HOPPING CONDUCTION AND POSSIBLE COULOMB GAP EFFECTS IN MANGANESE AND ZINC FERROUS FERRITE

Electrical conductivity and thermopower measurements have been made on zinc and manganese ferrous ferrite, Zn(x)Fe-xO4 and Mn(x)Fe3-xO4 with 0 less-than-or-equal-to X < 1, in the temperature range 100-300 K. The results of X < 0.5 are tentatively interpreted in terms of the formation of a Coulomb gap at low temperatures in an energy band of localized states. For X greater-than-or-equal-to 0.5 the electrical conductivity is sysmptomatic of nearest-neighbour hopping, and thermopower vareis aas In c where c = (1 - X)/2. The energy bandwidth of conduction states has been extracted from the thermopower and electrical conductivity data and is shown to be roughly proportional to the standard deviation of the Zn and Mn distributions [X(1 - X)]1/2. A polaron contribution to the conductivity activation energy of approximately 0.03 eV at room temperature has been deduced for Mn0.8Fe2.2O4 and Mn0.9Fe2.1O4