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

Abstract

Electrical conductivity and thermoelectric power measurements have been made on single crystals of nickel ferrous ferrite, Ni_xFe_3-xO₄ of compositions 0⩽x⩽0·9, 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 (x<0 4).