Magnetic structure and magnetoelastic coupling of GdNiSi3 and TbNiSi3

The series of intermetallic compounds RNiSi3 (R=rare earth) shows interesting magnetic properties evolving with R and metamagnetic transitions under applied magnetic field for some of the compounds. The microscopic magnetic structures must be determined to rationalize such rich behavior. Here, resonant x-ray magnetic diffraction experiments are performed on single crystals of GdNiSi3 and TbNiSi3 at zero field. The primitive magnetic unit cell matches the chemical cell below the Néel temperatures TN=22.2 and 33.2 K, respectively. The magnetic structure is determined to be the same for both compounds (magnetic space group Cmmm′). It features ferromagnetic ac planes that are stacked in an antiferromagnetic +−+− pattern, with the rare-earth magnetic moments pointing along the a⃗ direction, which contrasts with the +−−+ stacking and moment direction along the b⃗ axis previously reported for YbNiSi3. This indicates a sign reversal of the coupling constant between second-neighbor R planes as R is varied from Gd and Tb to Yb. The long b lattice parameter of GdNiSi3 and TbNiSi3 shows a magnetoelastic expansion upon cooling below TN, pointing to the conclusion that the +−+− stacking is stabilized under lattice expansion. A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of R sets the stage for the magnetic ground state instability observed along this series.