Itinerant ferromagnetism in the layered crystals LaCoOX (X=P,As)

The electronic and magnetic properties of cobalt-based layered oxypnictides, LaCoOX(X=P,As), are investigated. LaCoOP and LaCoOAs show metallic type conduction with room-temperature resistivities of ∼2×10⁻⁴ Ω cm, and the Fermi edge is observed by hard x-ray photoelectron spectroscopy. Ferromagnetic transitions occur at 43 K for LaCoOP and 66 K for LaCoOAs with spontaneous magnetic moments of 0.33μ_B and 0.39μ_B extrapolated to 0 K, respectively. Above the transition temperatures, the magnetic susceptibility exhibits a large temperature dependence. Provided that this temperature dependence follows the Curie-Weiss law, enhanced magnetic-moment values of ∼2.9μ_B for LaCoOP and ∼1.3μ_B for LaCoOAs are obtained. X-ray magnetic circular dichroism (XMCD) is observed at the Co L_2,3 edge, but not at the other edges. The calculated electronic structure shows a spin-polarized ground state with a magnetic moment of 0.52μ_B, ∼95% of which is localized on the Co ions. These results indicate that LaCoOX are itinerant ferromagnets and suggest that their magnetic properties are governed by spin fluctuation.