Nanometer sized effects on magnetic ordering in La-Ca manganites, probed by magnetic resonance

The X-band electron paramagnetic (EPR) and ferromagnetic resonance (FMR) measurements in the temperature range 5 K-600 K were used to explore the nanometer size effects on magnetic ordering in hole-and electron-doped compounds, belonging to the prototypical system of doped manganites-La _1-xCa_xMnO₃ (x = 0.1, 0.3, 0.5 and 0.6). To this end, the modelflttings of the temperature dependences of doubly integrated intensity and linewidth of EPR signal, as well as comparative analysis of both FMR data and the known results of neutron diffraction and magnetic measurements were performed. It appears that strongestfinite-size effects are observed for lowhole-doped x = 0.1 nano-crystals due to suppression of chemical/structural disorder characteristic for bulk. The x = 0.3 and 0.5 series demonstrate well pronounced effect of core/shell spin con-.guration on paramagnetic spin correlations and spin dynamics, as well as on low temperature magnetic ordering. The electron-doped x = 0.6 nano-crystals differ strongly from all other considered nano-samples-the antiferromagnetic charge ordered ground state was found to be stable in this case due to, supposedly, the local nature of double exchange coupling and resulting localization of carriers.