Complex Magnetic Structures of a Shastry-Sutherland Lattice TmB4 Studied by Powder Neutron Diffraction Analysis

Powder neutron diffraction analysis was performed on (TmB1)-B-11, which undergoes successive transitions at T-N1 = 11.7K, T* = 11 K, aid T-N2 = 10K. An incommensurate structure with two propagation vectors, k(2) [1 +/- k' +/- k" +/- k", 0] and k(2)' = [1 +/- k' +/- 3k", +/- k", 0] (k' = 0.1226 and k" similar or equal to 0.012). is suggested for phase II (T-3 <= T <= T-N1). Phase III (T-N2 <= T <= T-(sic)) is described by one propagation vector, k(3) = [1 +/- k',0,0]. The propagation vector for phase IV (T <= T-N2) further changes to a commensurate one characterized by k(4) = [100]. Four Tm moments in the c-plane order in a (+-+-) type, where + and - mean the directions of the moments parallel and antiparallel to the c-direction, respectively. The diffuse scattering intensity around the (100) magnetic reflection remains above T-N2 up to 20 K.