Magnetic impurities in glass and silver powder at milli- and microkelvin temperatures

In search of an ideal paramagnet for thermometry at very? low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fr3+, impurity concentration x = (180 +/- 10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 less than or equal to T less than or equal to 300 K by means of dynamic ac susceptibility, and at 1.6 less than or equal to T less than or equal to 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverised and mixed with silver powder of submicron grain size; the composite was subsequently compacted to a cylindrical sample by applying a pressure of a few kbar. This contact method which is applicable to other materials with bad thermal conductivity as well improved the accessible minimum temperature for the glass down to similar or equal to 0.6 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility chi of the magnetic Fe3+ impurities in the glass down to approximate to 0.6 mK, a broad maximum of chi at T similar or equal to 0.38 mK and a decrease towards even lower temperatures. Compared to the frequently used, highly diluted metallic spin glasses, the magnetic behaviour of the glass makes inductance thermometry applicable in a much larger temperature range. In addition ,we have investigated the low temperature magnetic purposes of a sample of compacted silver powder. The small amount of paramagnetic impurities in the AG particles (x = (4 +/- 1) ppm) exhibits a low temperature susceptibility which can be described by the Kondo effect with an unexpectedly small Kondo temperature of T-K similar or equal to 1 mK.