NMR With Superconducting-GMR Mixed Sensors

We have developed a very sensitive field sensor to detect magnetic signals in the femtotesla range. This mixed sensor is based on the association of a superconducting flux-to-field transformer with a magnetoresistive field sensor. The bandwidth of a GMR being wide and only limited in the GHz regime, where the ferromagnetic resonance of the free layer occurs, mixed sensors are good candidates for low field NMR. Advantages of mixed sensors are their robustness and their very fast recovery after the rf pulse (< 1 mus). We have first used these sensors for NQR detection in the MHz range. Here we show results obtained on low field NMR with frequencies up to 210 kHz without pre-polarization, detected with mixed sensors based on high-T_C superconductors.