Intrinsic pinning in the high-fieldCphase ofUPt3

Abstract

We report on the ac magnetic response of superconducting ${\mathrm{UPt}}_{3}$ in a dc magnetic field. At low fields $(H<{H}^{*}),$ the in-phase susceptibility shows a sharp drop at ${T}_{c}$ followed by a gradual decrease with decreasing temperature, while the out-of-phase component shows a large peak at ${T}_{c}$ followed by an unusual broad peak. As the $B\ensuremath{-}C$ phase line is crossed $(H>{H}^{*}),$ however, both the in-phase and out-of-phase susceptibilities resemble the zero-field Meissner curves. These features are only observed for relatively large ac excitation fields. We interpret these results in terms of a vortex pinning force which, while comparatively small in the $A/B$ phases, becomes large enough to effectively prevent vortex motion in the $C$ phase.