D ec 2 01 5 Magnetic critical properties and basal-plane anisotropy of Sr 2 IrO

The anisotropic magnetic properties of Sr2IrO4 are investigated, using longitudinal and torque magnetometry. The critical scaling across Tc of the longitudinal magnetization is the one 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 4-fold anisotropy K4 ≈ 1 10 5 erg mole. Although larger than for the cuprates, it is found too small to account for a significant departure from the isotropic 2D XY model. The in-plane torque also allows us to put an upper bound for the anisotropy of a field-induced shift of the antiferromagnetic ordering temperature.