Spin-Orbit-Enhanced Magnetic Surface Second-Harmonic Generation In Sr2iro4

An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr2IrO4 and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr2IrO4, our results altogether show that the anomalous SHG signal in parent Sr2IrO4 originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.