We present direct imaging of magnetic flux structures in the anisotropic, spin-triplet superconductor Sr 2 RuO 4 using a scanning µSQUID microscope. Individual quantized vortices were seen at low magnetic fields. Coalescing vortices forming flux domains were revealed at intermediate fields.Based on our observations we suggest that a mechanism intrinsic to the material stabilizes the flux domains against the repulsive vortex-vortex interaction. Topological defects like domain walls can provide this, implying proof for unconventional chiral superconductivity.
A scanning probe technique is presented: Scanning μ-superconducting quantum interference device (SQUID) force microscopy (SSFM). The instrument features independent topographic and magnetic imaging. The force microscope uses a piezoelectric quartz tuning fork as the detector and magnetic imaging is obtained by scanning μ-SQUID microscopy. The μ-SQUID is placed at the edge of a silicon chip attached to the tuning fork. A topographic vertical resolution of 0.02 μm is demonstrated and magnetic flux as weak as 10−3 Φ0 is resolved with a 1 μm diameter μ-SQUID loop. The SSFM operates in a dilution refrigerator in a cryogenic vacuum. Sample and probe can be cooled to 0.45 K.
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