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Title: Design, characterisation and optimisation of high Tc dc SQUIDs
Author: Colman, P. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic flux sensors known. In order to achieve a high field resolution, an antenna is coupled to the bare SQUID. Flux focusing, direct and flux transformer coupling schemes were investigated. Off-axis pulsed laser deposited YBa2Cu3O7-δ films with protective SrTiO3 (STO) cap layers on 24° STO bicrystal substrates were patterned into SQUID structures. A multilayer technology with YBCO and STO layers was used to fabricate trilayer flux transformers. The flux transformers was then coupled inductively to the bare SQUID in a flip-chip arrangement. Characterisation equipment was developed to measure SQUID parameters. Noise measurements with a bias reversal technique were employed to reduce the 1/f component, which dominates the noise at low frequencies. All noise data were obtained at 77 K. For a flux focusing SQUID, a flux noise level of φn = 2.8 μφ0Hz-1/2 at 10 kHz and φn = 11.5 μφ0Hz-1/2 at 1 Hz with bias reversal was achieved. The field sensitivity of the flux focused SQUID was limited by the small effective area, Aeff = 0.83 x 10-3 mm2, but this type of device was successfully integrated into the IRC cold sample scanning SQUID microscope which was used to image currents in wires and ferromagnetic link samples. Direct and flux transformer coupling schemes increased the effective areas to 0.23 mm2 and 1.08 mm2 respectively. The best white field sensitivity measured corresponded to 105 fTHz-1/2 for the directly coupled SQUID and 42 fTHz-1/2 for the flux transformer coupled SQUID. Planar, first-order directly coupled gradiometers were investigated as part of the collaboration with Oxford Instruments plc and a white gradient field sensitivity of 2.1 pTcm-1Hz-1/2 in an unshielded environment was achieved.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available