Ferromagnetic and antiferromagnetic interactions in lanthanum cobalt oxide at low temperatures

Systematic dc and ac magnetic susceptibility studies have been performed on single-phase LaCoO₃ powder samples. Evidence is presented in support of the existence of ferromagnetic (FM1, FM2) and antiferromagnetic (AFM) interactions at low temperatures. FM1 and FM2 are ferromagnetic interactions that occur at different temperature regimes and their proposed origin is different. Specifically, FM1 interactions are observed at very low temperatures (T<10K) and are attributed to superexchange interactions of the type CoIV-O-Coˢ (where $s=iii$ or 3+). FM2 interactions are observed for 20<T<100K and are attributed to coupling of localized moments on CoIV sites through RKKY interactions. The weak AFM interactions set in at T~28K are attributed to the strong electron correlations in the system. The existence of the weak AFM may constitute evidence in support of the proposal that this system is a Mott-Hubbard insulator. The fact that the onset temperature of AFM is different from that of the metal-to-insulator transition is attributed to the presence of magnetic impurities (CoIV). The existence of CoIV is based on the results of iodometric titrations, and attributed to La vacancies. A consistent qualitative explanation is provided for available magnetic and transport data.