Low frequency noise investigation of pT-level magnetic sensors by cross-spectral method

We present a simple method to estimate the noise of magnetic sensors running in the Earth's field range by establishing the cross-power spectrum density during ambient field operation and performing spectral subtraction. This method has advantages to the usual subtraction of two sensors outputs, mainly in requirements for synchronization of the sample rate and gain calibration. With this method, verified in simulation and measurements with AMR magnetometers, we could use a fluxgate as a second sensor in order to estimate the low-frequency noise of an HTS SQUID in the ambient field.