Magnetic properties of nanocrystalline ${\text{LaCoO}}_3$ 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 $T_C\approx 85\text{K}$, in agreement with recent observation on ${\text{LaCoO}}_3$ 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 ${\text{LaCoO}}_3$ nanoparticles is controlled by the unit-cell volume. This clear correlation suggests that the nature of ferromagnetic ground state of ${\text{LaCoO}}_3$ is likely related to orbitally ordered Jahn–Teller active ${\text{Co}}^{3+}$ ions with intermediate-spin (IS) state, which may persist in the expanded lattice at low temperatures. A robust orbital order presumed among the IS ${\text{Co}}^{3+}$ species can explain the very stable $T_C$ observed for ${\text{LaCoO}}_3$ samples prepared under different conditions: single crystal powders, nanoparticles, and thin films.

• Received 2 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.224421