Paramagnetic Spin Correlations and Spin Dynamics in Doped Manganites As the Precursors of Their Magnetic Ordering

To confront the magnetic ordering in bulk and nanometer-sized La1−xCaxMnO3 manganites with the Ca doping level x=0.1 and 0.6, the fitting of the temperature dependences of electron paramagnetic resonance linewidth and intensity to the theoretical models was combined with the magnetic measurements. In the x=0.1 case, the magnetic ordering in nanosamples changes as compared to bulk due to transition from the confined state of charge carriers in chemically disordered bulk to a mobile one in an impuritylike band in homogeneous nanocrystals. While both bulk and nano-x=0.6 samples demonstrate stable antiferromagnetic ordering in spite of strong ferromagnetic correlations in the paramagnetic state. This may be associated with the localization of doped electrons, which (together with the elastic interaction between Jahn-Teller) ions may be a prerequisite for electron-hole doping asymmetry in the phase diagram of La1−xCaxMnO3.