Size-dependent spin state and ferromagnetism in La_0.8 Ca _0.2 CoO₃ nanoparticles

Magnetic and structural properties of nanocrystalline low-doped La _0.8 Ca_0.2 CoO₃ cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5-320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in La_0.8 Ca_0.2 CoO₃ nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, T_C, when particle's size decreases from 23 to 8 nm. The effective magnetic moment μ eff was found also to decrease distinctly due to the strong interdependence between Co-O-Co interactions and Co spin state. The size-induced magnetic disorder drives the La_0.8 Ca_0.2 CoO₃ nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida-Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses TC, M_S, and coercive field H _C, like it is observed for bulk La_0.8 Ca_0.2 CoO₃. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while H_C diminishes with pressure much slower then in bulk material.