Two-dimensional ferromagnetic ordering and magnetoresistance in the layered perovskite La222xCa112xMn2O7

The resistivity and magnetization of layered perovskite La222xCa112xMn2O7 have been studied systematically using bulk samples with a wide doping concentration range 0<x<0.5 and epitaxial thin film samples with x50.3. The system becomes a metallic ferromagnet for doping concentrations 0.22<x<0.5. In this doping region, results for bulk samples as well as for thin films have clearly shown that there are two types of ferromagnetic ordering, possibly originating from an anisotropic exchange interaction. Magnetoresistance is observed over a wide temperature range below the ferromagnetic ordering temperatures. Based on these results, a model of temperature-dependent spin ordering is presented for a double perovskite ferromagnet, which suggests that there is a two-dimensional ferromagnetic alignment at temperatures between the two ordering temperatures. @S0163-1829~97!03433-4#