Generic model of the colossal magnetoresistance pyrochlore Tl/sub 2/Mn/sub 2/O/sub 7/

Abstract

The colossal magnetoresistance (CMR) exhibited by the pyrochlore compound Tl₂Mn₂O₇ was discovered in 1996. Important differences with CMR Mn-perovskite compounds regarding crystal structure and electronic properties were soon realized, indicating the possibility of different mechanisms for CMR being present. Recently, new pyrochlore compounds exhibiting CMR were prepared. The larger experimental background now existent, has received diverse interpretations. In the present work, we aim to contribute to the understanding of the properties of Tl₂Mn₂O₇ through the study of a generic model for the compound, including Hund and superexchange couplings as well as hybridization effects (HS model). The model includes two kinds of electronic orbitals, as widely believed to be present: one, directly related to the magnetism of the compound, involves localized Mn moments, represented by spins, and a narrow band strongly Hund coupled to them. Also, more extended electronic orbitals, related to the carriers in Tl₂Mn₂O₇, appear and hybridize with the narrow band. The possibility of a superexchange coupling between the localized spins is also allowed for. This generic model allows exploration of many of the proposals put forward by other researchers for Tl₂Mn₂O₇. As a first approach to the problem, here we employ a mean field approximation to study the magnetic phase diagram and electronic structure of the HS model, for various sets of parameters mentioned before in connection with the compound. Furthermore, we are able to exhibit similarities with predictions obtained before using an intermediate valence model for Tl₂Mn₂O₇, if appropriate sets of parameters are used. In particular, we show here many common features which can be obtained for the electronic structure around the Fermi level in the ferromagnetic phase. Due to the simplifications adopted in the present treatment the comparison between the models could not be extended to higher temperatures here.