Surface Observation and Magnetization Measurements of Grafoil Substrate

The important roles of impurity grains in graphite substrate in heat transfer mechanisms between 3He films and graphite substrate have been suggested. At sub-milli-Kelvin temperature ranges in particular, magnetic impurity grains of large diameter, about several micrometers are believed to play an important role in the heat transfer mechanism. However, the existence of such large grains is still controversial. Freshly cleaved surfaces of Grafoil are observed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses. Although results are preliminary, impurity grains with diameters of 0.5∼10 μm containing Al, Ca, Si, Fe, etc., are observed. Ferromagnetism is also observed after the subtraction of the diamagnetic contribution in magnetization measurements of Grafoil. The saturation magnetization coincides with the amount of magnetic impurities determined by ash analysis.