Magnetoresistance of RuO2-based resistance thermometers below 0.3 K Original Research Article

We have determined the magnetoresistance of RuO2-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 magnetoresistance 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.