Nanometer size effect on structural and magnetic properties of La _0.2Ca1/5/2013MnO₃

Structural and magnetic properties of La_0.2Ca _0.8MnO₃ nanoparticles with average size of 15-37 nm, prepared by the glycine-nitrate method, have been studied. Synchrotron experiments performed in the temperature range of 80-300 K have shown that a structural transition from room temperature orthorhombic Pnma to monoclinic P2₁/m space group, associated with orbital ordering, occurs below 200 K in the studied particles. This transition observed for the largest particles of 37 nm is very close to that of bulk, as seen by similar temperature variation of lattice parameters and orthorhombic strain. The transition is highly suppressed for smaller 15 nm particles. Horizontal and vertical shifts of magnetic hysteresis loops (M_Shift and H_EB), displayed in a field cooled process, indicate size dependent exchange bias effect. It is also shown that M_Shift and H_EB, as well as the remanent magnetization M_r and coercive field H_C at low temperatures, exhibit a non-monotonic size dependence for particles around 23 nm. These effects may be attributed to the changes in uncompensated spins at the surface, anisotropy or alternatively to a transition from a multi-domain to the single domain state.