Evolution of magnetic state in the La 1− xCa xMnO 3− γ ( x=0.30, 0.50) manganites depending on the oxygen content

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The crystal structure, magnetic and electrical properties of the La 1− x Ca x MnO 3− γ ( x=0.30, 0.50; 0⩽ γ⩽0.50) oxygen-deficient manganites have been studied. It is found that the compounds La 0.70Ca 0.30MnO 3− γ possess a long-range ferromagnetic order up to γ=0.06 and a cluster spin glass behavior at 0.06< γ⩽0.20. Antiferromagnetic state of La 0.50Ca 0.50MnO 3− γ ( γ=0) composition transforms into inhomogeneous ferromagnetic one at γ=0.04. The system converts into cluster spin glass state at γ=0.10. As oxygen deficit reaches the value γ=0.25, a new type of ferromagnetic phase appears. The fraction of this ferromagnetic phase is the highest in the composition γ=0.30. It is supposed that the compounds with γ⩾0.35 represent an antiferromagnetic medium with inclusions of the ferromagnetic phase. The strongly reduced samples exhibit a large magnetoresistance below the temperature, at which the spontaneous magnetization develops. The magnetic phase diagrams of both La 0.70Ca 0.30MnO 3− γ and La 0.50Ca 0.50MnO 3− γ systems have been constructed. We argue, that the oxygen vacancies are disordered in the La 0.70Ca 0.30MnO 3− γ system in the studied region of oxygen vacancies concentration (0⩽ γ<0.20) whereas for the La 0.50Ca 0.50MnO 3− γ they tend to order at γ>0.25 in a manner of Sr 2Fe 2O 5-type crystal structure. This study shows that Mn 3+–O–Mn 3+ ferromagnetic interaction may play an important role in the formation of magnetic state of manganites.