Abstract
We report the results of a search for a spontaneous magnetization in the superconducting B-phase of UPt3 which is predicted to exist because of broken time-reversal symmetry. We have measured the change of magnetic flux upon the superconducting transition in a field very close to zero by using a SQUID magnetometer. Our result gives the upper limit of 2 mG for the spontaneous internal field, one order of magnitude smaller than obtained by a recent μSR measurement. The small value favors the theory of orbital degeneracy of the pair wave function over the theory of spin degeneracy.