Magnetic order in honeycomb layered UPtGa studied by resonant X-ray scattering

Tabata, Chihiro; Kon, Fusako*; Murata, Ryoya*; Amitsuka, Hiroshi*; Nakao, Hironori*; Matsumoto, Yuji*; Kaneko, Koji   ; Haga, Yoshinori   

UPtGa 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/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 UPtGa undergoes an antiferromagnetic phase transition at a transition temperature of 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.