Transition to electron doping in La_1-xCa_xMnO ₃ manganite system: Size-induced effects on magnetic order, probed by electron resonance technique

The X-band electron magnetic resonance was measured on nano-sized half-doped La_0.5Ca_0.5MnO₃ and electron-doped La_0.4Ca_0.6MnO₃ manganite compounds at 5-600 K temperature range. The electron paramagnetic resonance parameters were fitted with known theoretical models. The low temperature ferromagnetic resonance data were analyzed together with the results of neutron diffraction and magnetic measurements. The size effects in half-doped and electron-doped nano-powders are strongly different. A suppression of bulk-like antiferromagnetic/charge ordered component and stabilization of frustrated ferromagnetic-like (FM) order were found for Ca0.5 series, in which the coreshell interaction and a spin disorder at the surfaces weaken upon increase in grain size. However, well pronounced FM spin correlations in the paramagnetic state of Ca0.6 nano-crystals have not induced the FM long-range ordering. The local nature of double exchange coupling and the resulting localization of carriers in electron-doped compounds seem to be responsible for this effect.