Neutron-diffraction studies of two-dimensional magnetic ordering in TbRu₂Si₂

We have studied the magnetic ordering of TbRU2Si2 by neutron diffraction. Below T-N, the order is confirmed to be one-dimensional, with a propagation vector Q = (3/13 0 0). At T-t similar to 5 K and T-t' similar to 4 K there are additional phase transitions giving rise to a number of magnetic satellites, not only on high-symmetry lines, but also on low-symmetry lines in the a*-b* reciprocal lattice plane. The satellites on highsymmetry lines are described by the fundamental modulation vector Q = (3/13 0 0) and its odd harmonics, while the satellites on low-symmetry lines are given by Q = (mtau ntau 0), where m and n are odd. The diffraction patterns show that, below Tt, the magnetic structures are never one-dimensional as previously reported, but the order is consistent with a two dimensional model having a 13a x 13a x c magnetic unit cell, containing two frustrated Tb sites, one at the corner and the other at the body-centered position. Below T-t', the order is more complex.