Progress in high-T-c dc SQUIDs

This paper reviews the development of magnetometers based on de Superconducting Quantum Interference Devices (SQUIDs) fabricated from thin films of YBa2Cu3O7-x (YBCO) and operated in liquid nitrogen at 77 K. The directly coupled magnetometer consists of a single layer of YBCO deposited on a SrTiO3 (STO) bicrystal patterned so that a pickup loop is connected to the body of the SQUID. These magnetometers achieve a magnetic field noise of under 100 fT Hz(-1/2) at frequencies down to below 1 Hz; the sensitivity may be improved by a factor of 3 or more by inductively coupling the device to a single-layer flux transformer on a 50 mm-diameter substrate. The process for depositing YBCO-STO-YBCO multilayers is reviewed. A fractional turn SQUID involving 16 loops and only 7 mm in diameter exhibited a magnetic field noise of 18 fT Hz(-1/2) at 1 kHz and 37 fT Hz(-1/2) at 1 Hz. A multilayer, multiturn flux transformer magnetically coupled to a SQUID in a flip-chip arrangement achieved a magnetic field noise of 8.5 fT Hz(-1/2) at 1 kHz and 27 fT Hz(-1/2) at 1 Hz; the area of the pickup loop was 10 x 10 mm(2).