Multilayer magnetometers based on high-Tc SQUIDs

Recent progress towards the development of a multilayer technology for the high transition temperature superconductor YBa2Cu3O7 − x (YBCO) is described. Early YBCOSrTiO3YBCO multilayers, fabricated with laser deposition and patterning of each layer, were plagued with excess magnetic flux noise at low frequencies. This noise was generated by the thermally activated hopping of flux vortices between pinning sites in the superconducting films. The progressive reduction in the level of this noise by improving the quality of the films is described. A key issue is the protection of the first YBCO film with a thin SrTiO3 cap before patterning. This process, which currently produces the lowest level of 1f noise, was used to fabricate a number of multilayer structures. A multiturn flux transformer coupled to a single-layer dc superconducting quantum interference device (SQUID) in a flip-chip arrangement achieved a magnetic field noise at 77 K of 8.5 fT Hz−12 at 1 kHz and 27 fT Hz−12 at 1 Hz; the pickup loop was 10 mm across. A monolithic 16-turn multiloop magnetometer with a diameter of 7 mm and operated at 77 K yielded 18 fT Hz−12 at 1 kHz and 37 fT Hz−12 at 1 Hz.