Scanning nanoscale superconducting quantum interference devices (nanoSQUIDs) are of growing interest for highly sensitive quantitative imaging of magnetic, spintronic, and transport properties of low-dimensional systems. Utilizing specifically designed grooved quartz capillaries pulled into a sharp pipette, we have fabricated the smallest SQUID-on-tip (SOT) devices with effective diameters down to 39 nm. Integration of a resistive shunt in close proximity to the pipette apex combined with self-aligned deposition of In and Sn, has resulted in SOTs with a flux noise of 42 nΦ0 Hz-1/2, yielding a record low spin noise of 0.29 μB Hz-1/2. In addition, the new SOTs function at sub-Kelvin temperatures and in high magnetic fields of over 2.5 T. Integrating the SOTs into a scanning probe microscope allowed us to image the stray field of a single Fe3O4 nanocube at 300 mK. Our results show that the easy magnetization axis direction undergoes a transition from the 〈111〉 direction at room temperature to an in-plane orientation, which could be attributed to the Verwey phase transition in Fe3O4.
Bibliographical noteFunding Information:
The authors thank M. L. Rappaport for technical assistance. This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grants no. 785971 and 802952), by the US-Israel Binational Science Foundation (BSF) (grant no. 2014155), and by the Weston Nanophysics Challenge Fund. EZ acknowledges the support of the Leona M. and Harry B. Helmsley Charitable Trust grant 2018PG-ISL006.
© 2020 The Royal Society of Chemistry.