Magnetic fields above the superconducting ferromagnet UCoGe

We study how coexisting states of superconductivity and magnetism interact with each other by local measurements of the magnetic fields, with micron scale spatial resolution, above a single crystal of the ferromagnetic superconductor UCoGe using scanning superconducting quantum interference device microscopy. Our measurements show that the spontaneous ferromagnetic transition at TC=2.5 K is characterized by Ising-like magnetization along the easy axis (c direction), with domain sizes of the order of 10μm, magnetization amplitudes of 45 G, and are consistent with estimates of domain-wall widths of several angstroms. The measured magnetization amplitudes are in agreement with bulk magnetization measurements, implying that domain reconstruction at the sample surface is negligible. In the superconducting state, which coexists with ferromagnetism below TSC=0.67 K, both diamagnetic screening and Meissner expulsion of flux, but no shrinkage of the ferromagnetic domains, are detected. Although we could not resolve individual vortices, our measurements provide evidence for the existence of the spontaneous vortex state in UCoGe.