Electrical conductivity and thermoelectric power of nickel ferrous ferrite. Variable-range hopping and the Coulomb gap

Electrical conductivity and thermoelectric power measurements have been made on single crystals of nickel ferrous ferrite, NixFe3-xO4 of compositions 0x0·9, in the temperature range 10 to 300 K. The temperature and concentration dependences of the conductivity and of the thermopower at high nickel concentrations (x 0·4) are discussed in terms of variable-range hopping and of nearest-neighbour hopping in an energy distribution of localized states, or the 'Anderson band'. It is argued that there is evidence for the formation of a Coulomb gap and for many-electron hopping in samples of low nickel content (