Surface spin-glass, large surface anisotropy, and depression of magnetocaloric effect in La(0.8)Ca(0.2)MnO(3) nanoparticles.

The surface magnetic behavior of La(0.8)Ca(0.2)MnO(3) nanoparticles was investigated. We observed irreversibility in high magnetic field. The surface spin-glass behavior as well as the high-field irreversibility is suppressed by increasing particle size while the freezing temperature T(F) does not change with particle size. The enhanced coercivity has been observed in the particles and we attributed it to the large surface anisotropy. We have disclosed a clear relationship between the particle size, the thickness of the shell, and the saturation magnetization of the particles. The large reduction of the saturation magnetization of the samples is found to be induced by the increase of nonmagnetic surface large since the thickness of the spin-disordered surface layer increases with a decrease in the particle size. Due to the reduction of the magnetization, the magnetocaloric effect (MCE) has been reduced by the decreased particle size since the nonmagnetic surface contributes little to the MCE. Based on the core-shell structure, large relative cooling powers RCP(s) of 180 J/kg and 471 J/kg were predicted for a field change of 2.0 T and 4.5 T, respectively, in the small particles with thin spin-glass layer.