SQUID-based setup for the absolute measurement of the Earth's magnetic field

We present a configuration of LTS dc SQUID magnetometers that is suited for an absolute measurement of the vector components of the Earth's magnetic field with a white noise level of about 6 fT Hz−1 2. Due to its periodic voltage-flux characteristic, a SQUID's output voltage generally corresponds to a set of equidistant fluxes or magnetic field strengths. To resolve this ambiguity, we introduce a configuration of coplanar SQUIDs integrated on a single chip, which exhibit effective areas differing by several orders of magnitude. The set of possible magnetic field strengths matching the output voltages of these SQUIDs is thereby significantly reduced and especially unique for magnetic field strengths less than a certain threshold value of about 10 μT in our current implementation. The SQUIDs are realized with 0.8 μm cross-type Josephson junctions that withstand high background fields of up to 3.9 mT during cool down and operation. A first one-dimensional experimental implementation successfully measured the modulation of the magnetic field component perpendicular to the sensor surface with amplitudes exceeding 50 μT. The overall dynamic range of the SQUID magnetometer system achieves 190 dB.