Correlation between Magnetic Properties and Mn\Co Atomic Order in LaMn0.5Co0.5O3+. 2. Magnetic and Calorimetric Properties

Correlation between magnetic properties and Mn\Co atomic order in LaMn0.5Co0.5O3+(theta) was investigated by powder X-ray diffraction, dc magnetization, ac magnetic susceptibility, and heat capacity measurements. The Mn\Co atomic order was controlled by annealing at some temperatures and successive quenching. The qualitative and quantitative magnetic properties depend on whether the annealing temperature is above or below 1400 K. The samples annealed below 1400 K showed a clear heat capacity anomaly and deviation from Curie-Weiss law due to ferromagnetic critical fluctuation. On the other hand, the samples quenched from above 1400 K showed an unclear heat capacity anomaly, downward deviation from Curie-Weiss law, and asymmetric M-H loop after highfield cooling. These results suggest that the longrange ordering of the Mn\Co atomic configuration occurs at around 1400 K, that the long-range ferromagnetic ordering occurs only when the long-range order of Mn\Co atomic configuration is present, and that ferromagnetic clusters form in a paramagnetic matrix when the long-range order of Mn\Co atomic configuration is absent. The frequency dependence of ac magnetic susceptibility, retardation to ac field, and coercive force showed a maximum when the sample was annealed at around 1400 K, which were discussed in relation to the fluctuation of Mn\Co atomic order