Incommensurate Magnetic Order in Sm_3BWO_9 with Distorted Kagome Lattice

We investigate the magnetic ground state of Sm_3BWO_9 with a distorted kagome lattice. A magnetic phase transition is identified at T_\rm N=0.75 K from the temperature dependence of specific heat. From ^11B nuclear magnetic resonance measurements, an incommensurate magnetic order is shown by the double-horn type spectra under a c-axis magnetic field, and absence of line splitting is observed for field oriented within the ab-plane, indicating the incommensurate modulation of the internal field strictly along c-axis. From the spin dynamics, the critical slowing-down behavior is observed in the temperature dependence of 1/T_1 with \mu_0H\perpc-axis, which is completely absent in the case with \mu_0H||c-axis. Based on the local symmetry of ^11B sites, we analyze the hyperfine coupling tensors and propose two constraints on the possible magnetic structure. The single ion anisotropy should play an important role in determination of contrasting ground states of Sm_3BWO_9 and Pr_3BWO_9.