Investigation of all niobium Nano-SQUIDs based on sub-micrometer cross-type Josephson junctions

TL;DR

This paper presents the development of highly sensitive niobium Nano-SQUIDs with sub-micrometer loops using cross-type Josephson junctions, achieving low flux noise and promising spin sensitivity for small spin system investigations.

Contribution

Introduction of Nb/AlOx/Nb cross-type Josephson junctions in Nano-SQUIDs, achieving lower flux noise and enhanced spin sensitivity compared to previous constriction-type junction devices.

Findings

Flux noise as low as 66 nΦ₀/Hz^{1/2} achieved

Spin sensitivity estimated below 7 μ_B/Hz^{1/2}

Potential for single spin resolution discussed

Abstract

We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlOx/Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n$\Phi_0$/Hz$^{ 1/2} $, well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield $S_\mu^{ 1/2} < 7 \mu_B$/Hz$^{ 1/2} $ in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.