Magnetic phase transformations and magnetotransport phenomena in La0.7Sr0.3Mn1 – xCoxO3 perovskite compounds

Resumo

The compositions La_0.7Sr_0.3Mn_{1 – x}Co_xO₃ (0.13 ≤ x ≤ 1) are studied by neutron diffraction, magnetometry, and measuring the magnetotransport properties. The substitution of cobalt ions for manganese ions is shown to decrease the magnetization and the Curie temperature from 270 K (x = 0.13) to 140 K (x = 0.33). As the cobalt ion content increases to x = 0.5, the Curie temperature increases to 190 K, the magnetization decreases, and the electrical resistivity increases. At x > 0.5, the temperature of transition into a paramagnetic state decreases to 68 K (x = 0.8) and then again increases to 225 K for the La_0.7Sr_0.3CoO₃ composition. The magnetoresistive effect in the range 0.3 ≤ x ≤ 0.4 reaches 97% and decreases gradually with increasing temperature without anomalies near the Curie point. At x ≤ 0.2, the magnetoresistive effect increases near the Curie temperature. The composition at x = 0.6 is stoichiometric, and no coherent magnetic contribution to neutron scattering is detected. The magnetic properties near x ∼ 0.5 are assumed to be caused by partial ordering of Co³⁺ and Mn⁴⁺ ions, and the Co³⁺ ions can be in both low- and high-spin states. The magnetic interaction between Co³⁺ ions in a high-spin state and Mn⁴⁺ is predominantly ferromagnetic, and the ferromagnetic part of the exchange interactions is close to the ferromagnetic part. These data are used to plot a magnetic La_0.7Sr_0.3Mn_{1 – x}Co_xO₃ phase diagram.