Pages scientifiques 2010

Measurements of very weak field (down to the femtotesla range) have been mainly addressed by low-Tc SQUIDS. We have proposed in 2004 the principle of a new sensor, based on spin-electronics, to offer extremely good sensitivity for measurements of magnetic fields over a wide range of frequency (dc to hundreds of MHz).

These sensors, called mixed sensors, are based on the association of a sensitive field sensor (Giant Magnetoresistance or Tunnel Magnetoresistance) and of an efficient flux-to-field transformer. The transformer is a cm size superconducting loop containg a micron size constriction. When an external field is applied perpendicular to the loop, a supercurrent is generated, to prevent the entrance of the field in the loop. This supercurrent reaches a very high density when passing through the constriction. The stray field lines due to this current can be locally hundreds to thousands times larger than the applied field. If one locates a GMR or TMR element on top or below the constriction, it will collect this locally enhanced field lines and a change in the resistance of the GMR (TMR) element can be measured (see Figure 1).

This device is fully made out of thin film technology, and can be either using a low-Tc (typically Nb) loop or a High-Tc (YBaCuO) loop. In the first case, liquid helium cooling is required whereas in the second one only nitrogen cooling is necessary.