Ames Laboratory Publications
Ames Laboratory
2012
A novel approach for x-ray scattering experiments in
magnetic fields utilizing trapped flux in type-II
superconductors
R. K. Das
Argonne National Laboratory
Z. Islam
Argonne National Laboratory
J. P. C. Ruff
Argonne National Laboratory
R. P. Sawh
University of Houston
R. Winstein
University of Houston
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Authors
R. K. Das, Z. Islam, J. P. C. Ruff, R. P. Sawh, R. Winstein, Paul C. Canfield, J.-W. Kim, and J. C. Lang
This article is available at Digital Repository @ Iowa State University: http://lib.dr.iastate.edu/ameslab_pubs/212
A novel approach for x-ray scattering experiments in magnetic fields utilizing trapped
flux in type-II superconductors
R. K. Das, Z. Islam, J. P. C. Ruff, R. P. Sawh, R. Weinstein, P. C. Canfield, J.-W. Kim, and J. C. Lang
Citation: Review of Scientific Instruments 83, 065103 (2012); doi: 10.1063/1.4725523
View online: http://dx.doi.org/10.1063/1.4725523
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/83/6?ver=pdfcov
Published by the AIP Publishing
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REVIEW OF SCIENTIFIC INSTRUMENTS 83, 065103 (2012)
A novel approach for x-ray scattering experiments in magnetic
fields utilizing trapped flux in type-II superconductors
R. K. Das,1 Z. Islam,1,a) J. P. C. Ruff,1 R. P. Sawh,2 R. Weinstein,2 P. C. Canfield,3
J.-W. Kim,1 and J. C. Lang1
1
X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave.,
Argonne, Illinois 60439, USA
2
Department of Physics, Texas Center for Superconductivity, University of Houston, Houston,
Texas 77004, USA
3
Department of Physics and Astronomy, Ames Laboratory, Iowa State University, Ames, Iowa 50010, USA
(Received 13 March 2012; accepted 8 May 2012; published online 8 June 2012)
We introduce a novel approach to x-ray scattering studies in applied magnetic fields by exploiting
vortices in superconductors. This method is based on trapping magnetic flux in a small disk-shaped
superconductor (known as a trapped field magnet, TFM) with a single-crystal sample mounted on or
at close proximity to its surface. This opens an unrestricted optical access to the sample and allows
magnetic fields to be applied precisely along the x-ray momentum transfer, facilitating polarizationsensitive experiments that have been impractical or impossible to perform to date. The TFMs used
in our study remain stable and provide practically uniform magnetic fields for days, which are sufficient for comprehensive x-ray diffraction experiments, specifically x-ray resonance exchange scattering (XRES) to study field-induced phenomena at a modern synchrotron source. The TFM instrument has been used in a “proof-of-principle” XRES study of a meta-magnetic phase in a rare-earth
compound, TbNi2 Ge2 , in order to demonstrate its potential. © 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4725523]
I. INTRODUCTION
The effects of magnetic fields (B) on material properties
are diverse and important to study from basic-science and
applied-research points of view.1–5 There are numerous
magnet research laboratories and academic research groups
worldwide focused on studies of materials in magnetic fields.
With the advent of modern synchrotron x-ray sources, significant efforts have also been invested to study field-induced
phenomena with x-rays.6–13 While these efforts have led
to many applications of x-ray spectroscopic and imaging
techniques to study field effects in materials,6–8 progress in
x-ray scattering studies of single-crystal samples in magnetic fields with split-pair and solenoid magnets have been
relatively slower due to two long-standing obstacles. First,
optical access required for scattering studies is restricted due
to magnet geometries. Second, it is practically impossible
to apply magnetic fields along the x-ray momentum transfer
(Q), which is frequently an ideal scattering geometry. In a
conventional split-pair magnet, the magnetic field is strictly
perpendicular to the scattering plane, whereas in the solenoid
magnet the magnetic field is typically in the scattering plane
but nearly orthogonal to Q (with the exception of back
scattering). A new approach that overcomes these geometric
limitations would enable one to thoroughly study any arbitrarily ordered magnetic and orbital structure, allow complete
exploitation of polarization-sensitive cross section by angular
dependence measurements about Q (with B||Q), and allow
precise determination of pure longitudinal magnetostriction
a) Author to whom correspondence should be addressed. Electronic mail:
zahir@aps.anl.gov.
