PAC measurements of the Verwey transition in magnetite

Abstract

Perturbed angular correlation (PAC) experiments with implanted ¹¹¹In tracers have recently been used to investigate magnetic phase transitions in metal oxides. Here we report on PAC measurements for ¹¹¹Cd in polycrystalline Fe₃O₄ in the neighborhood of the Verwey phase transition (T_V≈ 120 K). Perturbed angular correlation spectra were taken for implanted ¹¹¹In probes at temperatures between 9 and 850 K. The two observed Larmor frequencies are attributed to the two possible cation sites in the cubic inverse spinel lattice. For T > 120 K, the temperature dependence of both Larmor frequencies follows a Curie–Weiss power law ω_L(T)/ ω_L(0)=(1-T/T_N)^β, with the parameters T_N=848(2) K and β= 0.392(2). At the Verwey temperature we find a rapid change of both Larmor frequencies, with ω_L1 increasing from 178(2) MHz at 120 K to 191(4) MHz at 100 K, and ω_L2 decreasing from 173(4) MHz to 151(5) MHz. The Verwey transition also affects the widths of the frequency distributions, which more or less double below T_V. This possibly indicates the presence of several components with Larmor frequencies similar to those found in the previous Mössbauer data, or for electronic after-effects correlated with the semiconductivity of magnetite below T_V.