Magnetoresistance of RuO_2-based resistance thermometers below 0.3 K

摘要

We have determined the magnetoresistance of RuO_2-based resistors (Scientific Instruments (SI) RO-600) between 0.05 and 0.3 K in magnetic fields up to 8 T. The magnetoresistance is negative around 0.5 T and then becomes positive at larger fields. The magnitude of the negative magnetoresistance increases rapidly as the temperature is lowered, while that of the positive magneto-resistance has smaller temperature dependence. We have also examined the temperature dependence of the resistance below 50 mK in zero magnetic field. It is described in the context of variable-range hopping (VRH) conduction down to 15 mK. Hence, the resistors can be used as thermometers down to at least 15 mK.