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Investigation of the phase diagram of manganites La1-xCaxMnO3 (x > 0.5)

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1-xAxMnO3, where R stands for a rare earth element and A for an alkali or alkaline earth metal. One of the most prominent property of manganese oxides is a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). In these systems CMR can be as large as -99%, unlike magnetoresistance of most materials which is usually small (a few percent). Except CMR, manganese oxides are interesting because of their very rich phase diagrams as a consequence of the fine interplay of charge, spin, lattice and orbital interactions. CMR is present in the part of the phase diagram of manganese oxides with ferromagnetic metallic ground state and in the case of the investigated system La1-xCaxMnO3 for concentrations 0.2 < x < 0.5. While pure LaMnO3 and CaMnO3 are antiferromagnetic insulators and contain Mn3+, Mn4+ respectively, doped manganites La1-xCaxMnO3 have a variety of different phases and contain both Mn3+ and Mn4+ (a mixed valence state). Here we focused on the poorly explored part of the phase diagram x > 0.5 with a paramagnetic insulating phase at high, charge order phase (CO) at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on a series of thin film manganites (170 nm thick) of different concentrations 0.5 < x < 0.8 obtained by metalorganic aerosol deposition technique. We will show and discuss the results of magnetotransport properties by which we investigated the influence of the concentration x on resistivity, CO phase transition temperature, magnetoresistance and anisotropy of magnetoresistance.

manganites; colossal magnetoresistance; ferromagnetism; mixed valence state; charge order; magnetotransport

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