Characterization of manganese-oxide perovskites, exhibiting a colossal magnetoresistance
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Abstract
The complex phase diagrams of the La1-xSrxMnO3 ( 0 ≤ x ≤ 0.35), La 1- xCaxMnO3 ( 0 ≤ x ≤ 0.40 ), and R0.7A0.3MnO3 ( 0.949 ≤ t ≤ 0.997 ) systems, in which R is one or more the trivalent rare earth ions, A is one or two divalent alkaline earth, and t is tolerance factor, are presented; they were mapped out with measurements of resistivity ρ(T), thermoelectric power α(T), specific-heat Cp(T), and magnetization M(T) on a series of melt-grown single-crystal samples. A transition from localized to itinerant behavior of electrons of e-orbital parentage in the presence of a localized t3 configuration with spin S = 3/2 was shown to be characterized by an electronic phase segregation into orbitally ordered, electron-rich antiferromagnetic regions and orbitally disordered, conductive ferromagnetic regions that grow in an applied magnetic field. This two-phase electronic character in the crossover compositions gives rise to spin-glass behavior and a colossal magnetoresistance. Particular emphasis was placed on the evolution of the specific-heat data Cp(T) on crossing the transition from localized to itinerant electrons and the fact that Cp(T) is suppressed at a spin-glass freezing temperature.