Magnetic critical properties and basal-plane anisotropy of Sr₂IrO₄

The anisotropic magnetic properties of Sr₂IrO₄ are investigated, using longitudinal and torque magnetometry. The critical scaling across $T_{\text{c}}$ of the longitudinal magnetization is that expected for the 2D XY universality class. Modeling the torque for a magnetic field in the basal plane, and taking into account all in-plane and out-of-plane magnetic couplings, we derive the effective fourfold anisotropy $K_4\approx 1\times {10}^5$ erg mol⁻¹. Although larger than for the cuprates, it is found to be too small to account for a significant departure from the isotropic 2D XY model. The in-plane torque also allows us to set an upper bound for the anisotropy of a field-induced shift of the antiferromagnetic ordering temperature. (paper)