Description
We will present scanning superconducting quantum
interference device (SQUID) microscopy [1] and its
contribution to the study of the vortex phases in conventional
an unconventional superconductors (Sr2RuO4 and UCoGe).
For the case of UCoGe we will show 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 microSQUID
microscopy. Our measurements show that the spontaneous
ferromagnetic transition at TC =2.5K [2,3] is characterized by
Ising- like magnetization along the easy axis (c direction),
with domain sizes of the order of 10 ?m and 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. In the superconducting state,
which coexists with ferromagnetism below Tsc =0.67K, 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.
References
[1] JLTP 175, 861, (2014)
[2] Phys. Rev. Lett. 109, 237001 (2012)
[3] Phys.Rev. B 90, 184501 (2014)
