Crossover from antiferromagnetic to ferromagnetic ordering in TmCuSi by neutron diffraction

Abstract The previously reported ferromagnetic TmCuSi (P63/mmc) compound (TN=9 K) has been reinvestigated by neutron powder diffraction and was found to order antiferromagnetically at TN=6.5(1) K with a sine wave modulated structure q 1 =(q x ,0,0) with the Tm moments deviating from the c-axis. This phase is restricted to a narrow high-temperature range 5.1–6.5 K where the wave vector decreases linearly by only 9% (from qx=0.078 to 0.071, on cooling). However, below TC=5.8 K by a first-order transition the wave vector jumps to the value q 2 =0 giving rise to ferromagnetic ordering which dominates in the low-temperature range. The two phases coexist in temperature-dependent portions in the intermediate range 5.4±0.4 K. This transition is associated with a reorientation of the Tm moments towards the c-axis below TC. The saturation ordered moment value of 6.2(1) μB/atom at 1.5 K is reduced relatively to the Tm3+ free ion moment value of gμBJ=7 μB.