Magnetic critical properties and basal-plane anisotropy of Sr$_2$IrO$_4$

The anisotropic magnetic properties of Sr2IrO4 are investigated, using longitudinal and torque magnetometry. The critical scaling across T-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 approximate to 1x10(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.