Synchrotron-based high-pressure research in materials science Page: 4 of 8
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EXPERIMENTAL
We obtain magnetic field maps by scanning a magnetic sensor over the sample surface. A
schematic of the instrument is shown in Fig. la. The sample and sensor are immersed in liquid
nitrogen during the experiment. The sensor is attached to a brass strip mounted at the bottom of
a scan rod. The end of the rod protruding from the dewar is rastered by a high resolution x-y
scanner. A fixed pivot point near the sample translates the scanner motion into sensor motion
and reduces the total travel by about a factor of 5. Before scanning, the pivot rod is lowered
towards the sample so that the tension in the brass strip keeps the sensor in contact with the
sample surface. For the data shown here, 50 pm of Kapton plastic was inserted between the
sensor and sample to prevent sample damage during raster motion. Even with this protection
layer, however, the sliding motion generates about 7 G equivalent noise on the sensor signal.
Scratches are also visible on the plastic after each experiment. Voltage maps acquired from the
head are converted to magnetic field maps by calibrating against sources with known magnetic
field strength.
The sensors are permalloy-based magnetoresistive read-heads fabricated for use in computer
hard disk drives by a commercial manufacturer.8 The active area of the heads is less than 1 pm2
and room temperature sensitivities are typically better than 3 pV/G. Measurements at liquid
nitrogen temperature show a sensitivity of- 1 iV/G with a linear response measured up to 250
G when used in a constant current mode. The geometry of the heads makes them sensitive to the
surface normal component of the magnetic field.
Current density maps are obtained from the magnetic field maps by inverting the Biot-
Savart law using established Fourier space techniques.9 Similar methods were used to invert the
self-field data in the studies noted above. To facilitate inversion, images were acquired as 2n x
2n pixel maps. Before inversion, the images were also mirrored about the x- and y- axes to
produce a supercell with no offsets in the data at the image edges.
The samples for this work were YBCO deposited by pulsed laser deposition under typical
conditions' and patterned into bridges. Two types of substrate were used - SrTiO3 and textured
YSZ on Inconel ribbon. Sample "A" was YBCO deposited on single crystal SrTiO3. In order to
achieve an I, near 10 A, 300 nm of YBCO was deposited and patterned into a bridge 1 mm wide
and 5 mm long. The measured I, of sample A was 10 A (J, = 3.4 x 106 A/cm2). Sample "B" was
fabricated by depositing 600 nm of YBCO on a textured YSZ layer previously deposited by ion
beam assisted deposition on an Inconel ribbon. This was then patterned into a bridge structure 1
mm wide and 7 mm long. The thickness and width were chosen to provide I, near 10 A on this
substrate based on previous experience, however, the measured value was only 6 A (J, = 1 x 106
A/cm2), indicating some defect present in the sample.
For imaging, the fast scan direction of the raster motion was perpendicular to the bridge
length (the main transport current direction). The typical imaging orientation and coordinate
axes used for discussion are shown in Fig. lb. The only data smoothing applied was a small
vertical adjustment of adjacent scan lines to remove offsets. The typical imaging time for the
data was about 45 minutes. The current density maps we present will show only the x-
component of the current density (that flowing along the bridge length) since there is very little
current flowing across these particular samples.
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Synchrotron-based high-pressure research in materials science, article, Date Unknown; [Los Alamos, New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc935454/m1/4/: accessed April 20, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.