Phase Separation in CMR Materials: The Role of Spin Nanoclusters

Abstract

Manganese perovskites, with the general formula REMnO3 (RE=Rare Earth), display a wealth of new phenomena on substitution of part of the trivalent RE by a divalention of the alcaline earth series (Ca, Sr etc.). This doping results in the formation of a mixed valence system (Mn3+ , Mn4+) with a rich phase diagram with ferromagnetic insulators at low doping, ferromagnetic conductors, at higher doping and antiferromagnetic insulators at doping higher than 50%. The system displays magnetic phase separation in the phase boundaries while application of a magnetic field brings the system in its ground state which is usually the metallic ferromagnetic state. The result of this process is the appearance of colossal negative magnetoresistance (CMR) with many potential applications. There has been an intensive study of these systems during the past 5 years in order to elucidate this new phenomenon at the atomic level. The double electron exchange (Zener) and the Jahn-Teller effect are the basic principles governing the CMR effect, although there are basic questions that are still open. It has now been established, within the relevant scientific community, that these phenomena arise from an interplay of the charge, magnetic moment (spin and orbital) and lattice degrees of freedom. Among the many experimental techniques that have been employed for these studies, the spectroscopic techniques for hyperfine interaction studies are of prime interest since they provide information at the atomic level. In this work, we present the results of Moessbauer studies in La0.5Ca0.5MnO3 doped with 1% 57Fe and 1% 119Sn. These results show the coexistence of phases, in particular close to the phase boundaries, the dynamic transformation of one phase to the other and the role of the formation of spin-nanoclusters during this transformation.