SUBSTITUTION EFFECTS ON THE MAGNETIC PROPERTIES OF NI(ME,MN)2O4 AND LI(ME,MN)2O4: COMPARISON BETWEEN ME = CR AND FE

Manganese oxides of spinel structure, based on nickel (NiMn2O4) or lithium (LiMn2O4), were studied by magnetic measurements. Manganese was partially substituted by either chromium or iron, showing substantial variations of the magnetic properties with respect to the non-substituted compounds. Chromium enhances the antiferromagnetic interactions in both NiMn2-xCrxO4 and LiMn2-xCrxO4, while iron enhances the ferromagnetic ones, increasing Tc well above room temperature in NiMn2xFexO4 and LiMn2-xFexO4 (0 £ x £ 1). Titration analyses allowed us to estimate the Mnn+/Mnn+1 ratios in most cases, confirming the importance of the Mn3+-O-Mn4+ double-exchange ferromagnetism and the controlled-valence conduction mechanism in these materials