Upper critical field under hydrostatic pressure in UCoGe

We report on the pressure dependence of the upper superconductivity critical field H_c2 of the ferromagnetic superconductor UCoGe for field along the c axis, which is the easy magnetization axis. The surprising result is the upward curvature and strong enhancement of the upper critical field on cooling. This enhancement is even more pronounced above the critical pressure p_c in the paramagnetic phase than in the ferromagnetic phase below p_c. This effect cannot be explained by models based on the suppression of pair building magnetic fluctuations under magnetic field. That points out either decrease of the pair breaking mechanism under field or fluctuations favorable for the Cooper pairing linked to the wing structure of ferromagnetism near the critical pressure, where ferromagnetism is suppressed.