NanoSQUIDs: Basics & recent advances

Abstract

Superconducting Quantum Interference Devices (SQUIDs) are one of the most popular devices in superconducting electronics. They combine the Josephson effect with the quantization of magnetic flux in superconductors. This gives rise to one of the most beautiful manifestations of macroscopic quantum coherence in the solid state. In addition, SQUIDs are extremely sensitive sensors allowing to transduce magnetic flux into measurable electric signals. As a consequence, any physical observable that can be converted into magnetic flux, e.g., current, magnetization, magnetic field or position, becomes easily accessible to SQUID sensors. In the late 1980's it became clear that downsizing the dimensions of SQUIDs to the nanometric scale would encompass an enormous increase of their sensitivity to localized tiny magnetic signals. Indeed, nanoSQUIDs opened the way to the investigation of, e.g., individual magnetic nanoparticles or surface magnetic states with unprecedented sensitivities. The purpose of this review is to present a detailed survey of microscopic and nanoscopic SQUID sensors. We will start by discussing the principle of operation of SQUIDs, placing the emphasis on their application as ultrasensitive detectors for small localized magnetic signals. We will continue by reviewing a number of existing devices based on different kinds of Josephson junctions and materials, focusing on their advantages and drawbacks. The last sections are left for applications of nanoSQUIDs in the fields of scanning SQUID microscopy and magnetic particle characterization, putting special stress on the investigation of individual magnetic nanoparticles.CONTENTS arXiv:1609.06182v4 [cond-mat.supr-con]

Grooved Dayem Nanobridges as Building Blocks of High-Performance YBa2Cu3O7−δ SQUID Magnetometers

“…In this respect, the recent technological advances in nano-fabrication enabled the realization of nanoSQUIDs with white flux noise levels well below 1 µΦ 0 / √ Hz , 7-14 opening the way to single spin detection, a milestone of experimental physics. 15,16 SQUIDs made of High critical Temperature Superconductors (HTS) have a much wider temperature range of operation (from mK to above 77 K) compared to their Low critical Temperature Superconductor (LTS) counterparts, greatly simplifying their practical applications. HTS SQUID magnetometers are promising candidates for future on-scalp magnetoencephalography systems.…”

Noise of a superconducting magnetic flux sensor based on a proximity Josephson junction

“…Measuring noise provides an uncompromising test of microscopic and nanoscopic superconducting sensors [1][2][3][4][5] , such as superconducting quantum interference devices (SQUIDs), for ultra-sensitive detection of weak and local magnetic signals. A hybrid superconducting magnetometer 6, 7 based on the proximity effect 8 (superconducting quantum interference proximity transistor, SQUIPT 9 ) has demonstrated in experiments high responsivity to magnetic flux [9][10][11][12] and theoretically 13 the noise is predicted to be very low, comparable to or below 50 nΦ 0 /Hz 1/2 obtained with state-of-the-art nanoSQUIDs 4,5 .…”

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

“…The use of cheap and abundant liquid nitrogen furthermore simplifies the cryogenic requirements in terms of cooling and thermal insulation as compared to LTS-based systems. The realization of HTS Josephson junctions, the key ingredient of a SQUID, has been a topic of intense research during the last three decades [7][8][9][10][11] . The state-of-the-art HTS SQUIDs operating near 77 K typically use either bicrystal or step edge grain boundary Josephson junctions.…”