0034-6748/2012/83(6)/065103/7/$30.00
effects. We report efforts at the Advanced Photon Source
to develop such a magnet instrument that exploits magnetic
properties of bulk superconductors (SC) with the potential to
reach applied fields beyond 17 T.14
It is well known that type-II superconductors can trap
magnetic flux in the so-called vortex state.15 If the trapped
flux is stable over time then the magnetized superconductor can be used as a “permanent” magnet (known as trapped
field/flux magnet, TFM). Worldwide research in this area has
accelerated16 since the discovery of high-temperature superconductors over two decades ago, and recent breakthroughs
in the understanding of pinning centers (used to trap the fluxoids in place)17–19 have made practical applications of superconducting disks as strong permanent magnets conceivable.
The ability to grow high-quality large melt-textured high-Tc
superconductors (e.g., YBa2 Cu3 O7 (YBCO) and other related
compounds) and to control the introduction of strong pinning
centers in them by chemical18 or irradiation17, 19 methods have
reached a high degree of sophistication. The texturing process
orients a preferred crystalline axis (e.g., c axis) of the micrograins in the superconductor, allowing high critical current
density as well as increasing fracture toughness, which is crucial for withstanding Maxwell stress of the trapped flux. Indeed it has been shown that a superconducting disk (∼27 mm
diameter and 15 mm thick) is capable of trapping magnetic
fields in excess of 17 T,14 a result that foresees availability
of magnetic fields beyond those available from commercial
split-pair magnets for scattering studies.
We have developed a melt-textured YBCO-based TFM
instrument in order to circumvent the two long-standing impediments to x-ray scattering studies discussed above. In
this approach a YBCO SC disk is magnetized by trapping
83, 065103-1
© 2012 American Institute of Physics
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FIG. 1. Sample mounting scheme. In this case the sample is attached directly to the surface of the TFM. Magnetic field (B) direction is normal to the
sample surface as indicated by a vertical arrow.
quantized magnetic flux on pinning centers. In the most
straightforward configuration, as shown in Fig. 1, a tiny
single-crystal sample is mounted directly on the TFM surface.
The advantages of this geometry are clear: (1) optical access is
over the entire hemisphere, and (2) magnetic fields can be applied along the x-ray momentum transfer (B||Q) in the symmetrical reflection geometry.20 In addition, the TFM is compact, and it can be cooled with a small closed-cycle (or liquid
He flow) cryostat for use in any x-ray experimental stations at
modern synchrotron beamlines. Since the magnetic fields of a
TFM are confined within close proximity of the cryostat cold
finger, no significant stray fields are present to interfere with
motors or sensitive electronics in the experimental stations.
The TFM instrument described in Sec. II would, therefore,
be ideal for numerous synchrotron x-ray scattering studies of
electronic, magnetic, orbital, and structural properties in magnetic fields. In Sec. III, we report characterization measurements of TFMs and results of a preliminary x-ray magnetic
scattering study on TbNi2 Ge2 demonstrating the use of the
instrument.
II. EXPERIMENTAL DETAILS
The core of this instrument is a YBCO-based TFM grown
by a melt-textured method. Platinum (Pt) powders were introduced to create flux pinning centers. During melt texturing they react with Y2 BaCuO5 impurities (a by-product of the
growth) to reduce their size and refine their shapes, which are
believed to be responsible for their very high trapped magnetic fields and critical current density properties. The TFMs
were cut to desired sizes, and their trapped field profile was
measured at liquid nitrogen (LN2 ) temperature to check for
signatures of structural flaws before being incorporated into
the TFM instrument for further characterization and x-ray
measurements. TFMs produced in this way18 achieve fields
of about 0.5 T at LN2 temperatures in a TFM 20 mm in diameter and 8 mm thick. Other processes provide fields of, e.g.,
2.0 T at similar size and temperature.17 Both current
density and trappable field increase rapidly at lower temperatures. Present results19 indicate that further pinning center
development will make possible 8-T TFMs at this size and
temperature.
