Sputtered Mo66Re34 SQUID-on-Tip for High-Field Magnetic and Thermal Nanoimaging

Sputtered Mo₆₆Re₃₄ SQUID-on-Tip for High-Field Magnetic and Thermal Nanoimaging

Kousik Bagani, Jayanta Sarkar, Aviram Uri, Michael L. Rappaport, Martin E. Huber, Eli Zeldov, and Yuri Myasoedov

Phys. Rev. Applied 12, 044062 – Published 28 October 2019

Abstract

Scanning nanoscale superconducting quantum interference devices (SQUIDs) are gaining interest as highly sensitive microscopic magnetic and thermal characterization tools of quantum and topological states of matter and devices. We introduce a technique of collimated differential-pressure magnetron sputtering for versatile self-aligned fabrication of SQUID-on-tip (SOT) nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point source, deposition. The technique provides access to a broad range of superconducting materials and alloys beyond the elemental superconductors employed in the existing thermal deposition methods, opening the route to greatly enhanced SOT characteristics and functionalities. Utilizing this method, we have developed molybdenum-rhenium ( ${Mo}_{66} {Re}_{34}$ ) SOT devices with sub-50-nm diameter, magnetic flux sensitivity of 1.2 $μ Φ_{0} / H z^{1 / 2}$ up to 3 T at 4.2 K, and thermal sensitivity better than 4 $μ K / H z^{1 / 2}$ up to 5 T—about five times higher than any previous report—paving the way to nanoscale imaging of magnetic and spintronic phenomena and of dissipation mechanisms in previously inaccessible quantum states of matter.

Received 21 June 2019
Revised 7 August 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.044062

Physics Subject Headings (PhySH)

Methods in superconductivity Quantum transport Superconductivity

SQUID Superconducting devices

Resistivity measurements Scanning probe microscopy Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Kousik Bagani^1,*, Jayanta Sarkar^1,‡, Aviram Uri¹, Michael L. Rappaport², Martin E. Huber³, Eli Zeldov^1,†, and Yuri Myasoedov¹

¹Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel
²Physics Core Facilities, Weizmann Institute of Science, Rehovot 7610001, Israel
³Departments of Physics and Electrical Engineering, University of Colorado Denver, Denver, Colorado 80217, USA

^*kousik.bagani@weizmann.ac.il
^†eli.zeldov@weizmann.ac.il
^‡Present address: Department of Applied Physics, Aalto University, Espoo, FI-02150, Finland.

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Vol. 12, Iss. 4 — October 2019

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Images

Figure 1
(a) A schematic drawing of the collimated differential-pressure sputtering method. The sputter gun is sealed in a sputtering chamber and the collimator allows the sputtered elements to deposit on the quartz pipette placed under it in the main UHV chamber. P is pressure gauge. (b) Photograph showing the sputtering chamber, collimator, and the tube for thickness monitoring. (c) Cross section of the collimator showing positions 1 to 3 of the quartz pipette during the three deposition steps. (d) Side view of the collimator showing the geometry of the three deposition steps at different pipette angles. (e),(f) Scanning electron microscope images of sputtered Mo₆₆Re₃₄ SOTs showing two superconducting leads with a gap between them and the loop at the apex with two weak links forming the SQUIDs with effective loop diameters of 105 nm (e) and 49 nm~(f).
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Figure 2
Current-voltage characteristics of Mo₆₆Re₃₄ SOTs at 4.2 K. (a) Measured $I_{SOT}$ vs $V_{b}$ and (b) the corresponding calculated $I_{SOT}$ vs $V_{SOT}$ curves of device A (Ø49 nm) at different indicated magnetic fields. Inset in (a) shows the SOT measurement circuit schematic, where $R_{b} = 1 k Ω$ , $R_{s} = 1 Ω$ , $R_{p} \approx 0.5 Ω$ , and $R_{sh} \approx 5 Ω$ . (c) Differential resistance $d V_{SOT} / d I_{SOT}$ as a function of $B_{a}$ and $I_{SOT}$ of device B showing SQUID interference pattern corresponding to an effective loop diameter of 79 nm. (d) Magnetic response function $d I_{SOT} / d B_{a}$ of device B plotted as a function of $B_{a}$ and $V_{b}$ .
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Figure 3
Noise characteristics of SOT device B (Ø79 nm) at 4.2 K. (a) Spectral density of the current noise $S_{I}^{1 / 2}$ at $B_{a}$ of 0 and 3 T exhibits no significant change in white noise level with magnetic field. (b) Flux noise $S_{Φ}^{1 / 2}$ (left axis) and spin noise $S_{n}^{1 / 2}$ (right axis) as function of magnetic field showing flux noise in the sensitive regions in the range of $S_{Φ}^{1 / 2} = 0.9$ to 1.5 µΦ₀/Hz^1/2 and spin noise of $S_{n}^{1 / 2} = 15$ to 30 µ_B/Hz^1/2 up to 3 T.
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Figure 4
Thermal response characteristics of Mo₆₆Re₃₄ SOTs. (a) $I_{SOT}$ vs $V_{b}$ characteristics of device C measured at zero field and different temperatures ranging from 4.2 to 7 K. (b) $I_{SOT}$ vs T measured at a constant bias voltage $V_{b} = 0.04 V$ [dashed line in (a)]. The SOT thermal response $d I_{SOT} / d T$ is obtained from the slope of the linear fit between 4.2 and 5 K (red). (c) $S_{T}^{1 / 2} as$ function of $B_{a}$ showing thermal noise of 1 to 2 µK/Hz^1/2 in the sensitive regions in magnetic fields up to 4 T in device D (Ø65 nm) and thermal noise of 2.5 to 3.8 µK/Hz^1/2 in the entire field range of up to 5 T in device E (Ø77 nm).
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