Unconventional strong pinning in multiphase superconductors

Upt(3) and thoriated UBe13 are the only known examples of superconductors t hat show more thin one superconducting phase below T-c. In both materials, a second jump in the specific heat is clearly observed at a temperature T-c 2 < T-c. By means of magnetic relaxation measurements in UPt3 and thoriated UBe13. we found vortex creep transitions of about three orders of magnitud e at T-c2, From a regime of thermally activated creep to a low temperature regime of no creep in a time scale of 10(5) s. This lack of creep points to new type of extremely strong pinning structures and new type of vortices i n the low temperature superconducting phases of these unconventional superc onductors. Recently. we detected similar vortex transition in Sr5RuO4 at T approximate to 30 mK. (C) 2000 Elsevier Science B.V. All rights reserved.