbor models, in the mean-field approximation, suffice to account for all electronic and magnetic critical temperatures, in the Ising two-state and Heisenberg formulations, respectively.