Key Components for High Tc SQUID Magnetometers
Doctoral thesis, 1993
This thesis describes the development of key components for superconducting quantum interference device (SQUID) magnetometers in the high temperature superconductor YBa2Cu3O7 (YBCO). Such magnetometers are made for liquid nitrogen temperature operation, as opposed to low Tc devices that are confined to liquid helium. The work was divided into four parts: growth of thin films in YBCO, development of Josephson junctions, investigation of SQUID sensors made from these junctions, and trilayer work for flux transformers.
Initially, the investigated film growth process was coevaporation of Y, BaF2 and Cu on substrates of SrTiO3 followed by a separate high temperature annealing step. The films grown in this way had critical temperatures up to 90 K and critical current densities up to 106 A/cm2 at 77 K. The critical current density increased for decreasing film thickness. This was related to the film morphology, which consisted of a c-axis oriented bottom layer covered with a-axis oriented grains on the surface. Later, laser deposition became the growth method of preference.
Josephson junctions were made by laser-depositing thin films of YBCO on bicrystal substrates of Y-ZrO2 or SrTiO3. The critical current density at 77 K could be adjusted from 100 A/cm2 to 106 A/cm2 by choosing the appropriate misorientation angle in the bicrystal. The junctions followed the RSJ-model with noise rounding for angles larger than 22 .degreee.. For lower angles, a flux flow like behavior was seen.
SQUID loops with these junctions were made. They could be operated up to the critical temperature of the films (90 K) with modulation depths in accordance with predictions. The noise levels for SQUIDs with 70 pH inductance were around 1029 J/Hz at 77 K, and a level of 4.5 x10-29 J/Hz at 10 Hz and 85 K indicate low 1/f noise. The 1/f noise was traced to originate mainly from the Josephson junctions at low temperatures and to flux noise at temperatures close to Tc.
Crossovers and vias for flux transformers were made from trilayers, where two YBCO layers were separated by an insulating layer of PrGaO3 and of SrTiO3/PrGaO3 multilayers. The crossovers contained grain boundaries which gave low critical current values. The grain boundaries originated from the growth properties of YBCO on PrGaO3.