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口頭

Magnetic order in honeycomb layered U$$_2$$Pt$$_6$$Ga$$_{15}$$ studied by resonant X-ray scattering

田端 千紘; 今 布咲子*; 村田 怜也*; 網塚 浩*; 中尾 裕則*; 松本 裕司*; 金子 耕士; 芳賀 芳範

no journal, , 

U$$_2$$Pt$$_6$$Ga$$_{15}$$ is a recently discovered compound in a material search in the U-T-X (T: transition metal, X: Al, Ga) ternary system. It crystallizes in a hexagonal structure (space group P6$$_3$$/mmc), where U-Ga layers stacked along the c-axis with c/2 spacing, and between them, Pt and Ga form nested triangular lattice layers. A detailed single-crystal structural analysis shows, however, that the hexagonal structure is an average structure, and that the actual one has a kind of crystallographic disorder: 1/3 of U sites on the triangular lattice are not occupied, resulting in a honeycomb layer formed by the rest occupied U sites, with a larger orthorhombic unit cell. From a microscopic viewpoint, occupied U sites do not have inversion center, which makes this material of interest as a new candidate material for the emergence of unconventional cross-correlation phenomena. From measurements of bulk physical properties such as magnetization and specific heat, it was revealed that U$$_2$$Pt$$_6$$Ga$$_{15}$$ undergoes an antiferromagnetic phase transition at a transition temperature of $$T_N$$ $$sim$$ 26 K. To clarify a microscopic picture of this magnetic ordering, we have performed resonant X-ray scattering (RXS) experiments to observe resonant magnetic reflections. Using 3.72 keV synchrotron X-rays, we observed diffractions of resonant scatterings at the M4 absorption edge of U at BL-11B Photon Factory of KEK. The reflections with the q = (1/6, 1/6, 0) propagation vector were observed below TN at the resonant energy of the M4 absorption edge. The polarization dependence of the RXS signal strongly suggests that the ordered magnetic moments of U are parallel to the hexagonal caxis, which is consistent with the strong magnetic anisotropy observed in magnetization measurements.

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

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

no journal, , 

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.

口頭

ハニカム層状化合物U$$_{2}$$Pt$$_{6}$$Ga$$_{15}$$の磁気秩序

田端 千紘; 金子 耕士; 芳賀 芳範; 今 布咲子*; 日比野 瑠央*; 太田 玖吾*; 松本 裕司*; 網塚 浩*; 中尾 裕則*

no journal, , 

ハニカム層状構造化合物U$$_{2}$$Pt$$_{6}$$Ga$$_{15}$$は、Uイオンが形成する擬六方対称のハニカム層が、乱れを伴って積層した結晶構造をとる。単位格子は面内方向に変位した2枚のハニカム層を含み、この変位によって六回回転対称が破られるため、結晶構造は直方晶で記述される。しかしながら、120$$^{circ}$$ずつ回転した3ドメインが積層乱れを与える形で混在するために、バルクの単結晶は見かけ上の六回回転対称を有している。積層乱れに加え、2枚のハニカム層間の距離が比較的大きいことから、2次元的なハニカム磁性体の観点から興味が持たれる本系について、その低温磁気秩序を共鳴X線散乱と中性子回折によって調べた。実験の結果、六方晶表記においてq=(1/6, 1/6, 0)の伝搬ベクトルで記述される磁気反射をTN=26K以下で観測した。この磁気秩序は、積層方向の相関距離が短い異方的な磁気相関長で特徴付けられ、層間の磁気相関が比較的弱いことが示された。さらに、中性子回折強度の半定量的解析と群論的考察から磁気構造を決定し、ハニカム面内の交換相互作用のフラストレーションが主要な役割を担っていることを明らかにした。

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