We report on the development of a new family of magnetic field sensors with exceptionally low magnetic field noise, as low as 0.3 fT Hz −1/2 . Beside this, they exhibit high usable voltage swings of more than 150 μV pp and tolerable background fields during cool-down of up to 6.5 mT. In operation mode they recover completely from magnetization pulses of up to 76 mT, which makes them well suited for applications such as low-field magnetic resonance imaging.With respect to their easy and reliable use as well as their field resolution in the sub-fT Hz −1/2 range, the presented SQUID sensors are adequate for many applications, such as in geophysics or in biomagnetism.

We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6 µm. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.

We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb crosstype Josephson tunnel junctions results in white flux noise levels as low as 66 n 0 /Hz 1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S  1/2 < 7 B /Hz 1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution. _____________________________________________________________________________________________

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.

Abstract-We report on the development of an ultralow noise thin-film based SQUID magnetometer. A niobium thin-film pickup coil is connected to the input coil of a SQUID current sensor. The low capacitance of the used sub-micrometer crosstype Josephson junctions enable superior noise performance of the device. Application scenarios e.g. in geophysics and ultra-low field magnetic resonance imaging are discussed.