Investigation of magnetic after-effects in magnetite near the verwey temperature

The magnetic relaxation in polycrystalline, non-stoichiometric magnetite Fe3−ΔO4 (5 × 10−6 < < Δ < 3 × 10−2) is investigated near the Verwey temperature (TV ≈ 120 K). The analysis of the measurements is based on a phase transition of second order from a state of long range ionic order below TV to a state of delocalized, itinerant electrons. A relaxation process with a logarithmic time dependence is observed in the temperature range between 40 to 140 K. which is ascribed to electron hopping processes of the type Fe2+ – e− ⇄ Fe3+ in the ordered phase. The relaxation time of this process is given by an Arrhenius equation τ = τ0 exp (Q/kT) with activation parameters τ0 ≈ 10−14 s; 0.15 < Q < 0.22 eV, depending upon vacancy concentration. The relaxation strength of this process depends on the degree of long range ionic order and is further modulated by anomalous relaxation effects which occur at the critical temperatures of crystallographic (TC ≈ 105 K) and ionic (TV ≈ 124 K) transitions.