Magnetic field induced spin-flop transition in Na${}_x$CoO${}_2$ (0.5$<$x$<$0.55)

Abstract

The isothermal magnetoresistance (MR) with magnetic field (H) parallel to and perpendicular to ab plane is systematically studied on the single crystal Na${}_{0.52}$CoO${}_2$ with charge ordering at $\sim 50$ K and an in-plane ferromagnetism below 25 K. The isothermal MR behavior with H $\parallel$ ab plane and H $\perp$ ab plane is quite different. When H $\parallel$ ab plane, the MR is always negative and the in-plane ferromagnetic behavior is enhanced. While the MR with H $\perp$ ab plane changes from negative to positive with decreasing temperature or increasing H, and the in-plane ferromagnetic behavior is suppressed. A striking feature is that the MR with H $\perp$ ab plane shows a hysteresis behavior below 25 K, which is absent for the case of H $\parallel$ ab plane. These results provide strong evidence for a spin-flop transition of small moments of Co${}^{3.5-\delta }$ sites induced by H $\perp$ ab plane, leading to a metamagnetic transition for small moments of Co${}^{3.5-\delta }$ sites. These complex magnetism suggests an unconventional superconductivity in Na${}_x$CoO${}_2$ system because the Na${}_x$CoO${}_2$ around x=0.5 is considered to be the parent compound of superconductivity.

arXiv comment6 pages, 6 figures

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