Mössbauer Studies for Exploring CMR and TMR Perovskites

Doped transition metal compounds with complex electronic and magnetic structures show a wide range of new physical phenomena like high-temperature superconductivity, room temperature magnetic semiconductivity, or colossal magnetoresistance (CMR). Following the discovery of the effect of colossal negative magnetoresistance in manganese based perovskites, several different classes of transition metal compounds such as Sr2FeMoO6 double perovskites or La1-xSrxCoO3 perovskites were found to exhibit unusually high magnetoresistance (MR) partly as a peak around their Curie-temperature (CMR effect), partly as an increasing feature with decreasing temperature (tunneling-type magnetoresistance, TMR)(Table1). The most important difference between these two types of MR is their temperature dependence: while CMR effect manifests only around the magnetic temperature as a peak in the MR vs. T plot, TMR increases monotonically with decreasing temperature. Due to the complexity of the underlying physical and chemical processes in these materials, the understanding of their electronic and magnetic structure is one of the most vital topics in condensed matter physics nowadays. The first models aiming to shed light on the CMR effect found in manganite perovskites, and to explain the unusually strong correlation between the magnetic state of the material and its electric transport properties were based on the theories of double exchange model and strong electron-lattice interactions. The former was based on the fact that in La1-xCaxMnO3 perovskites the doping divalent ions (usually Ca) introduce a number of x extra electrons to the system, either oxidizing Mn ions into Mn or creating oxygen vacancies, although for low doping rates the latter effect was found to be negligible. As a Mossbauer Studies for Exploring CMR and TMR Perovskites