The electrical conductivity and thermoelectric power of nickel ferrous ferrite Magnetic ordering effects

Abstract

Abstract The conductivity σ and thermoelectric power θ of single crystals of the ferrimagnetic system nickel ferrous ferrite (NixFe3-xO4, with 0 ≤ x ≤ 0·9) have been measured in the temperature range 300 to 1150K. The measurements are interpreted in terms of nearest-neighbour hopping between localized states distributed in energy around the Fermi energy, the finite width of the distribution (or band) arising from ionic and electronic disorder. The measured conductivity is compared with the empirical expression σ= ΓT −1exp(-W/kT), where W is the hopping energy and Γ is a constant. A cusp in d1n(σT)/d(1/T) and an associated thermopower anomaly at the Néel temperature T N are attributed to the onset of magnetic disorder. A ‘two-band’ model has been developed in which the energy levels of spin-up and spin-down electrons are split by the internal exchange field, which gives rise to magnetic ordering. The model explains the major features of the magnetic anomalies. An observed increase in Γ with temperature is shown to be consistent with the two-band model and with effective-medium treatments of hopping conduction. It is also demonstrated that the concentration dependence of the thermoelectric power in the paramagnetic region may be explained in terms of a band of localized states.