Tunable magnetic properties and magnetocaloric effect of off-stoichiometric LaMnO₃ nanoparticles

The crystal and electronic structures and the magnetic and magnetocaloric properties of off-stoichiometric LaMnO₃ nanoparticles (NPs) with various particle sizes D = 20–100 nm were studied. The Rietveld refinement revealed that all NPs were crystallized in the rhombohedral structure, with varied structural parameters dependent on D. Magnetization (M) measurements indicated a considerable difference between zero-field-cooled and field-cooled magnetizations at temperatures below ferromagnetic-paramagnetic (FM-PM) phase transition, particularly for the samples with D = 25–40 nm. These results are ascribed to spin-glass-like behaviors and magnetic inhomogeneity. We also found the possibility of tuning the FM-PM phase transition temperature (T_C) from 77 to 262 K, which is dependent on both D and W (the e_g-electron bandwidth). Under an applied field of H = 50 kOe, the absolute maximum magnetic entropy change that achieved around T_C can be improved from 4.02 J kg⁻¹ K⁻¹ for D = 40 nm to 6.36 J⋅ kg⁻¹⋅ K⁻¹ for D = 100 nm, corresponding to the relative-cooling-power values of 241–245 J⋅ kg⁻¹. We also analyzed the data of M and magnetic entropy change based on theoretical models to further understand the magnetic property and phase-transition type of the NP samples.