Frustrated\nMagnetism and Spin Anisotropy in a Buckled\nSquare Net YbTaO<sub>4</sub>

The interplay between\nquantum effects from magnetic frustration, low-dimensionality, spin–orbit\ncoupling, and crystal electric field in rare-earth materials leads\nto nontrivial ground states with unusual magnetic excitations. Here,\nwe investigate YbTaO<sub>4</sub>, which hosts a buckled square net\nof Yb<sup>3+</sup> ions with <i>J</i><sub>eff</sub> = 1/2\nmoments. The observed Curie–Weiss temperature is about −1\nK, implying an antiferromagnetic coupling between the Yb<sup>3+</sup> moments. The heat capacity shows no long-range ordering down to\n0.10 K, instead shows a field-dependent broad maximum indicative of\nshort-range correlations. The magnetic entropy recovered and magnetization\nmeasurements confirm a spin–orbit driven <i>J</i><sub>eff</sub> = 1/2 Kramers doublet ground state. Point charge calculations\nshow that the Yb<sup>3+</sup> ions do not host the quintessential <i>XY</i> spin anisotropy observed in typical Yb-based quantum\nmagnets like NaYbO<sub>2</sub>, YbMgGaO<sub>4</sub>, and pyrochlores\nbut rather exhibit an almond-shaped anisotropy with an easy axis.\nThus, YbTaO<sub>4</sub> can serve as a model system to study frustrated\nmagnetism in a square lattice using the <i>J</i><sub>1</sub>–<i>J</i><sub>2</sub> Heisenberg model. This work\nalso emphasizes the significance of small perturbations to the local\ncrystal electric field that can alter the spin anisotropy and change\nthe collective behavior of the system.