Pressure-induced suppression of ferromagnetic phase in LaCoO₃ nanoparticles

Magnetic and structural properties of nanocrystalline LaCoO₃ with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below T_C ≈ 85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas T_C remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO₃ nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ∼11 kbar. Remarkably, the T_C does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO₃ nanoparticles, likely related to the intermediate-spin (IS) Co³⁺ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co-O bonds.