Sensitive Josephson magnetometry of flux quantization in a normal conducting hole in a narrow $YBa2Cu3O7$ line

A small $YBa2Cu3O7$ Josephson junction on a $24°$ symmetric $SrTiO3$ bicrystal is used as a sensitive magnetometer with micrometer spatial resolution in magnetic fields of up to 1 mT. The dependence of its critical current on the external magnetic flux is used to measure the local magnetic field. In the narrow line of 4 μm width leading to the Josephson junction we prepared a normal conducting area of about 2.5 μm diameter. This was achieved by heating the $YBa2Cu3O7$ locally with a focused laser beam to lower the oxygen content and thus suppress superconductivity at 77 K. We investigate the flux quantization in this normal conducting “hole” by cooling the whole device in different magnetic fields, reducing this external field to zero, and measuring the resulting flux. This way, superconducting properties of a hole in a superconducting film have been determined, which are important for the operation of hole-patterned magnetometers based on direct current superconducting quantum interference devices in static magnetic fields.