Low noise YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x multilayers for improved superconducting magnetometers

Abstract

We have fabricated YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x (YBCO–STO–YBCO) trilayers, in which each layer is patterned photolithographically, capping the first YBCO film with an in situ STO film. Atomic force microscopy demonstrates that the capping process dramatically improves the quality of the surface of the second layer, allowing the growth of an upper YBCO film with a substantially reduced level of low-frequency flux noise. A magnetometer with a multiturn flux transformer coupled to a dc superconducting quantum interference device achieved a magnetic field noise of 74 fT Hz−1/2 at 1 Hz, improving to 31 fT Hz−1/2 at 1 kHz.

Squid Fundamentals

“…It is believed that the excess noise arose predominantly from the upper YBCO film which had a poorer crystalline quality than YBCO films deposited directly on a substrate. However, we have recently developed processing techniques that substantially reduce the 1/f noise, and we begin with a brief summary of our current technique [127].…”

“…37. The effective area of the magnetometer is given by In our most recent series of such devices [127,134,135] we used 16-turn input coils. generally patterning the pickup loop and input coil in the lower YBCO film and the crossover connecting the innermost tum to the pickup loop in the upper YBCO film.…”

High transition-temperature SQUID magnetometers and practical applications

“…Frank Ludwig was instrumental in developing the final "standard" version of this process which I will briefly describe [45]. The thicknesses and deposition parameters for each layer are listed in Table 4.1.…”

“…As described in Parts I and 111, a magnetometer consisting of a square-washer SQUID inductively coupled to a flux transformer in either a flipchip [45] or integrated [64] arrangement combines high magnetic field sensitivity with small sensor size. The outer dimension of the SQUID washer is usually made large, say 500 pm, to facilitate efficient coupling to the multiturn input coil.…”

“…The flux transformers were fabricated using the multilayer process that involved in situ capping of the lower YBCO f i l m with a thin layer of STO [45] as described in Chapter 4.…”