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In this paper, we have studied the effect of size reduction on charge ordering (CO) and magnetic phase separation in La 0.5 Ca 0.5 MnO 3 . The magnetic ground state at low temperature shifts from ferromagnetic (nanoparticles) to CO-antiferromagnetic (bulk) with increasing annealing temperature. The studied samples exhibit relevant phenomena such as Griffiths phase and training effect. Magnetic field cycling can stabilize ferromagnetic or antiferromagnetic domains, depending on the temperature value as well as the sample's nature. For the compound sintered at 1200 °C, step-like metamagnetic transitions of martensitic type were observed at 4 K, indicating the presence of weak spin-phonon coupling. The disappearance of magnetic phase separation at 4 K (the sample is purely antiferromagnetic) can be a possible origin for the occurrence of such transitions. Lowering the sintering temperature leads to the disappearance of martensitic transitions. The obtained results indicate that the occurrence of step-like metamagnetic transition of martensitic type necessarily requires the onset of a long-range CO-antiferromagnetic state.

Magnetotransport properties of La0.5-xHoxCa0.5MnO3 (x=0.05 and 0.1) polycrystalline samples were investigated in order to study the effect of magnetic Ho 3+ ions on the stability of the charge ordering (CO) and magnetic phase coexistence in pristine La0.5Ca0.5MnO3. Our samples were synthesized by using sol-gel method. Temperature dependence of resistivity shows an insulating behavior with high resistivity values, confirming the presence of long-range CO. The application of 7 T magnetic field slightly reduces resistivity values and demonstrates relatively small magnetoresistance (MR) values (-MR(T) did not exceed 14 % under 7 T applied field). This observation indicates that the CO in the Ho-based specimens is strong, which indicates that Ho-substitution for La in La0.5Ca0.5MnO3 gives more strength to the CO.

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