Magnetization and exchange in nonstoichiometric magnetite

Measurements of saturation magnetization and remanence in Fe_3(1-δ){O}₄$ single crystals with (0.000 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 {δ}>0.0039 is consistent with the change from a first- to second-order Verwey transition, associated wtih splitting of the octahedral electronic levels. A self-reversal of the remanent moment is observed at TV, 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.