Abstract
An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr$_2$IrO$_4$ 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 two-fold 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 Sr$_2$IrO$_4$, our results altogether show that the anomalous SHG signal in parent Sr$_2$IrO$_4$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.
Highlights
An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr2IrO4 and attributed to a hidden odd-parity bulk magnetic state
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 show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern
Summary
The key feature of the SHG data in Sr2IrO4 is the onset of a new radiation process below the Néel temperature (TN ∼ 230 K) that lowers the rotational symmetry about the c axis from C4 to C1 This is incompatible with its reported Néel structure [1,17,18,19], in which a canting-induced net ferromagnetic moment in each layer is stacked along the c axis in a − + +− order so as to preserve C2 symmetry [Fig. 1(a)]. In the presence of a weak in-plane field (H < Hc), no change is observed in the high-temperature EQ intensity, but there is slight enhancement of the low-temperature SHG intensity along with the emergence of a peak structure just below TN [Fig. 1(d)] This is reminiscent of bulk magnetization data acquired under similar field conditions, which exhibits a ferromagnetic upturn at TN followed by a sharp drop as the − + +− state is stabilized [inset Fig. 1(d)]. This is further corroborated by the high-field (H > Hc) SHG data [Fig. 1(e)] that shows a large intensity increase relative to the low-field case and a disappearance of the peak
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