Single-layer and integrated YBCO gradiometer coupled SQUIDs

For many SQUID applications such as biomagnetism or non-destructive evaluation it is convenient or even necessary to work without the restrictions of a magnetically shielded room. This contribution deals with two sensors appropriate for this purpose. In the first concept we present a flip chip arrangement of a single-layer flux transformer and a single-layer SQUID, taking advantage of a simple technology. The SQUID was prepared on a 15x15 mm2 SrTiO3 24 deg. bicrystal and located in the common line of two-parallel-loop arrangements. The flipped antenna was designed as a two-parallel-loop gradiometer with 26 mm baseline on a 10x40 mm2 LaAlO3 single-crystal substrate. A field gradient sensitivity of 1 nT cm-1Φ0 was obtained. We could demonstrate a field gradient resolution of 20 fT cm-1 Hz1/2 at 1 kHz in an unshielded environment. In the second concept we integrated both the flux antenna and the SQUID on a SrTiO3 bicrystal. The tighter coupling scheme results in smaller devices for similar field gradient sensitivities. The integrated SQUID is designed as a 3x8 mm2 device on a 10x10 mm2 bicrystal substrate. The remaining space is used for test structures and SQUIDs without antennae, in order to control the technology as well as the SQUID design. Parallel processed dummy substrates were used to monitor the quality of film growth by x-ray analysis. The quality of our SQUID design will be discussed on the basis of the measured field gradient sensitivity and noise. The reliability of the devices is demonstrated by an NDE type measurement. (author)