Advanced SQUID-on-lever scanning probe for high-sensitivity magnetic microscopy with sub-100-nm spatial resolution

TL;DR

This paper presents the development of nanometer-scale SQUID-on-lever probes with sub-100-nm resolution, enabling high-sensitivity magnetic imaging of skyrmions and other nanoscale magnetic structures at low temperatures.

Contribution

The authors fabricated robust, high-resolution SQUID-on-lever probes with integrated functionalities, surpassing previous limitations in spatial resolution and magnetic field range for scanning SQUID microscopy.

Findings

Achieved spatial resolution better than 100 nm

Demonstrated magnetic imaging of skyrmions and modulated patterns

Operates in magnetic fields up to 0.5 T at 4.2 K

Abstract

Superconducting quantum interference devices (SQUIDs) are exceptionally sensitive magnetometers capable of detecting weak magnetic fields. Miniaturizing these devices and integrating them onto scanning probes enables high-resolution imaging at low-temperature. Here, we fabricate nanometer-scale niobium SQUIDs with inner-loop sizes down to 10 nm at the apex of individual planar silicon cantilevers via a combination of wafer-scale optical lithography and focused-ion-beam (FIB) milling. These robust SQUID-on-lever probes overcome many of the limitations of existing devices, achieving spatial resolution better than 100 nm, magnetic flux sensitivity of $0.3~\mu\Phi_0/\sqrt{\rm{Hz}}$, and operation in magnetic fields up to about 0.5 T at 4.2 K. Nanopatterning via Ne- or He-FIB allows for the incorporation of a modulation line for coupling magnetic flux into the SQUID or a third Josephson junction for shifting its phase. Such advanced functionality, combined with high spatial resolution, large magnetic field range, and the ease of use of a cantilever-based scanning probe, extends the applicability of scanning SQUID microscopy to a wide range of magnetic, normal conducting, superconducting, and quantum Hall systems. We demonstrate magnetic imaging of skyrmions at the surface of bulk Cu$_2$OSeO$_3$. Analysis of the point spread function determined from imaging a single skyrmion yields a full-width-half-maximum of 87 nm. Moreover, we image modulated magnetization patterns with a period of 65 nm.