Anisotropic magnetoresistance in charge-ordered Na0.34(H3O)0.15CoO2 : Strong spin-charge coupling and spin ordering

Angular-dependent in-plane magnetoresistance (AMR) for single crystal Na_0.34(H₃O)_0.15CoO₂ with charge ordering is studied systematically. The anisotropic magnetoresistance shows a twofold symmetry at high temperature with rotating H in the Co-O plane, with a sixfold symmetry below a certain temperature (T_ρ) . In the meantime (below T_ρ ), the positive MR arms are developed in AMR with a rotating magnetic field (H) in the plane consisting of the current and c axis. The variation of AMR symmetry with temperature arises from the subtle changes of the spin structure. These results provide direct evidence for the itinerant electrons directly coupled to the localized spins.