Low-noise, second-harmonic magnetoresistive imaging using SAL-biased AMR sensors in a bridge configuration

This work demonstrate sensors that accomplish this same end by modulating the bias on soft-adjacent-layer (SAL) biased AMR devices and sensing the second harmonic signal. The 2f technique allows us to move the signal above the 1/f noise regime and into a less noisy bandwidth. Noise response of these sensors and demonstrated the scalability of the noise with the volume of the sensor has been measured. Using the Johnson noise limit, the sensitivity of 1 pT/radicHz is achievable in these devices at reasonable power levels. In addition, the 2f detection is capable of providing high-contrast magnetic field images that reject thermal asperities. This is due to the fact that the interaction of the bias current and magnetic moment of the sensor is independent of the sign of the bias current. Lock-in technique is used to generate 2f signal from the differential output of the bridge. A custom MR looper was built to characterize these sensors. Applications include arrays of bridges for high-resolution scanned MR microscopy including room temperature magneto-cardiograms.