Magnetic toroidal order of UNi4B studied by resonant X-ray scattering

UNi$_4$B is a layered intermetallic compound with a crystal structure belonging to the space group $Cmcm$, where uranium atoms form distorted triangular lattices. Below T$_N$ ~ 20 K, a vortex-like magnetic order, called toroidal order, is realized by a partial ordering of 2/3 of the uranium sites forming a pseudo honeycomb network in the triangular lattice [1]. The toroidal order simultaneously breaks the time-reversal and spatial-inversion symmetry of the system. In such a state, an electromagnetic effect is expected to appear even in a metallic system [2], and it was demonstrated by an experiment of magnetization measurement under electric current in UNi$_4$B [3]. However, the current-induced magnetization was observed even in the direction of the current which was assumed not to induce magnetization based on the assumed orientation of the toroidal moment. It has been pointed out that the assumed toroidal structure may be distorted from the ideal 120 degree structure (Fig. 1) [3,4].
　In order to clarify the origin of the current-induced magnetization observed in UNi$_4$B, we have investigated the detailed magnetic structure of this system by resonant X-ray scattering (RXS). The experiments were performed at the Photon Factory beamline BL-11B of the High Energy Accelerator Research Organization (KEK) using synchrotron radiation X-rays with the energy of the $M_4$ absorption edge of uranium. As a result of the experiment, satellite reflections described by the propagation vector of q = (0, 2/3, 0), which was consistent with the vortex-like magnetic structure (i.e. the toroidal order), were observed at several reciprocal lattice points under the resonance condition below T$_N$. The greatly enhanced RXS signals allowed observation of weak superlattice reflections, which were revealed to have a linear temperature dependence, unlike the usual upper convex behavior of an order parameter observed for stronger reflections. Such two different types of temperature dependence are sometimes considered as an implication of an existence of a secondary order parameter besides the primary order parameter describing the magnetic order [5]. However, in the present UNi$_4$B case, they are likely to originate from a common magnetic order, since all those superlattice reflections are described by the identical propagation vector and there is no difference in the energy dependence of the RXS signal. We investigated the distribution pattern of the reflections exhibiting the upper convex temperature dependence and the linear temperature dependence. The observed pattern revealed that the two types of temperature dependence can be explained by a canted magnetic order from the ideal 120 degree structure.