Cobalt magnetism in a superstructured metallic antiferromagnet Na0.825CoO2

Abstract

The Na atomic ordering and the local Co magnetism, in a metallic antiferromagnet made of Na${}_{0.825}$CoO${}_{2}$, are investigated by nuclear magnetic-resonance (NMR) techniques. Multiple well-resolved NMR peaks of ${}^{23}$Na and ${}^{59}$Co are observed, which suggests a superlattice structure forming in this Na-vacant cobaltate. Examining clues from published x-ray data, we refine the lattice superstructure so as to obtain the best description for the peaks seen in our NMR data. Magnetic Co ions are found to form a striped structure, which produces nontrivial magnetism. Analyzing the NMR frequency shift for each peak, we observe a frequency-shift anomaly, i.e., the breakdown of linearity between the NMR shift (that is, the local susceptibility) and the bulk susceptibility. We find that the shift anomaly can be described by an additional susceptibility component emerging at $\ensuremath{\sim}$60 K. Slow spin dynamics are observed near a metamagnetic transition where the spins flip between the antiferromagnetic and ferromagnetic orders. Unlike in ordinary spin glasses, this glassy behavior is induced by a magnetic field, which causes competition between the in-plane ferromagnetic and out-of-plane antiferromagnetic interactions, in a magnetic-frustrated triangular lattice.