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2012

A novel approach for x-ray scattering experiments inmagnetic fields utilizing trapped flux in type-IIsuperconductorsR. K. DasArgonne National Laboratory

Z. IslamArgonne National Laboratory

J. P. C. RuffArgonne National Laboratory

R. P. SawhUniversity of Houston

R. WinsteinUniversity of Houston

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AuthorsR. K. Das, Z. Islam, J. P. C. Ruff, R. P. Sawh, R. Winstein, Paul C. Canfield, J.-W. Kim, and J. C. Lang

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A novel approach for x-ray scattering experiments in magnetic fields utilizing trappedflux in type-II superconductorsR. 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 Articles you may be interested in Flux-cutting and flux-transport effects in type-II superconductor slabs in a parallel rotating magnetic field Low Temp. Phys. 37, 947 (2011); 10.1063/1.3672157 A portable high-field pulsed-magnet system for single-crystal x-ray scattering studies Rev. Sci. Instrum. 80, 113902 (2009); 10.1063/1.3251273 Voltage from mechanical stress in type-II superconductors: Depinning of the magnetic flux by movingdislocations Appl. Phys. Lett. 93, 042503 (2008); 10.1063/1.2960337 Synchrotron Xray Powder Diffraction and Absorption Spectroscopy in Pulsed Magnetic Fields with MillisecondsDuration AIP Conf. Proc. 902, 103 (2007); 10.1063/1.2723633 Synchrotron x-ray powder diffraction studies in pulsed magnetic fields Rev. Sci. Instrum. 77, 063903 (2006); 10.1063/1.2216914

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REVIEW OF SCIENTIFIC INSTRUMENTS 83, 065103 (2012)

A novel approach for x-ray scattering experiments in magneticfields 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

1X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave.,Argonne, Illinois 60439, USA2Department of Physics, Texas Center for Superconductivity, University of Houston, Houston,Texas 77004, USA3Department 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 exploitingvortices in superconductors. This method is based on trapping magnetic flux in a small disk-shapedsuperconductor (known as a trapped field magnet, TFM) with a single-crystal sample mounted on orat close proximity to its surface. This opens an unrestricted optical access to the sample and allowsmagnetic fields to be applied precisely along the x-ray momentum transfer, facilitating polarization-sensitive experiments that have been impractical or impossible to perform to date. The TFMs usedin our study remain stable and provide practically uniform magnetic fields for days, which are suf-ficient for comprehensive x-ray diffraction experiments, specifically x-ray resonance exchange scat-tering (XRES) to study field-induced phenomena at a modern synchrotron source. The TFM instru-ment has been used in a “proof-of-principle” XRES study of a meta-magnetic phase in a rare-earthcompound, TbNi2Ge2, 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 propertiesare diverse and important to study from basic-science andapplied-research points of view.1–5 There are numerousmagnet research laboratories and academic research groupsworldwide focused on studies of materials in magnetic fields.With the advent of modern synchrotron x-ray sources, sig-nificant efforts have also been invested to study field-inducedphenomena with x-rays.6–13 While these efforts have ledto many applications of x-ray spectroscopic and imagingtechniques to study field effects in materials,6–8 progress inx-ray scattering studies of single-crystal samples in mag-netic fields with split-pair and solenoid magnets have beenrelatively slower due to two long-standing obstacles. First,optical access required for scattering studies is restricted dueto magnet geometries. Second, it is practically impossibleto apply magnetic fields along the x-ray momentum transfer(Q), which is frequently an ideal scattering geometry. In aconventional split-pair magnet, the magnetic field is strictlyperpendicular to the scattering plane, whereas in the solenoidmagnet the magnetic field is typically in the scattering planebut nearly orthogonal to Q (with the exception of backscattering). A new approach that overcomes these geometriclimitations would enable one to thoroughly study any arbi-trarily ordered magnetic and orbital structure, allow completeexploitation of polarization-sensitive cross section by angulardependence measurements about Q (with B||Q), and allowprecise determination of pure longitudinal magnetostriction

a)Author to whom correspondence should be addressed. Electronic mail:zahir@aps.anl.gov.

effects. We report efforts at the Advanced Photon Sourceto develop such a magnet instrument that exploits magneticproperties of bulk superconductors (SC) with the potential toreach applied fields beyond 17 T.14

It is well known that type-II superconductors can trapmagnetic flux in the so-called vortex state.15 If the trappedflux is stable over time then the magnetized superconduc-tor can be used as a “permanent” magnet (known as trappedfield/flux magnet, TFM). Worldwide research in this area hasaccelerated16 since the discovery of high-temperature super-conductors over two decades ago, and recent breakthroughsin the understanding of pinning centers (used to trap the flux-oids in place)17–19 have made practical applications of super-conducting disks as strong permanent magnets conceivable.The ability to grow high-quality large melt-textured high-Tc

superconductors (e.g., YBa2Cu3O7 (YBCO) and other relatedcompounds) and to control the introduction of strong pinningcenters in them by chemical18 or irradiation17, 19 methods havereached a high degree of sophistication. The texturing processorients a preferred crystalline axis (e.g., c axis) of the micro-grains in the superconductor, allowing high critical currentdensity as well as increasing fracture toughness, which is cru-cial for withstanding Maxwell stress of the trapped flux. In-deed it has been shown that a superconducting disk (∼27 mmdiameter and 15 mm thick) is capable of trapping magneticfields in excess of 17 T,14 a result that foresees availabilityof magnetic fields beyond those available from commercialsplit-pair magnets for scattering studies.

