Magneto resistance of RuO2-based resistance thermometers below 0.3 K

We have determined the magnetoresistance of RuO2-based resistors (Scientifi c 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 positi ve at larger fields. The magnitude of the negative magneto resistance incre ases rapidly as the temperature is lowered, while that of the positive magn etoresistance has smaller temperature dependence. We have also examined the temperature dependence of the resistance below 50 mK in zero magnetic fiel d. It is described in the context of variable-range hopping (VRH) conductio n down to 15 mK. Hence, the resistors can be used as thermometers down to a t least 15 mK. (C) 2001 Elsevier Science Ltd. All rights reserved.