Magnetic studies and removal of non-stoichiometry in MnO and CoO

A major impediment in accurately determining the magnetic properties of the transition-metal oxides (MnO, CoO, NiO, FeO) has been the presence of non-stoichiometry (NS) in the available crystals. From studies in crystals of MnO and CoO, we report two important developments: First, the temperature-dependent magnetic susceptibility (χ) provides an excellent probe for detecting NS and second, a method has been developed to remove NS. From these studies, the presence of Mn3O4 in MnO and that of cation vacanies in CoO is inferred. In MnO and CoO, NS has been removed to undetectable levels by annealing the single crystals in the presence of Mn and Co atmospheres, respectively, at 1300 K for about 200 hours. Possible applications of these techniques to other systems are also discussed.