Vortex pinning and stability in the low field, superconducting phases of UPt_3

We use an array of miniature Hall probe magnetometers to probe the entry and flow of flux in a single crystal torus or the heavy fermion superconductor UPt_3. Local measurements over the hole are exquisitely sensitive to vortex motion anywhere in the torus, and they permit us to detect avalanches restricted to and with a sharp onset in the lower temperature superconducting phase. Computer simulations support a mechanism dependent upon the degenerate nature of the superconducting order parameter.