Rev. Sci. Instrum. 83, 065103 (2012)
The primary instrumentation consists of (1) a small cryostat for the TFM and sample, (2) cryogenic Hall probes, and
(3) an external superconducting magnet. For x-ray scattering studies, the TFM first needs to be magnetized and then
kept below its superconducting transition temperature, Tc .
There are different ways to magnetize (or “activate”) a TFM.
While in situ use of pulsed magnetic fields and flux pumping are desirable, at present these techniques remain an active
field of research on their own right.21–24 Commonly, an external resistive or superconducting magnet is used with a TFM
cooled either in an applied field (field cooled, FC), or in zero
field with subsequent application of fields at low temperatures
(zero-field cooled, ZFC). While we are able to employ a ZFC
method for activating a TFM using our setup described below, there is a significant drawback: applied field needs to be
at least twice the maximum field that a TFM can trap in order
to guarantee full flux penetration into the bulk. In this paper
we describe the implementation of the FC method because it
ensures a uniform flux penetration into the bulk, the maximum field trapped for a given TFM being limited only by the
maximum field available from the external magnet.
The experimental setup for activating a TFM is shown
in Fig. 2. A “cryogen-free” superconducting split-pair magnet is used for applying external fields. The maximum field
at gap center is 4 T, while at the center of each coil of the
pair the field reaches ∼5 T, which in principle sets an upper
field limit for activation. However, the outer vacuum jacket of
this horizontal magnet was modified to install a room temperature (RT) insert into the magnet bore, restricting the magnet
to operate at or below ∼3.5 T. A small light-weight cryostat,
model SRDK-101 from Sumitomo Heavy Industries, is used
for cooling the TFM and sample. The TFM cryostat has a narrow vacuum shroud around the cold finger that fits into the RT
insert. During a TFM activation the cryostat is mounted on a
translation stage such that it can slide inside the RT bore of
the external magnet (Fig. 2) and be positioned at the magnet center where the volume of field uniformity is the largest.
The magnetic field of the external magnet is in the horizontal plane and along the axis of the TFM cryostat. The inset
of Fig. 2 shows a small TFM (black square) of size 8 mm
× 8 mm in area and 4 mm thick. The TFM is mounted on
the cold finger with a base temperature of ∼3.5–4.5 K. The
sample can be attached directly to the TFM surface. A Hall
probe array (HPA) with seven sensors (model MULTI-7U
from AREPOC) is mounted on the TFM near the sample to
monitor the magnetic field during and after the activation process. Two calibrated cernox temperature sensors and a cartridge heater are installed to monitor and control the TFM
(and sample) temperature. The HPA can be read out using an
AD25PCI DAQ board and PciDAQ software from AREPOC.
For activation, the cryostat is translated into the RT bore of
the horizontal magnet and cooled down to 100 K, above the
Tc of YBCO. Next, the applied magnetic field is ramped up
to a desired field value followed by a cool down of the TFM
to the base temperature. At the base temperature the external magnetic field is ramped down to zero, leaving the TFM
magnetized. The entire activation process takes less than 2 h.
X-ray resonance exchange scattering (XRES) (Ref. 25)
studies were carried out on the 6-ID-B beamline at the
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Das et al.
Rev. Sci. Instrum. 83, 065103 (2012)
FIG. 2. Experimental setup for activation of TFMs. The cryostat is mounted on a translation stage. There are three feedthrough ports on the cryostat for electrical
connections for Hall probes, temperature sensors, and a heater. The inset shows the TFM (black square) of size 8 mm × 8 mm in area and 4 mm thick.
Advanced Photon Source. The measurements were carried
out at the Tb LIII (7.514 keV) edge with a large resonant
enhancement of the magnetic cross section. The monochromatic beam from a Si (111) double-bounce monochromator
was vertically focused using a mirror. The horizontal size of
the incident beam was determined by slits before the sample. A NaI-scintillator point detector was used for recording
scattering intensity of charge Bragg and magnetic superlattice peaks. However, no polarization analyses were necessary
to observe magnetic superlattice peaks. TbNi2 Ge2 crystals
were grown out of a high-temperature metallic solution.26, 27
A single crystal was oriented and cut in the shape of a 1 mm
× 1 mm square plate with c axis normal to the flat facet. The
plate was thinned down to ∼200 μm by polishing one side.
