Scanning magnetic imaging of ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$

We magnetically imaged the $ab$-plane surface of single crystals of the unconventional superconductor ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$, including one sample with an array of microholes, using scanning superconducting quantum interference device and Hall probe microscopy in a dilution refrigerator at low applied magnetic fields. The images show dilute trapped vortices, as would be expected in conventional type-II superconductors, and no other magnetic features. We found no direct signs of the spontaneous magnetization that would be expected in a time-reversal symmetry-breaking (TRSB) superconductor. These measurements set upper limits on the presence of TRSB signatures in this material.

Figure 1

A SEM image of the ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ crystal used for the Hall probe imaging. An array of holes was milled in the sample using a FIB. The hole spacing is $20\mu \mathrm{m}$.

Figure 2

(Color online) Scanning Hall probe image at $T=100\mathrm{mK}$ of the sample shown in Fig. 1, cooled in a background field of $\sim 25\mathrm{mG}$. The background has been line normalized to remove $1∕f$ noise. (a) Image with a color scale showing the full measured magnetic field range. Isolated trapped vortices dominate the image. (b) Cross section taken along the darkened line in (a). (c) Image with an expanded color scale, showing that there are no obvious features in the noise. (d) Cross section taken along the lightened line in (c).

Figure 3

Hall probe scans at $T=100\mathrm{mK}$ of a sample field-cooled in moderate magnetic fields. The vortices appear to form lines that are spaced much more closely than the FIB-milled hole pattern. There are also regions at the top and right-hand side of the imaged area in which the density of vortices is much lower than the average in the images.