The Origin of the Magnetic Channel of the Thermal Boundary Resistance Between Liquid 3He and Ag Sinters at Very Low Temperatures

At temperatures below about 10mK, the temperature dependence of the thermal boundary resistance between liquid3He and metal sinters deviates significantly from the behaviour expected from the acoustic mismatch theory (RK∝ T−3). This behaviour is explained in terms of the existence of a magnetic channel for the coupling between the3He quasiparticles and the sinter; the origin of this coupling, however, is unknown so far. In our studies of the magnetic properties of Ag sinters made of powders of submicron grain size we have detected and identified a magnetically ordering subsystem which, although it contributes only a few ppms to the total mass, dominates the magnetic properties of the sinters. We present the results of our investigation of the magnetic properties of these Ag sinters which strongly deviate from the behaviour of bulk silver, and discuss a possible mechanism how a magnetically ordered subsystem may open up the magnetic channel of the thermal boundary resistance to liquid3He at very low temperatures. The main conclusion of our study is that our results should allow the tuning of the thermal boundary resistance between liquid3He and sintered metal heat exchangers at very low temperatures.