XRES measurements were performed in reflection geometry
on the unpolished as-grown surface. During these x-ray measurements the activation setup was moved inside the experimental station. The sample was attached to a TFM mounted
on the cryostat (Fig. 2) and used on the diffractometer to perform scattering measurements. For activation the TFM cryostat was removed from the diffractometer without interfering
with cooling. After magnetizing the TFM, the cryostat was
put back on the diffractometer in exactly the previous orientation. The sample realignment took about half an hour and
the x-ray measurements continued until a need to activate the
TFM to higher fields arose. Note that trapped fields were monitored using the HPA intermittently during the x-ray measurements in order to minimize heating of the sample.
III. RESULTS AND DISCUSSION
Preliminary measurements of trapped flux were done in
LN2 to check for overall quality of the field distribution.
These measurements were done soon after activation (typically within half an hour) of TFMs in LN2 using an electromagnet. Magnetization measurements were done by scanning
a single Hall probe over the TFM surface at a fixed distance
(i.e., z = 0.8 mm). The field distribution (Fig. 3) revealed a
well-defined central peak field characteristic of a single magnetized TFM without any signs of structural flaws such as
cracks, voids, multi-crystalline growth, or pinning center nonuniformity.15 The appearance of a rounded triangular-shaped
peak centered on the TFM implies that trapped fields are approaching the maximum value for this magnet at LN2 temperature. For the small square TFMs used, this value is less than
0.5 T at LN2 temperatures. The magnet was then installed in
the cryostat for low temperature measurements and x-ray use
(see inset in Fig. 2).
We have characterized several TFMs of the same size
and shape discussed above. Figure 4(a) shows a typical behavior of magnetic fields as a function of time through the
activation process and beyond. Application of the external
magnetic field of 3.5 T at 100 K, cool down of the TFM
to ∼4 K, followed by a ramp down to zero, and stability
of the trapped field over time are indicated by arrows in
Fig. 4(a). After the external field is ramped down to zero,
the Hall probe reading corresponds to a central field value at
z = 0.8 mm from the TFM surface due to trapped flux. In
this case the maximum trapped field measured by the array was ∼2.15 ± 0.05 T, which remained practically unaffected over the entire 16 h of measurements. Figure 4(b)
shows the trapped field on an expanded scale, which exhibits a well-known logarithmic decay due to flux creep, B(t)
= B(t0 ) − α ln( tt0 ), where α is a constant related to flux-flow
rate.28 However, the decay is slow and provides ample time
for x-ray scattering studies. We note that even a ∼3% decay,
which is practically insignificant for experiments of interest,
would take five days, a typical duration of x-ray scattering
experiments at a synchrotron facility.
The magnetic field gets larger on decreasing distance to
the TFM surface. The maximum magnetic field on the surface, based on our measurements at z = 0.8 mm and the model
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Rev. Sci. Instrum. 83, 065103 (2012)
FIG. 3. Magnetic field distribution at 0.8 mm above a TFM surface at 77 K. The profile was measured soon after activation. (Left) A single-dome field profile,
as expected for a single-grain bulk YBCO. (Right) Same data in a 2D color map.
as described in Ref. 16, is close to ∼2.8 T. Measurements of
the degree of uniformity of the magnetic field over the surface
of the TFM are shown in Fig. 4(c) for different trapped field
values (corresponding to the maximum of each curve). The
trapped field also decreases with radial distance away from
the center toward the edge of the TFM. The uniformity decreases with increasing trapped field value; for a trapped field
of 2.8 T on the surface, the field is uniform over an area of
∼2 mm × 2 mm. Figure 4(d) shows that the TFM is demagnetized by simply warming above Tc . Since warming reduces
the value of the trapped field, one can use this property as
a means of performing lower field measurements following
those at higher fields. One can warm up to an appropriate
temperature to reduce the field (e.g., 1 T) and cool down to
base temperature and repeat the process to lower fields, allowing one to carry out an entire experiment with a single
activation.
