The low-temperature resistivity and Seebeck coefficient of fluorine-substituted magnetite

The resistivity and thermopower of the system Fe3O4_ x F x (0 ≤ x ≤ 0.15) have been measured in the temperature range 20 to 300 K. A sharp Veruey transition occurs in the concentration range 0 ≤ x ≤ 0.15 and a thermal hysteresis is observed in both the resistivity and thermopower at the transition. At the lowest temperatures a T −1/4 law in the conductivity and the behaviour of the thermopower are attributed to the onset of impurity conduction. Measurements on samples in the concentration range 0.05 ≤ x ≤ 0.15, which do not show a sharp Verwey transition, are interpreted in terms of hopping between ‘Anderson-localized’ states.