GMR-based sensors for ultra-sensitive magnetometry

Ultra-sensitive magnetometry addresses the detection of extremely small magnetic signals, typically in the femtotesla range, and is required for magnetic brain signal acquisition (Magnetoencephalography) and Low-Field Magnetic Resonance Imaging (LF-MRI). Superconducting Quantum Interference Devices (SQUIDs) have been since forty years the most appropriate devices for femtotesla detection. Recently other systems have been proposed, based on atomic magnetometry or on spin electronics. In the latter system, by combining a sensitive magnetoresistive field sensor and a superconducting flux-to-field transformer, we have achieved detection levels in the femtotesla range in the thermal noise. Here we present the principle and performances of these devices, and present modulation techniques that may be applied to reduce the low frequency noise, which is the main drawback for biomagnetic measurements.