Ferromagnetism in LaCoO3 -: art. no. 014402

A systematic investigation of the low-temperature magnetic properties of LaCoO3 has demonstrated a ferromagnetism with T(c)approximate to85 K from surface cobalt atoms. The experimental investigation involved comparison of the magnetic susceptibility of (1) a single crystal, (2) a powder ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 400 degreesC followed by a later anneal at 1000 degreesC. The low-temperature magnetic susceptibility was found to have three contributions: a Curie-Weiss paramagnetism, a thermally driven spin-state transition, and a surface-related ferromagnetism with T(c)approximate to85 K. The ferromagnetic component has a remanence and coercivity at 5 K that increases dramatically with increasing surface/volume ratio of the different samples. The presence of the surface ferromagnetism explains the discrepancies of the low-temperature magnetic susceptibility reported by different groups. An anion coordination at surface Co(III) ions that differs from that of the bulk cobalt is shown to be capable of stabilizing higher spin states. A T(c)approximate to85 K is argued to be too low for ferromagnetic coupling by oxidized clusters, and possible mechanisms for a ferromagnetic coupling between higher-spin Co(III) ions are discussed.