CONDUCTION ABOVE THE VERWEY TRANSITION IN MAGNETITE

Abstract

The conductivity of magnetite, measured by several investigators, is an increasing function of temperature above the Verwey temperature (120°K), reaching a maximum near room temperature. The total increase is roughly a factor of five. We have used the results of an itinerant‐electron model previously introduced1, to calculate the conductivity in the temperature range mentioned above.2 This is the region of temperature in which the bands are not yet split by the charge ordering. On a one (half‐filled) band model, the conductivity surely decreases as temperature increases. However, the more realistic calculation described in reference 1 reveals two almost flat conduction bands just touching the valence band at the center of the Brillouin zone.3 Conduction is then by holes, whose number is an increasing function of temperature. This increase is linear in T/h for T/h < 2, with a slope of 1.5 where h is the valence‐band width and T the temperature, except for extremely low temperatures. Thus, the number of holes increases linearly with temperature in the region of interest here. We expect the mobility of holes to be temperature‐dependent as well, due to interband scattering by optical phonons and spin waves. Because the flat bands are so accessible, we also expect the mobility to be small. This is again in qualitative agreement with experiment. Thus the observed T dependence of conductivity above the Verwey temperature in magnetite is understandable in the band picture, though there is actually no gap in the electron energy spectrum.