An offset temperature coefficient in SQUID sensors

Output offsets in SQUID sensors which accompany changes in the sensor temperature have been measured under varying conditions of magnetic stress applied to the sensor films. It is shown that there is an offset temperature coefficient (OTC), characterizing a reversible variation of the offset with temperature, which depends upon magnetic stress. Further, the variations of the OTC with magnetic stress are shown to be reversible for stress values below a critical value. Above this critical value, an irreversible change in the OTC occurs. This change can be annealed out by heating the sensor above its critical temperature. Two mechanisms which are capable of producing an OTC which behaves in this manner are discussed in quantitative terms. One mechanism involves temperature dependent pinning forces and the other involves the temperature dependent shielding currents which flow in response to magnetic stress. A simple technique for minimizing the deleterious effects of the OTC on magnetic measurements is described.