Thick film thermometers with predictable R- T characteristics and very low magnetoresistance below 1 K

A new type of RuO 2-based thick film resistor has been investigated for use as a thermometer in the millikelvin temperature range. The resistance versus temperature behaviour between 25 and 800 mK can be described with an accuracy of better than 0.5% by the relation found for conduction by variable-range hopping: R( T) = R 0 exp[( T 0/ T) α ], with α = 0.345. This relation was found to be valid down to at least 6 mK with an accuracy of a few per cent. The low temperature resistance behaviour can be predicted within 2% starting from the room temperature resistance value. The thermometers show a small negative magnetoresistance effect from which the apparent temperature change exceeds 2% only at temperatures below 0.1 K and in fields of more than 5 T. No dependence of the magnetoresistance effect on the orientation of the field has been found.