Magnetic field dependence of the (3)He to silver thermal boundary resistance.

We have measured the thermal boundary resistance between liquid (3)He and a compressed silver powder heat exchanger for 1mK5mK in magnetic fields 2mT9.4T at various sample pressures. We observe the boundary resistance initially to increase as the field is raised from zero and to saturate for fields above 0.2T. This behavior has never before been reported, and is in sharp contrast to behavior observed for thermal conduction between liquid (3)He and platinum powder. In addition, we find the boundary resistance at constant field and temperature to increase with increasing sample pressure, in contradiction to the predictions of acoustic models of the boundary conduction.