Frustrated Magnetism and Spin Anisotropy in a Buckled Square Net YbTaO₄

The interplay between quantum effects from magnetic frustration, low-dimensionality, spin-orbit coupling, and crystal electric field in rare-earth materials leads to nontrivial ground states with unusual magnetic excitations. Here, we investigate YbTaO₄, which hosts a buckled square net of Yb³⁺ ions with J_eff = 1/2 moments. The observed Curie-Weiss temperature is about -1 K, implying an antiferromagnetic coupling between the Yb³⁺ moments. The heat capacity shows no long-range ordering down to 0.10 K, instead shows a field-dependent broad maximum indicative of short-range correlations. The magnetic entropy recovered and magnetization measurements confirm a spin-orbit driven J_eff = 1/2 Kramers doublet ground state. Point charge calculations show that the Yb³⁺ ions do not host the quintessential XY spin anisotropy observed in typical Yb-based quantum magnets like NaYbO₂, YbMgGaO₄, and pyrochlores but rather exhibit an almond-shaped anisotropy with an easy axis. Thus, YbTaO₄ can serve as a model system to study frustrated magnetism in a square lattice using the J₁-J₂ Heisenberg model. This work also emphasizes the significance of small perturbations to the local crystal electric field that can alter the spin anisotropy and change the collective behavior of the system.