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Cryogenic scanning Hall-probe microscope with centimeter scan range and submicron resolution
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/content/aip/journal/rsi/76/10/10.1063/1.2072438
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16.Two positions with large gradients in the Hall voltage in different directions were examined. At the first, was was , and was . At the second, was was 34, and . These gradients may have been due to an electrical interaction between the sample and sensor, rather than magnetic vortices. They were, however, approximately constant over a 600 nm diameter and reproducible between scans before and after the vibration measurement.
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23.See EPAPS Document No. for the complete set of grain boundary images assembled into a movie. This document can be reached through a direct link in the online articles's HTML reference section or via the EPAPS homepage (http://www.aip.org/pubservs/epaps.html).[Supplementary Material]
24.
24.The ADC card actually acquires the two wave forms by scanning its ADC between the two inputs, thus the samples are interleaved in time rather than simultaneous. The resultant time shift is corrected before averaging the Hall or sample voltage wave forms.
25.
25.Instead of finding zero crossings, one could fit a sinusoid to to determine its frequency and phase, which could be used to break into cycles. We found, however, that showed slight, but statistically significant, variations in frequency within each wave form causing the sinusoid to misrepresent some cycles.
26.
26.The ADC card is rated to settle from full range to 1 least-significant bit within . In the interval used here, it would settle to 4 mV, which translates to a magnetic field of 0.1 G, from the maximum value of 1 V.
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28.
28.Such a magnet was designed and built into a Desert Cryogenics probe station by Dr. Douwe Monsma in Professor Charles Marcus’s laboratory at Harvard University.
29.
29.See, for example, vacuum-compatible stepper motors from tectra GmbH Physikalische Instrumente, http://www.tectra.de, or servo motors from Aerotech, Inc., http://www.aerotech.com
30.
30.For example, Physik Instrumente (PI) GmbH & Co. KG, http://www.pi.ws, makes closed-loop, stick-slip piezo actuators.
31.
31.See EXFO Burleigh Products Group Inc.’s motors, http://www.exfo.com, or Bookham Inc. New Focus division’s , http://www.newfocus.com
http://aip.metastore.ingenta.com/content/aip/journal/rsi/76/10/10.1063/1.2072438
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Figures

Image of FIG. 1.

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FIG. 1.

(a) Sketch of the large area scanning Hall-probe microscope. A flow cryostat cools the sample. A Hall sensor is rastered over the sample surface. The sensor position is controlled by an external, stepper motor-based stage. The cryostat allows optical access from above and, via mirrors, from the sides. (b) Magnetic image of a millimeter long YBCO strip carrying 100 mA. This image uses only a fraction of the instrument’s centimeter scan range.

Image of FIG. 2.

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FIG. 2.

Sketch of the three-axis stage that moves the sensor arm. Stepper motors drive micrometer screws to generate linear motion. For the and axes, worm gears first reduce the motors’ rotation; for the axis, the motor is coupled directly to the micrometer screw.

Image of FIG. 3.

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FIG. 3.

(a) Sketch of the Hall sensor mounted on a foil cantilever that, together with a fixed metal plate above it, acts as a capacitor used to sense touchdown of the sensor on the sample. (b) Photo of the same, tilted to reveal the bottom of the assembly, which faces the sample. (c) Capacitance vs sensor height above the sample surface indicates when the sensor has touched down. For , capacitance abruptly starts to increase with decreasing sensor arm height. is defined by this measurement with 30 nm standard deviation. (d) Sketch showing the sensitive area (Hall cross) and its placement near a corner, defined by photolithography and wet etching that creates a step several microns deep followed by manual polishing to remove most of the remaining material. As in (b), the viewpoint is tilted to reveal the sensor’s bottom face. (e) Scanning electron micrograph (SEM) of the Hall cross tilted 45° as in (d). For this sensor, similar to that used in Sec. III, the area is covered by a layer of gold to screen electric fields.

Image of FIG. 4.

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FIG. 4.

Temperatures and pressure after insertion of the precooled liquid-helium transfer line into the cryostat. The sample stage cools from room temperature to near its final temperature of 4 K in less than 10 min. Other parts of the cryostat take longer to equilibrate.

Image of FIG. 5.

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FIG. 5.

(a) Top view photograph of a square section of a thin film of YBCO varnished to the sample stage. (b)–(d) Magnetic images of the square at several length scales, from its full width down to submicron resolution. Each white dashed box in (a)–(c) indicates the area scanned in the succeeding image. The pixel spacing in (d) is 200 nm. (e)–(f) After more careful sensor alignment, single rows of points near a vortex were scanned along the , then directions. These data (black points) were fitted (gray lines) with a model of the vortex field, depicted in (g).

Image of FIG. 6.

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FIG. 6.

(Color online) Imaging current-induced flux penetration into a grain boundary: (a)Illustration of the sample geometry. The approximate area of the magnetic images is outlined as a dashed box. A bridge passes current across the boundary while the and leads measure voltage; the other leads are not used. (b) Current (dashed purple line) and voltage (solid green line) during a 500 mA rms, 400 Hz cycle. Select times are marked with labeled dots, and the corresponding magnetic images are shown in (c)–(s). Flux penetrates along the grain boundary well before the bulk of the superconductor admits vortices. The complete set of frames can be downloaded as a movie from Ref. 23.

Image of FIG. 7.

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FIG. 7.

(Color online) Time traces recorded during imaging of ac current-induced fields. At each pixel, (a) the applied current cycles many times while the magnetic field is recorded. (b) and (c) show the magnetic response at two different pixels. (d)–(f) The cycles are overlaid and averaged. Points from the averaged wave forms are assembled into the images shown in Figs. 6(c)–6(s) (g), (h) Plotting the averaged current and field against each other yields a hysteresis loop whose area indicates the amount of flux trapping. (b), (e), and (g) correspond to a pixel above the bridge edge where hysteresis is small indicating that a nondissipative Meissner response dominates. In contrast, (c), (f), and (h) are measured at a pixel along the grain boundary where flux is forced in by the applied current and remains after the current is removed leading to a very hysteretic response. The areas of the hysteresis loops for all pixels are assembled in (j) providing a map of ac losses that emphasizes flux penetration along the grain boundary and the bridge edges.

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/content/aip/journal/rsi/76/10/10.1063/1.2072438
2005-10-04
2014-04-24

Abstract

We have constructed a scanning Hall-probe microscope that combines a scan range with 200 nm positioning resolution by coupling stepper motors to high-resolution drivers and reducing gears. The instrument is uniquely suited for efficient magnetic imaging of mesoscopic devices, media, and materials, operating from 4 K to room temperature with fast turn-around time. Its potential for studying dissipation in coated conductors—high- superconducting tapes—is demonstrated via model systems. We image an entire sample of , then zoom in to individual fluxons. Flux penetration into a single artificial grain boundary is imaged with field resolution and time resolution by averaging over cycles of ac driving current. Using the resulting magnetic movie, we map out ac power losses.

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Scitation: Cryogenic scanning Hall-probe microscope with centimeter scan range and submicron resolution
http://aip.metastore.ingenta.com/content/aip/journal/rsi/76/10/10.1063/1.2072438
10.1063/1.2072438
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