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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.

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