Na0.75CoO2—an itinerant magnet?

• Published in: Physica. C, Superconductivity Vol. 460-462; no. 1; pp. 495 - 496
• Main Authors: Eschrig, Helmut, Koepernik, Klaus
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 01-09-2007; Elsevier Science
• Subjects: Cobaltates; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Fermi surface; Magnetic properties and materials; Magnetism; Nonmetallic ferromagnetic materials; Physics; Studies of specific magnetic materials; Cobaltates; 71.15.Mb; 71.20.Be; 75.50.Ee; Magnetism; Fermi surface; 71.18.+y; Photoelectron spectra; Virtual crystal approximation; Sodium oxides; 75.50.Ee Cobaltates; Spin correlations; Stacking sequence; Curie-Weiss law; Ferromagnetic materials; Cobalt oxides; Intercalation compounds; Neutron diffusion; Magnetic susceptibility; Itinerant model; Superconductors; Thermoelectric power; Density functional method; Local spin density approximation; Ferromagnetism
• ISSN: 0921-4534; 1873-2143
• DOI: 10.1016/j.physc.2007.03.078

NaxCoO2 for x≈0.75 is a rather unusual metal. It is an …ABAB… stack of CoO2 sheets intercalated with Na atoms. It has a large Curie–Weiss magnetic susceptibility. Neutron scattering sees spin correlations which are ferromagnetic in plane and antiferromagnetic in stack direction. Below about 22K it orders in this way with an unusual small moment of about 0.2μBohr per Co atom. Its Fermi surface measured by photoemission is a cylinder around the stack axis. Density functional calculations in local spin density approximation are presented which treat the partial Na occupation in a well justified virtual crystal approximation. They provide all those findings and yield an understanding as itinerant magnetism. In addition, a ferromagnetic order also in stack direction, which is a very clear half-metall, is nearly energetically degenerate with the antiferromagnetic ground state, with dramatic Fermi surface reconstruction. This could yield a clue to the anomalous thermopower and its strong field dependence.