Spin fluctuations and 3D itinerant magnetism in NaxCoO2: Slave‐boson mean‐field study

Abstract

AbstractThe in‐plane ferromagnetic (FM) fluctuations and three‐dimensional (3D) itinerant magnetism on the 3D triangular lattice of NaxCoO2 are studied based on a single‐band Hubbard model using the Kotliar–Ruckenstein (KR) slave‐boson mean‐field (SBMF) technique. By analyzing the instabilities of the FM and layer‐type antiferromagnetic (AFM) susceptibilities, the in‐plane FM fluctuations in NaxCoO2 with x < 0.63 are identified as the effects of both the strong Coulomb repulsion and the frustration on the Co ion triangular lattice. The saddle‐point solutions suggest that the experimental layer‐type AFM state is favorable for x > 0.75 due to the band top edge van Hove singularity arising from the next‐nearest‐neighbor (NNN) hopping. We also discuss the potential connection between the Curie–Weiss metallic behavior and the crossover from the weak FM phase to the layer‐type AFM phase at 0.63 < x < 0.75. Our results reveal the crucial role of the modified effects of the Coulomb interactions with doping x.