We report on the a.c. magnetic response of superconducting UPt_3 in a d.c.
magnetic field. At low fields (H < H^*), the in-phase susceptibility shows a
sharp drop at Tc 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-C phase line is crossed (H>H^*), however, both the
in-phase and out-of-phase susceptibilities resemble the zero-field Meissner
curves. 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.Comment: Modified discussion, slight changes to figures, accepted in PRB Rapid
Communications. RevTex file, 5 figure