PHASE TRANSITIONS IN La_1-xCa_xMnO_3-x/2 MANGANITES
Troyanchuk I.O., Trukhanov S.V., Szymczak H., Przewoznik J., Barner K.

Received: December 8, 2000

PACS: 72.15.Gd, 75.30.Kz, 75.70.Pa

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The crystal structure parameters, magnetic and electrical properties of La_1-xCa_xMnO_3-x/2 reduced manganites with are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at . The compositions in the range show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogeneous ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1-xCa_xMnO_3-x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. The introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unconventional features for strongly reduced manganites such as a large ferromagnetic component and high magnetic ordering temperature as well as a large magnetoresistance in spite of absence of Mn³⁺-Mn⁴⁺ pairs. For the explanation of these results, the oxygen vacancies are supposed to be ordered.