Transport properties and magnetism of a helically Hund-coupled conductor:: β-MnO2

Transport properties and magnetic susceptibility have been investigated for single crystals of the magnetic conductor beta-MnO2. Resistivity, Hall coefficient, and thermoelectric power reveal the behavior of n-type metal with an anomalously small mobility. The transport properties are accompanied by a large anomaly around T-N(approximate to 92 K) below which a helical magnetic long-range order forms. This indicates that the e(g) conduction electrons, originating from a slight off-stiochiometry, are strongly coupled with localized magnetic moments in the t(2g) orbitals by means of Hund's rule. Below T-N, the magnetoresistance shows a large anisotropy reflecting the helical magnetic structure. Below about 50 K, the mobility gradually decreases, suggesting a carrier localization. The anisotropic measurement of magnetic susceptibility on a single crystal reveals the behavior of proper-type helical magnetism qualitatively, but shows a significant deviation from a molecular-field analysis based on a localized spin model.