Monolithic 77 K dc SQUID magnetometer
We report the first fabrication of a monolithic dc SQUID magnetometer that operates above 77 K. We have used bi‐epitaxial grain boundary Josephson junctions in YBa2Cu3O7 (YBCO) to produce the SQUID itself while the flux transformer consists of heteroepitaxial layers of YBCO/SrTiO3/LaAlO3/SrTiO3/YBCO. In the circuit fabricated, the SQUID is built on top of the epitaxial layers of the flux transformer. We have used dry etching, ion‐beam cleaning, and photolithographic processing to pattern all the layers. Via contacts and step coverage of the epitaxial wire layers have been achieved without significant degradation of the superconducting properties of any of the three YBCO layers. The magnetometer enhances the magnetic field sensitivity of the bare SQUID by a factor of 127, giving an effective area, dΦ/dB, of 1.9 mm2.