Magnetic phase transitions in Ln1–xSrxCo0.5Mn0.5O3 (Ln = La, Pr, Nd, and Eu)

Resumo

The system of La_1–xSr_xCo_0.5Mn_0.5O₃ solid solutions have been studied by neutron diffraction and the measurement of the magnetization. The composition with x = 0 has the monoclinic structure (space group P2₁/n); the compositions with 0.15 ≤ x ≤ 0.6 are rhombohedral (R\(\bar 3\)c), and the composition with x = 0.75 is cubic (Pm\(\bar 3\)m). All the compositions with x ≤ 0.75 are shown to contain a ferromagnetic component. The stoichiometric composition with x = 0.5 undergoes a sharp ferrromagnet–paramagnet transition at 240 K; however, no coherent magnetic contribution to the neutron diffraction has been detected. The magnetic data are interpreted, assuming that the ferromagnetism at x ≤ 0.15 is due to positive superexchange Co²⁺–O–Mn⁴⁺ interaction, while, at x ≥ 0.25, the superexchange interaction between ions Co³⁺ in a high-spin state and ions Mn⁴⁺ is responsible for the ferromagnetic component.