M I Faley, D Meertens, U Poppe, R E Dunin-Borkowski
Journal of Physics, Conference Series
The fabrication process and physical properties of graphoepitaxially engineered high-T c direct current superconducting quantum interferometer devices (DC SQUIDs) are studied. Double buffer layers, each comprising a graphoepitaxial seed layer of YBa 2 Cu 3 O 7-x and an epitaxial blocking layer of SrTiO 3 , were deposited over textured step edges on (001) surfaces of MgO substrates. Scanning electron microscopy and high-resolution transmission electron microscopy were used to investigate the
... ostructural properties of DC SQUIDs with graphoepitaxial Josephson junctions. Both direct coupled and inductively coupled high-T c DC SQUIDs with graphoepitaxial step edge junctions and flux transformers were studied. 3 edge Josephson junctions demonstrated an ∼ 5 nT/Φ 0 field-to-flux transformation coefficient and a magnetic field resolution of ∼ 50 fT/√Hz at 77 K. An increase in the size of the pick-up loop to about 25 mm leads to an improvement in the field-toflux transformation coefficient to approximately 1.5 nT/Φ 0 , and to a magnetic field resolution of ∼ 15 fT/√Hz at 1 kHz and 77 K. This value is comparable to that reported in Ref.  for similar frequency and temperature conditions. A moderate improvement of sensitivities can be obtained by inductive coupling of the direct coupled SQUIDs to single layer flux transformers made from relatively thick superconducting films  . Such coupling leads also to a reduction in the pick-up loop inductance, which improves the sensitivity and operational stability of the sensor even if the flux concentrator has a similar area to that of the pick-up loop of the SQUID. Much greater sensitivity improvement was achieved by the implementation of inductive coupling of the SQUIDs to a multiturn input coil of 8-mm or 16-mm superconducting flux transformers  . Magnetometers with 8-mm flux transformers demonstrated field-to-flux transformation coefficient of approximately 1.2 nT/Φ 0 and magnetic field resolution of ∼ 12 fT/√Hz at 77 K. Magnetometers with 16-mm flux transformers observed field-to-flux transformation coefficients of ∼ 0.45 nT/Φ 0 and magnetic field resolutions of ∼ 5 fT/√Hz at 77 K. The noise spectrum of these magnetometers was white down to the frequencies of about 10 Hz and achieved of ∼ 20 fT/√Hz at 1 Hz.