A low noise HTS SQUID magnetometer with an on-chip pickup loop coupled via an intermediate flux transformer

For many SQUID applications robust, compact magnetometers are required with low flux noise and high effective area at 77 K. Low flux noise is achieved by using a low inductance SQUID. A low inductance SQUID will also have a high transfer function, allowing simplified direct readout schemes to be used. For maximum field sensitivity the SQUID needs to be coupled to a pickup loop of large area and therefore large inductance. We have modelled, designed and fabricated a novel HTS magnetometer consisting of a low inductance (<16 pH) dc SQUID coupled to a 28 nH pickup loop flux transformer fabricated on the same 1 cm² substrate. Efficient coupling was achieved by using a flip-chipped intermediate flux transformer. This configuration produces a magnetometer with a high sensitivity per unit area. The magnetometers incorporate PrBa₂Cu₃O₇ isolation layers and two 2 μm diameter CAM variant junctions. Thick YBa₂Cu₃O₇ layers were used to improve coupling and decrease the demagnetisation factor wherever possible. Our device had a transfer function of 913 μV/Φ₀ which allows direct readout without any matching or additional positive feedback arrangements. The measured effective area at 77 K was 0.58 mm² (3.6 nT/Φ₀). The magnetometer white noise was 18 fT/$\text{Hz}$ and at 1 Hz was 380 fT/$\text{Hz}$. This was reduced at 1 Hz to 120 fT/$\text{Hz}$ using bias reversal. The measured white noise was higher than the design value and is largely due to Johnson noise from the PrBa₂Cu₃O₇ isolation layer used in our devices.