Crystal field splittings and magnetic ground state of the square-lattice antiferromagnets YbBi2ClO4 and YbBi2IO4 with Jeff = 1/2

Abstract: We report on the crystal field level splitting and magnetic ground state of the Jeff = 1/2 square lattice antiferromagnets YbBi2ClO4 and YbBi2IO4 using powder inelastic neutron scattering (INS) and neutron diffraction measurements. Both compounds exhibit a well-isolated $\Gamma_{7}$ doublet ground state under a tetragonal crystal field environment, confirming a robust Jeff = 1/2 picture with slight XY-type anisotropic character in the g-tensor. Notably, the ground state wave functions closely resemble the $\Gamma_{7}$ doublet expected in the perfect cubic limit, consistent with the nearly cubic ligand configuration of eight O2- ions surrounding Yb3+. Below TN =0.21 K, YbBi2IO4 exhibits a stripe long-range magnetic order characterized by an ordering wave vector qm = (1/2, 0, 0) or its symmetry-equivalent (0, 1/2, 0), with magnetic moments aligned along qm. The ordered moment is approximately 79 % of the classical prediction, significantly larger than expected from the isotropic J1-J2 model, suggesting the possible involvement of exchange anisotropy in explaining this observation. We show that symmetry-allowed XXZ and bond-dependent anisotropic exchange terms in a square lattice can play a critical role in stabilizing the stripe order and suppressing the moment reduction as observed. These findings establish YbBi2ClO4 and YbBi2IO4 as unique platforms for exploring rich Jeff = 1/2 magnetism from two less investigated perspectives: (i) on a square lattice and (ii) within a (nearly) cubic ligand environment.