Antiferromagnet-ferromagnet and structural phase transitions in La 0.88 MnO x manganites

The crystal and magnetic structure as well as elastic, magnetic, and electrotransport properties of La 0 . 8 8 MnO x (2.82≤x≤2.96) manganites have been investigated as a function of oxygen content by x-ray, neutron diffraction, Young's modulus, magnetization, and resistivity measurements. The crystal structure of the compounds has been found to be orthorhombic at x<2.91 and monoclinic at x≥2.91. The gradual transition from an orbitally ordered state to an orbitally disordered one is observed with increasing temperature or oxygen content. The magnetic properties of the samples have been found to correlate with the type of orbital state. The dynamic orbital correlations favor ferromagnetic state, while A-type antiferromagnetic state is typical for the static Jahn-Teller distortions. The orthorhombic compounds have been found to exhibit a semiconducting behavior, and show a second-order transition into paramagnetic state. The metallic conductivity appears starting from the x=2.92 monoclinic compound and, simultaneously, a first-order paramagnet-ferromagnet magnetic phase transition is observed. The properties are discussed in the framework of structurally driven phase separation model.