RF Response of Superconducting-GMR Mixed Sensors, Application to NQR

When coupled to a Giant Magneto-Resistive (GMR) sensor, a superconducting loop containing a constriction can be a very sensitive device for magnetic field detection . The bandwidth of a GMR sensor is wide with only a limitation in the GHz regime mainly due to the ferromagnetic resonances of the free layer. We have studied the radiofrequency response of this mixed sensor and we present femtotesla-range sensitivity obtained up to 10 MHz. An application of this kind of sensor is Nuclear Quadrupolar Resonance (NQR) and low field Nuclear Magnetic Resonance (NMR). We present results showing that mixed sensors can be better than optimized resonant coils for frequencies below 10 MHz opening new possibilities not only for magnetic imaging and low field Nuclear Magnetic Resonance but also for a lot of applications of magnetic sensing in the RF domain.