According to the results presented in Fig. 4 the trapped
magnetic field is stable over time and centrally uniform over
millimeter-length scales parallel to the TFM surface. We note
that typical x-ray beam sizes of ∼100 × 100 μm2 are routinely available on undulator beamlines of third-generation
synchrotron facilities. Furthermore, for resonant scattering
such as XRES and many high-resolution diffraction studies, x-ray penetration depths are of the order of μms (for
energies 3–15 keV) for numerous materials. Therefore, the
magnetic field of a TFM in the scattering volume of such
x-ray measurements is practically constant. Note that although in reflection geometry the scattering volume is near
the top surface of the crystal, for thin samples the probed
volume remains very close to the TFM surface. Magnetic
fields quoted for x-ray data below are those for the scattering
volume.
In order to illustrate the use of TFM for synchrotron experiments we have performed XRES studies on TbNi2 Ge2 ,
which is perhaps the most interesting member of the RNi2 Ge2
family of compounds.27, 29 Magnetic properties in this material arise from the Tb 4f moments located at the corners and at
the body center of the tetragonal crystal structure (I4/mmm)
with an easy axis of magnetization along the c direction. On
cooling below TN ∼ 17 K the material becomes an incommensurate (IC) amplitude modulated (AM) anti-ferromagnet
(AFM), characterized by magnetic scattering at τ 1 = (0, 0,
τ z ). TbNi2 Ge2 undergoes a second transition below Tt ∼ 9 K
into a commensurate structure described by a set of three
wave vectors (equal-moment, EM, phase). For both phases,
ordered magnetic moments are strictly along the c axis. In
applied magnetic field along the c axis at low temperatures
a rich phase diagram (Fig. 5) with clearly defined boundaries and the possible existence of several critical points have
been determined by thermodynamic measurements. However,
in order to access these “meta-magnetic” phases (MPs) one
needs to apply field along c and also have optical access for
x-ray magnetic scattering measurements along c* = [0, 0, 1],
which is the direction of the primary ordering vector τ 1 . Our
instrument, with the c axis of TbNi2 Ge2 orthogonal to the
TFM surface (i.e., B||c), offers an ideal geometry for such
measurements.
We performed the measurements at 0 T and 2.0 T magnetic fields at 4.5 K (below Tt ). Note that at 4.5 K, 2 T
should be sufficient to access meta-magnetic phase MPII,
which exists over a large field and temperature range. Relevant reciprocal-lattice scans are shown in Figs. 5(b) and 5(c).
As expected, in zero field the low-temperature EM phase
is characterized by τ 1 = (0, 0, 34 ), τ 2 = ( 21 , 12 , 0), and τ 3
= ( 21 , 12 , 12 ) wave vectors, as observed in [0, 0, L] and [ 12 , 12 , L]
scans. We have increased fields up to ∼1.5 T and observed
no changes in the EM phase, consistent with the phase diagram. However, at 2 T we were able to access a field-induced
meta-magnetic phase. At this field the same scans now
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Das et al.
Rev. Sci. Instrum. 83, 065103 (2012)
Trapped Field ( T @ z = 0.8 mm)
) b)
B(t) = B(t0) -
ln(t/t0)
t0 = 1 hour
2.08
B(t0) = 2.09 T
= 0.00853 T
2.07
2.06
2.05
1
10
100
t (hour)
FIG. 4. (a) Trapped magnetic field vs time measured with the HPA. Various steps in FC activation process and trapped field stability with time are indicated by
the arrows. (b) Data on an expanded scale for Hall probe 4, located at the center of the TFM and which records the maximum trapped field value. (c) Trapped
magnetic field as a function of radial distance from the TFM center. Different colored lines are for FC activation to different maximum values of trapped fields
at the center. Trapped field is practically constant around the center over an area of ∼2 mm × 2 mm. (d) TFM can be fully demagnetized by simply warming
above Tc . Measurements can be performed at lower fields by warming up to some T < Tc to reduce trapped field, followed by cooling, as indicated by arrows.
