Flux Noise Reduction of HTS SQUID Using Josephson Junctions Made by FIB

Abstract

Superconducting quantum interference devices (SQUIDs), which are the most ultrasensitive magnetic sensors, are generally made by microfabrication of superconducting thin films. The bicrystal junction, which is often used in high-temperature superconducting (HTS) Josephson junctions (JJs), has problems expensive bicrystal substrates, and the fact that JJs can only be arranged in a fixed straight line [1-2]. Because the HTS thin film is a type-II superconductor, magnetic fluxes penetrates the HTS thin film and moves for the Lorentz force, which increases the flux noise at low frequencies.In this study, we investigated a method to fabricate low-noise SQUID magnetic sensors by Ga focused ion beam (FIB) irradiation of high-temperature superconducting thin films. When HTS thin film is irradiated with FIB, point defects are introduced into the HTS crystal structure, which leads to changing into normal conduction. The dependence of the critical current change ratio IC/IC0 on the dose was shown in Fig.1. The dependence of the normal resistance RN are also depicted. The thickness of the YBa2Cu3O7-δ (YBCO) film was 50 nm and it was protected by a thin Au layer. IC/IC0 decreased with irradiation fluence and changed into normal conduction at 2.0×1017 ions/cm2; after this point RN increased steeply.HTS nanobridge JJ, were fabricated by FIB irradiation. Figure 2 shows the V-I characteristics of a nanobridge with an HTS film thickness of 50 nm and a bridge width of 120 nm. The critical current was reduced to 264.5 μA and shows a clear Shapiro-steps under microwave irradiation with frequency of 10 GHz. As a method to reduce the magnetic flux noise, we also investigated the introducing of normal conducting pinning sites (anti-dots) into HTS thin films by FIB irradiation. We have confirmed that anti-dots with diameters of several hundred nm can be formed by point irradiation of HTS thin films with FIB. In the future, we will confirm SQUID operation at a nanobridge JJ with an YBCO film thickness of 50 nm and investigate the improvement of SQUID noise characteristics by introducing anti-dots.  Fig. 1. Dependence of IC/IC0 and RN on FIB irradiation fluence.  Fig. 2. V-I characteristics of the HTS nanobridge with a width of 120 nm.