Electrical and magnetic properties of 3d-transition metal vanadates

Abstract

The electrical and magnetic properties of vanadates of the general formula (MO)_n .V₂O₅, where M=Mn, Co, Ni, Cu and Zn, and n=1, 2 and 3, are discussed. Conduction in these materials occurs due to small deviations from the stoichiometric composition in the form of anion-(n-type) or cation-(p-type) vacancies. The mechanism of electrical transport is of a thermally activated hopping of charge carriers on equivalent cation lattice sites. It has been found that the meta vanadates (n=1) are alln-type while the pyro-(n=2) and ortho-(n=3) compounds of Mn, Co and Ni arep-type; but Cu and Zn analogues remainn-type only. The vanadates obey Curie-Weiss law at 80-340 K. The effective magnetic moments for MV₂O₆ are nearly the same as spin only values for the magnetic M²⁺ ions, and diminish with increasing MO contents. The predominant exchange interactions are the weak 90°MOM super-exchange and M-O-O-M super-super exchange. The Weiss constant is negative (antiferro-) for all the compounds except Co₃V₂O₈ which has a positive (ferro-) value. Cooperative phenomena, if any, have not been observed in this temperature range.