Phase transitions in La1-xCaxMnO3-x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La1-xCaxMnO3-x/2 reduced manganites with 0 less than or equal to x less tha n or equal to 0.5 are established. These investigations contribute to the u nderstanding of magnetic interactions in manganites without Mn4+ ions. It i s found that these manganites show a long-range antiferromagnetic order up to x = 0.09 and transform into spin glasses at 0.09 < x less than or equal to 0.35. The compositions in the range 0.35 < x less than or equal to 0.5 s how a strong increase in the spontaneous magnetization and critical point a ssociated with the appearance of spontaneous magnetization and can therefor e be viewed as inhomogenious ferromagnets. The magnetic and crystal structu re peculiarities of La0.5Ca0.5MnO2.75 are established by the neutron diffra ction method. The strongly reduced samples show a large magnetoresistance b elow the point where the spontaneous magnetization develops. The magnetic p hase diagram of La1-xCaxMnO3-x/2 is established by magnetization measuremen ts. The magnetic behavior is interpreted assuming that the Mn3+-O-Mn3+ magn etic interaction is anisotropic (positive-negative) in the orbitally ordere d phase and isotropic (positive) in the orbitally disordered phase. Introdu ction of the oxygen vacancies changes the magnetic interaction sign from po sitive to negative, thereby leading to a spin glass state in strongly reduc ed compounds. The results obtained reveal unusual features of strongly redu ced manganites such as a large ferromagnetic component, a high magnetic ord ering temperature, and a large magnetoresistance despite the absence of Mn3 +-Mn4+ pairs. In order to explain these results, the oxygen vacancies are s upposed to be ordered. (C) 2001 MAIK "Nauka/Interperiodica".