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Magnetic properties of nanocrystalline LaCoO 3 with particle size of 25, 30, 32, and 38 nm, prepared by the citrate method, were investigated in temperature range 2-320 K, magnetic field up to 50 kOe, and under hydrostatic pressure up to 11 kbar. All nanoparticles exhibit weak ferromagnetism below T C Ϸ 85 K, in agreement with recent observation on LaCoO 3 particles and tensile thin films. It was found that with decreasing particle size, i.e., with increasing the surface to volume ratio, the unit-cell volume increases monotonically due to the surface effect. The ferromagnetic moment increases as well, simultaneously with lattice expansion, whereas T C remains nearly unchanged. On the other hand, an applied hydrostatic pressure suppresses strongly the ferromagnetic phase leading to its full disappearance at 10 kbar, while the T C does not change visibly under pressure. It appears that the ferromagnetism in LaCoO 3 nanoparticles is controlled by the unit-cell volume. This clear correlation suggests that the nature of ferromagnetic ground state of LaCoO 3 is likely related to orbitally ordered Jahn-Teller active Co 3+ ions with intermediate-spin ͑IS͒ state, which may persist in the expanded lattice at low temperatures. A robust orbital order presumed among the IS Co 3+ species can explain the very stable T C observed for LaCoO 3 samples prepared under different conditions: single crystal powders, nanoparticles, and thin films.

Magnetic properties of compacted La 0.8 Ca 0.2 MnO 3 manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures T C Ϸ 231 K and T C Ϸ 261 K, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances T C of La 0.8 Ca 0.2 MnO 3 nanoparticles at a rate dT C / dP Ϸ 1.8-1.9 K / kbar, independently on the average particle size. Pronounced irreversibility of magnetization below T irr Ϸ 208 K and strong frequency dependent ac susceptibility below T C for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled ͑ZFC͒ and field-cooled magnetization. These features indicate the appearance of spin-glasslike state, partially reminiscent the behavior of La 1−x Ca x MnO 3 crystals, doped below the percolation thresholdx Ͻ x C = 0.225. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of La 1−x Ca x MnO 3 crystals with x Ͼ x C . The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent 1.89Ϯ 0.56. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.

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