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Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena
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10.1063/1.4731656
/content/aip/journal/rsi/83/7/10.1063/1.4731656
http://aip.metastore.ingenta.com/content/aip/journal/rsi/83/7/10.1063/1.4731656
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

(a) A drawing of the rotator used to hold and rotate the tip during deposition based on the design of Yoo et al. 22 (b) A schematic of the three step thermal deposition of a SOT. The tip in each step is offset for clarity from its actual position above the source. Inset: a SEM micrograph showing a head-on view of the SOT. The darker regions are quartz covered with not more than 3 nm of aluminum, below its percolation threshold, and the brighter regions are the deposited aluminum. The dark spot in the center surrounded by the bright ring is the hole in the SOT loop.

Image of FIG. 2.
FIG. 2.

(a) Schematic of the measurement circuit. A room-temperature voltage source in series with a cold 5 kΩ resistor applies a nearly constant current to the SOT shunted by a small resistance, R b . When the SOT is in the superconducting state, the entire current passes through it and is measured via inductive coupling by the SSAA. When it switches to the voltage state, most of the current flows through R b , while a small fraction continues to flow through the SOT itself. R s is a small, unwanted, series resistance, typically less than 1 Ω. (b) I SOT vs. H for different values of V in showing the quantum oscillations of the current with a period that corresponds to an SOT loop diameter of 245 nm. (c)–(d): I SOT vs. V in for different values of applied magnetic field, H. The field is shown in terms of the flux in the SOT in units of the flux quantum, Φ0.

Image of FIG. 3.
FIG. 3.

(a) Image of a tuning fork after the vacuum can had been removed. The outline of the tip is superimposed on the right prong in a red dashed line. The blue blob near the end of the prong shows the location and approximate size of the epoxy drop used to glue the tuning fork to the tip; (b) A SEM micrograph of a tip glued to a prong of a tuning fork; (c) The tip/tuning fork assembly. The tip is positioned between two stainless steel blocks, one with a groove, acting as an electrode, and the other having a BeCu spring connected to it, acting as a counter-electrode by pressing against the tip. The tuning fork itself is glued onto the dither piezo with Bondmaster E32 and then glued to the tip with Araldite 2020.

Image of FIG. 4.
FIG. 4.

A comparison among room temperature, atmospheric pressure, and low temperature (300 mK), low pressure resonance curves of the tuning fork. The room temperature resonance is typically wider and its resonant frequency is slightly lower than the low temperature one.

Image of FIG. 5.
FIG. 5.

The microscope assembly without its outer shell. The lower part includes the coarse x and y positioners and the tip holder while the upper part includes the z positioner, the xyz scanners and the sample holder.

Image of FIG. 6.
FIG. 6.

(a) An optical image of the Al serpentine deposited on a Si/SiO2 substrate. The arrows indicate the direction of the current flow in the serpentine. (b) A SEM image of a similar serpentine made of Nb that contains periodic constrictions (narrowing in the line shape).

Image of FIG. 7.
FIG. 7.

AC magnetic field B ac at a height of 1 μm above an aluminum serpentine carrying an ac current of 2 mA at 510 Hz under an applied dc field of 90 Oe. (a) A 30 × 30 μm2 magnetic image showing two strips of the serpentine carrying current in opposite directions as indicated by the arrows. The circular “depressions” along the central axes of the strips are normal (not superconducting) regions in the intermediate state of type-I superconductor. The applied ac current circumvents the normal regions giving rise to the observed “depressions” in the local B ac . (b) Line scans from (a), with lines “2” and “3” passing through “depressions”.

Image of FIG. 8.
FIG. 8.

(a) Optical image of the NbSe2 crystal patterned into a serpentine using a focused ion beam. (b) Image of the magnetic signal at tip-sample separation of 2.5 μm exemplifying the screening of the magnetic field in the superconductor at applied field of 140 Oe. The dark regions are two NbSe2 strips that partially screen the magnetic field and the bright strip is the FIB etched region between the two superconducting strips.

Image of FIG. 9.
FIG. 9.

Meissner-like signal from a Nb serpentine at an applied field of 250 Oe. (a) The induced field, B z , scanned a few microns above the sample. The dark (38 G) regions are the Nb strips while the bright (250 G) regions are the silicon substrate. (b) A cut along the x-axis, marked by the blue line in (a). The shielding currents screen the superconducting Nb strips from the applied magnetic field. The trapped field of 38 G in the Nb strip is due to field-cooling in the presence of the remnant field in the magnet. (c) AC signal measured by the SOT at 32 kHz due to the oscillations of the TF and the SOT along x axis. (d) A cross section along the x-axis, marked by the red line in (c). Since the measured ac signal is determined by the gradient of the dc field, the spikes correspond to the sudden change in the local field when crossing the edges of the superconducting strip. The offset values of B ac apparently result from electrical pickup at 32 kHz and a contribution from in-plane field B x because the tip is not exactly perpendicular to the sample surface.

Image of FIG. 10.
FIG. 10.

(a) The dc magnetic signal B z in Nb film measured after field-cooling the sample in 20±7 Oe. Individual vortices are visible as bright spots. (b) The corresponding ac magnetic signal in the same scan area due to the oscillation of the SOT in the x direction at 32 kHz. The vortices are visible as a pair of bright/dark signal corresponding to the gradient dB z /dx of the vortex field. (c) A topographic measurement of the surface of the Nb film, 3 × 3 μm2, in the same setup (different scan area).

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/content/aip/journal/rsi/83/7/10.1063/1.4731656
2012-07-09
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena
http://aip.metastore.ingenta.com/content/aip/journal/rsi/83/7/10.1063/1.4731656
10.1063/1.4731656
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