(a)
FIG. 5. (a) H-T phase diagram of TbNi2 Ge2 with magnetic field applied along the c axis (adapted from Ref. 27). The dashed–dotted line shows the H-T path
followed in the present study. Note the end points of the thick lines and the areas shaded by ovals where two or more phases coexist. (b) (0,0,L) scans at 0 T
(black dotted line) and at 2 T (red solid line) at 4.5 K. We reproduce previous observations of the 0 T phase, with ordering vector (0, 0, 34 ). At 2 T the ordering
vector is IC (0, 0, ∼0.765) with a weaker satellite at (0, 0, ∼0.796), as indicated by arrows. (c) ( 12 , 12 , L) scans at 0 T (black dotted line) and at 2 T field (red
solid line) at 4.5 K. EM phase magnetic peaks associated with ( 12 , 12 , 0) and ( 12 , 12 , 12 ) give way to higher order satellites as indicated by arrows.
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Das et al.
revealed profound changes to the wave vectors and their intensity. The EM structure τ 1 became IC (0, 0, ∼0.765) with the
appearance of a weaker τ 1 = (0, 0, ∼ 0.796) satellite,
while the half-order peaks characteristic of AFM planes were
strongly suppressed. The presence of a weak τ 2 peak and
the absence of τ 3 peaks indicate that this state is distinct from
the EM or the AM phases observed in zero field. Note that
since all the MP phases have net FM components, it is expected that the AFM superlattice peaks in general will decrease in intensity. Interestingly, the width of the stronger IC
peak was larger at 2 T, indicating that the correlation length
of the field-induced state also decreased compared to that of
the EM phase. This loss of correlation is in contrast to an
increase in correlation in the EM-to-AM phase transition at
Tt in zero field. Furthermore, weak peaks due to combinations of IC and τ 2 peaks appear, indicating a more complex
magnetic structure than the EM phase. We note that since we
cooled the sample in applied field, the zero-field IC phase may
persist down to lower temperatures (“supercooled”) than the
phase diagram in Fig. 5 suggests, effectively shifting the MP
boundaries. So, it is possible that two magnetic phases coexist, which appear to be distinct from EM and AM structures in
zero fields. The exact relationship of these two MPs observed
with XRES to MPI and MPII as identified using magnetization measurements remains to be clarified.
IV. CONCLUDING REMARKS
We have developed a paradigm-shifting approach to xray scattering studies in magnetic fields using trapped flux in
type-II superconductors. This novel method utilizing TFMs
uniquely facilitates photon-in and photon-out scattering experiments with unrestricted optical access over the entire
hemisphere and with a capability of applying magnetic fields
along the x-ray momentum transfer, respectively. It is easily portable due to the compact and lightweight design of the
TFM cryostat. These capabilities have been demonstrated by
XRES measurements of a field-induced meta-magnetic state
in TbNi2 Ge2 .
We note that our current instrument has a base temperature of ∼4 K with highest available magnetic field of ∼3 T
from a small TFM. However, the potential for larger fields
and further innovative applications is evident. With the use
of larger-sized TFMs, or better designed pinning centers,19
fields in excess of 20 T appear to eventually be within reach.
Additionally, a pair of TFMs stacked with a gap in between
or a TFM with an axial hole can offer conventional “splitpair” and “solenoid” geometries, should that be desirable. It
is possible to activate TFMs to perform higher field measurements first, followed by successive warming to appropriate
temperatures to reduce fields for lower field measurements.
Although one needs to keep track of field-temperature history
of the sample, this method offers a wide field range with a
single activation of the TFM. Finally, it is possible to independently cool the TFM and sample by placing them on two
separate thermal stages of a single cryostat with the sample
mounted on a linear positioner to adjust its distance from the
TFM surface. Such a scheme is being implemented, which
will enable one to keep the sample at any desired temperature
Rev. Sci. Instrum. 83, 065103 (2012)
and change magnetic field simply by varying sample distance
while the TFM remains magnetized during the course of the
experiment.
ACKNOWLEDGMENTS
We appreciate having fruitful discussions with G.
Shenoy, U. Welp, B. W. Veal, and D. Robinson of Argonne
National Laboratory, and with D. Parks of University of
Houston (UH). The work performed at the Advanced Photon Source was supported by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences
under Contract No. DE-AC02-06CH11357. Work at UH on
TFM development is supported by The Welch Foundation
(Grant No. E-1380), the U.S. Army Research Office (USARO), and the State of Texas via the Texas Center for Superconductivity at UH. Work at Ames Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering. Ames Laboratory is operated for the U.S. Department
of Energy by Iowa State University under Contract No. DEAC02-07CH11358.
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