Magnetic Observations of the Superfluid-Superfluid Phase Transition in Liquid Helium Three

We report observations of the first-order phase transition between the two superfluid phases of ^3He. We supercool a long cylindrical sample of the higher-temperature A phase in a magnetic field in a simple dc magnetometer. We then introduce B phase at one end of the sample and observe its growth into the supercooled A phase via the magnetometer signal. For temperatures above a certain value, rm T_{nom }, the phase interface travels up the tube at a fairly constant velocity, in reasonable agreement with the theory of Leggett and Yip. For temperatures below rm T_{nom} there arises a new interaction between the rapidly moving phase interface and the magnetization of the sample. Our observations allow some characterization of this new interaction.