Magnetism and phase transitions in LaCoO₃

Neutron scattering and magnetometry measurements have been used to study phase transitions in LaCoO₃ (LCO). For H ≤ 100 Oe, evidence for a ferromagnetic (FM) transition is observed at T_c ≈ 87 K. For 1 kOe ≤ H ≤ 60 kOe, no transition is apparent. For all H, Curie-Weiss analysis shows predominantly antiferromagnetic (AFM) interactions for T > T _c, but the lack of long-range AFM order indicates magnetic frustration. We argue that the weak ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and nanoparticles. The lattice strain is present at the bulk surfaces and at the interfaces between the LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly affected by the Co-O-Co angle (γ), in agreement with recent band calculations which predict that ferromagnetic long-range order can only take place above a critical value, γ_C. Consistent with recent thin film estimations, we find γ_C = 162.8°. For γ > γ_C, we observe power-law behavior in the structural parameters. γ decreases with T until the critical temperature, T_o ≈ 37 K; below T_o the rate of change becomes very small. For T < T _o, FM order appears to be confined to regions close to the surfaces, likely due to the lattice strain keeping the local Co-O-Co angle above γ_C.