Resonant X-ray scattering study of the nonmagnetic order of 5f electrons in a pseudo-Kagome layered system URhSn

擬カゴメ層状化合物URhSnにおける非磁性5f電子秩序の共鳴X線散乱による研究

田端 千紘; 今 布咲子*; 網塚 浩*; 清水 悠晴*; 青木 大*; 中尾 裕則*

Tabata, Chihiro; Kon, Fusako*; Amitsuka, Hiroshi*; Shimizu, Yusei*; Aoki, Dai*; Nakao, Hironori*

Magnetic materials with no inversion center at the magnetic site have attracted much attention from the viewpoints of chiral magnetic orders and electromagnetic effects in recent years. URhSn has a crystal structure without spatial inversion symmetry (hexagonal, space group P-62m) consisting of distorted Kagome layers, and according to previous reports on polycrystalline samples, it undergoes a two-step phase transition at To = 54 K and TC = 16 K. While the low-temperature phase is considered a ferromagnetic phase with spontaneous magnetization along the c-axis, the order parameter of the high-temperature phase has remained unknown for decades. No symmetry breaking has been detected for this mysterious phase by microscopic measurements such as powder neutron diffraction and Mossbauer spectroscopy. We recently succeeded in growing high-quality single crystals of this compound and clarified its detailed bulk properties (specific heat, magnetization, electrical resistivity, and thermal expansion). Interestingly, the high-temperature phase below To isstabilized by a magnetic field parallel to the c-axis, despite the absence of a c-axis spontaneous magnetization component. This peculiar behavior indicates neither a simple ferromagnetic nor antiferromagnetic scenario. We investigated the order parameter of this unknown phase using a resonant X-ray scattering (RXS) technique. On the resonance condition at the M4 absorption edge of U, we observed a slight increase in the intensity of a fundamental Bragg reflection below To, suggesting an order parameter with q = 0 that does not break the translational symmetry of the crystal. The observed RXS signal can be interpreted by either case of order parameters of magnetic dipoles and electric quadrupoles. Given the Sn-Mossbauer and Sn-NMR reports of no internal magnetic field associated with the order, an electric quadrupolar order is more plausible.