Size-dependent spin state and ferromagnetism in La0.8Ca0.2CoO3 nanoparticles Academic Article uri icon

abstract

  • Magnetic and structural properties of nanocrystalline low-doped La0.8Ca0.2CoO3 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 La0.8Ca0.2CoO3 nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, TC, when particle’s size decreases from 23 to 8 nm. The effective magnetic moment μeff 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 La0.8Ca0.2CoO3 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, MS, and coercive field HC, like it is observed for bulk La0.8Ca0.2CoO3. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while HC diminishes with pressure much slower then in bulk material.

publication date

  • January 1, 2010