The nucleation of the first-order phase transition of superfluid ${}^3\mathrm{He}-B$ from superfluid ${}^3\mathrm{He}-A$ is quite remarkable since it requires a seed of the order of a micron. We have studied this nucleation for ${}^3\mathrm{He}$ confined to a very dilute silica aerogel. This dirty superfluid behaves in a manner similar to previous reports for the pure superfluid. But we have discovered a novel magnetically driven nucleation switch acting on the pure superfluid- $B$ phase. Last, we find the surprising result that the proximity effect between the pure and dirty superfluids at their interface is insufficient to nucleate the $B$ phase in either superfluid.

• Received 9 October 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.045505