Observation of the Meissner-Ochsenfeld Effect and the Absence of the Meissner State in UCoGe

Abstract

We present low field magnetization and susceptibility measurements made on a single crystal of the ferromagnetic superconductor UCoGe. The interplay between ferromagnetism and superconductivity comes into view in the study of hysteresis along the c axis (easy magnetization axis). The Meissner state (perfect diamagnetism) could not be observed in very low magnetic fields for all three crystallographic directions, implying that the sample is always in the mixed state. Notwithstanding, the Meissner-Ochsenfeld effect (reversible flux expulsion) occurs and is found to be anisotropic. For the c axis in low fields, it is proportional to the bulk magnetization M (and thus to the population of domains) and not to the applied magnetic field H. On a microscopic level, our interpretation of these results implies that flux is expelled independently from each domain proportional to its volume.

Phase diagram of UCoGe

“…According to the experimental results reported in [16][17][18] this field at ambient pressure is larger than the lower critical field field H c1 in UCoGe. So, the complete field screening is not realised, and the phase transition occurs directly to the superconducting mixed state.…”

Phase transitions in the domain structure of ferromagnetic superconductors

“…On experiments [7] in UCoGe the diamagnetic response was observed and this may be explained by the strong vortex pinning -the vortex configuration is frozen and only the surface Meissner current contributes to the screening.…”

“…In real FS compounds these pinning effects may be, of course, rather strong. For example, the recent magnetization measurements in U CoGe [7] show an important vortex pinning effect, overcoming the magnetic domain pinning. The interplay between these two types of pinning makes the physics of the domain structure in FS very reach.…”

MicroSQUID Force Microscopy in a Dilution Refrigerator

“…Imaging at the base temperature of 0.2 K was achieved for the study of the ferromagnetic superconductor UCoGe [25] . We showed that by controlling the shape of the very end of the MicroSQUID-tip we can reduce the MicroSQIUID-sample distance to at least 420 nm while scanning reliably, opening the path to the precise determination of the penetration depth in superconductors.…”