Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

Abstract

In search of an ideal paramagnet for thermometry at very low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fe³⁺, impurity concentration x = (180±10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 ≤ T ≤ 300 mK by means of dynamic ac susceptibility, and at 1.6≤T ≤ 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverized 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 ≃0.1 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility χ of the magnetic Fe³⁺ impurities in the glass down to ≈0.6 mK, a broad maximum of χ at T ≃ 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 properties 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 ≃ 1 mK.