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 N\'eel 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 {\it ac} planes that are stacked in an antiferromagnetic +−+− pattern, with the rare-earth magnetic moments pointing along the a direction, which contrasts with the +−−+ stacking and moment direction along the b 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 {\it 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.
@article{arxiv.1907.09830,
title = {Magnetic structure and magnetoelastic coupling of GdNiSi3 and TbNiSi3},
author = {R. Tartaglia and F. R. Arantes and C. W. Galdino and D. Rigitano and U. F. Kaneko and M. A. Avila and E. Granado},
journal= {arXiv preprint arXiv:1907.09830},
year = {2019}
}