Conduction band polarization in some CMR materials: Evolving guidelines for new systems

Abstract

First principles electronic structure calculations reveal certain important common features in the conduction band polarization of many of the newly examined colossal magnetoresistance (CMR) materials. Most CMR compounds seem to possess a localized, magnetic band slightly below the Fermi energy. This localized band transfers polarization to a relatively broad conduction band. The nature of the two bands in different systems can be quite distinct. In the perovskite-derived manganese oxides, the magnetic band is derived from Mn t2g states while the conduction band is derived from Mn eg states. In the chalcospinel Fe0.5Cu0.5Cr2S4, the Cr t2g states which are below EF polarize conducting Fe d states. In Gd metal and the newly discovered CMR material GdI2, the metallic Gd d states are polarized by the underlying f levels. In double perovskites (elpasolites) such as Sr2FeMoO6, Fe states similarly polarize the broader Mo-derived conducting d states. In the pyrochlore Tl2Mn2O7, localized, magnetic Mn d states polarize the conduction band derived from bonding Tl–O states. In all the systems except Gd metal, the conduction band seems to be strongly spin differentiated. The systematics obtained here provide guidelines for the design of new CMR systems.