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

TL;DR

The paper addresses the challenge of high-sensitivity, nanometer-scale magnetic imaging at low temperatures by introducing wafer-scale Nb SQUID-on-lever probes (SoL) integrated on planar cantilevers. The authors combine optical lithography with Ne- and He-FIB milling to create robust 2JJ and 3JJ SQUID geometries with on-tip modulation or phase-bias circuitry, achieving sub-100 nm spatial resolution and a flux sensitivity of $0.3~\mu\Phi_0/\sqrt{\mathrm{Hz}}$ in fields up to $\sim 0.5~\text{T}$ at $4.2~\text{K}$. Imaging of Cu$_2$OSeO$_3$ reveals skyrmion textures with a point-spread function of $87$ nm and a 65 nm magnetic modulation, demonstrating the platform’s capability to resolve nanoscale magnetic features. The integration of on-tip circuitry and multi-JJ configurations enables flexible readout modes (flux-locked and phase-locked) and robust operation across large field ranges, promising broad impact for studying magnetism, superconductivity, and quantum Hall phenomena at the nanoscale.

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 $0.3~μΦ_0/\sqrt{\rm{Hz}}$, 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.

Figures (4)

• Figure 1: Planar SQUID-on-lever with advanced functionality. (a) Optical micrograph of part of an SOI wafer with 21 patterned cantilever chips, which can be individually broken out. (b) False-colored scanning electron microscopy (SEM) image of part of a cantilever chip with patterned Nb lines extending to the cantilever (left) that protrudes beyond the chip body. (c) False-colored SEM image of a cantilever. Three Nb lines (blue) connect to a Nb triangle at the apex (left). (d), (e): False-colored SEM images (left) and corresponding circuit diagrams (right) illustrate examples of SQUID circuits that were subsequently patterned via Ne- and He-FIB milling by cutting trenches and a hole (diameter $d=36$ nm in (d) and 19 nm in (e)) into the Nb triangle. In (d), a modulation current $I_\mathrm{mod}$ can be applied to couple flux into the 2-JJ SQUID. In (e) a control current $I_\mathrm{ctrl}$ can be used to shift the phase of the 3-JJ SQUID. Each SEM image has been taken at an angle of 45 $^\circ$.
• Figure 2: Electric transport properties of 2-JJ SQUID-on-lever. 4-point measurements at $T=4.2$ K for a SQUID milled by Ne-FIB with hole diameter $d = 36$ nm and JJ widths $w_1 = w_2 = 55$ nm before and $w_1 = 48$ nm and $w_2 = 42$ nm after recutting by Ne-FIB. (a) Critical current $I_\text{c}$ vs modulation current $I_\text{mod}$ before and after recutting the JJs. Arrows indicate maximum and minimum positive critical currents $I^+_\text{c,max}$ and $I^+_\text{c,min}$, respectively. (b) Current-voltage characteristics before and after recutting, recorded at $I_\text{mod}$ values that provide $I^+_\text{c,max}$ and $I^+_\text{c,min}$. Before recutting, the IVCs show a pronounced hysteresis (arrows indicate sweep direction of current), which is almost absent after recutting. (c) Voltage $V$ vs $I_\text{mod}$ oscillations, measured after recutting for different bias currents (from $-25$ to 25 µ A in 1 µ A steps).
• Figure 3: Electric transport and noise properties of 3JJ-SoL. Results obtained at $T=4.2K$ with the configuration in (a) are shown in (b)--(d) for a 3JJ-SoL milled by Ne-FIB ($w_1 = w_2 = 50nm$, $w_3 = 75nm$); (c) also includes results from a 3JJ-SoL milled by He-FIB ($w_1 = w_2 = 28nm$, $w_3 = 37nm$), which was later used for imaging. Both devices have a hole diameter $d = 15nm$. (a) Readout circuit used for 3JJ-SoL characterization and imaging. (b) $I_\text{SQUID}(V_\text{b})$ curves at various magnetic fields $B$, with $I_\text{ctrl}=0$ for the 3JJ-SoL milled by Ne-FIB ($R_\text{b}=6.2~\text{k}\Omega$, $R_\text{s}=1~\Omega$). (c) $I_\text{c}(B)$ for the 3JJ-SoL milled by He-FIB with $I_\text{ctrl}=0$ and for the 3JJ-SoL milled by Ne-FIB with $I_\text{ctrl}=0$ and 30 µ A. (d) Rms spectral densities of flux noise $S_\Phi^{1/2}$ and field noise $S_B^{1/2}$ at 12 kHz vs applied magnetic field $B$ for the 3JJ-SoL milled by Ne-FIB. The two curves correspond to the most sensitive working points found with $I_\text{ctrl}=0$ at $V_\text{b}=0.15~$V and with $I_\text{ctrl}=30~$µ A at $V_\text{b}=0.4~$V.
• Figure 4: Magnetic field microscopy. (a) Schematic of SQUID-on-lever scanning probe over a skyrmion. (b) $B_z(x,y)$ measured $35 \pm 10$ nm above the surface of $\text{Cu}_2 \text{OSeO}_3$ in the low-temperature skyrmion (LTS) phase at $T = 5$ K. Green dotted frames indicate zoomed in regions in (c) and (d). (c), (d) $B_z (x,y)$ and $\text{d}B_{z}/\text{d}z (x,y)$ of skyrmion clusters. The scale bars of this image and all following ones correspond to 100 nm. (e) Line cuts corresponding to the white dotted lines indicated in (c) and (d). Points indicate measured data, while lines represent Gaussian fits. (f) $\text{d}B_{z}/\text{d}z (x,y)$ and $PSF$ of the SoL extracted from the measurement of a single skyrmion with 2D Gaussian fit FWHM. (g) $B_z (x,y)$ of the in-plane helical phase. Fourier space analysis yields a periodic spacing of $65 \pm 5$ nm.