We have developed a melt-textured YBCO-based TFMinstrument in order to circumvent the two long-standing im-pediments to x-ray scattering studies discussed above. Inthis approach a YBCO SC disk is magnetized by trapping

0034-6748/2012/83(6)/065103/7/$30.00 © 2012 American Institute of Physics83, 065103-1

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065103-2 Das et al. Rev. Sci. Instrum. 83, 065103 (2012)

FIG. 1. Sample mounting scheme. In this case the sample is attached di-rectly to the surface of the TFM. Magnetic field (B) direction is normal to thesample surface as indicated by a vertical arrow.

quantized magnetic flux on pinning centers. In the moststraightforward configuration, as shown in Fig. 1, a tinysingle-crystal sample is mounted directly on the TFM surface.The advantages of this geometry are clear: (1) optical access isover the entire hemisphere, and (2) magnetic fields can be ap-plied along the x-ray momentum transfer (B||Q) in the sym-metrical reflection geometry.20 In addition, the TFM is com-pact, and it can be cooled with a small closed-cycle (or liquidHe flow) cryostat for use in any x-ray experimental stations atmodern synchrotron beamlines. Since the magnetic fields of aTFM are confined within close proximity of the cryostat coldfinger, no significant stray fields are present to interfere withmotors 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 ofelectronic, magnetic, orbital, and structural properties in mag-netic fields. In Sec. III, we report characterization measure-ments of TFMs and results of a preliminary x-ray magneticscattering study on TbNi2Ge2 demonstrating the use of theinstrument.

II. EXPERIMENTAL DETAILS

The core of this instrument is a YBCO-based TFM grownby a melt-textured method. Platinum (Pt) powders were in-troduced to create flux pinning centers. During melt textur-ing they react with Y2BaCuO5 impurities (a by-product of thegrowth) to reduce their size and refine their shapes, which arebelieved to be responsible for their very high trapped mag-netic fields and critical current density properties. The TFMswere cut to desired sizes, and their trapped field profile wasmeasured at liquid nitrogen (LN2) temperature to check forsignatures of structural flaws before being incorporated intothe TFM instrument for further characterization and x-raymeasurements. TFMs produced in this way18 achieve fieldsof about 0.5 T at LN2 temperatures in a TFM 20 mm in diam-eter and 8 mm thick. Other processes provide fields of, e.g.,2.0 T at similar size and temperature.17 Both currentdensity and trappable field increase rapidly at lower temper-atures. Present results19 indicate that further pinning centerdevelopment will make possible 8-T TFMs at this size andtemperature.

The primary instrumentation consists of (1) a small cryo-stat for the TFM and sample, (2) cryogenic Hall probes, and(3) an external superconducting magnet. For x-ray scatter-ing studies, the TFM first needs to be magnetized and thenkept 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 pump-ing are desirable, at present these techniques remain an activefield of research on their own right.21–24 Commonly, an exter-nal resistive or superconducting magnet is used with a TFMcooled either in an applied field (field cooled, FC), or in zerofield with subsequent application of fields at low temperatures(zero-field cooled, ZFC). While we are able to employ a ZFCmethod for activating a TFM using our setup described be-low, there is a significant drawback: applied field needs to beat least twice the maximum field that a TFM can trap in orderto guarantee full flux penetration into the bulk. In this paperwe describe the implementation of the FC method because itensures a uniform flux penetration into the bulk, the maxi-mum field trapped for a given TFM being limited only by themaximum field available from the external magnet.

The experimental setup for activating a TFM is shownin Fig. 2. A “cryogen-free” superconducting split-pair mag-net is used for applying external fields. The maximum fieldat gap center is 4 T, while at the center of each coil of thepair the field reaches ∼5 T, which in principle sets an upperfield limit for activation. However, the outer vacuum jacket ofthis horizontal magnet was modified to install a room temper-ature (RT) insert into the magnet bore, restricting the magnetto operate at or below ∼3.5 T. A small light-weight cryostat,model SRDK-101 from Sumitomo Heavy Industries, is usedfor cooling the TFM and sample. The TFM cryostat has a nar-row vacuum shroud around the cold finger that fits into the RTinsert. During a TFM activation the cryostat is mounted on atranslation stage such that it can slide inside the RT bore ofthe external magnet (Fig. 2) and be positioned at the mag-net center where the volume of field uniformity is the largest.The magnetic field of the external magnet is in the horizon-tal plane and along the axis of the TFM cryostat. The insetof Fig. 2 shows a small TFM (black square) of size 8 mm× 8 mm in area and 4 mm thick. The TFM is mounted onthe cold finger with a base temperature of ∼3.5–4.5 K. Thesample can be attached directly to the TFM surface. A Hallprobe array (HPA) with seven sensors (model MULTI-7Ufrom AREPOC) is mounted on the TFM near the sample tomonitor the magnetic field during and after the activation pro-cess. Two calibrated cernox temperature sensors and a car-tridge heater are installed to monitor and control the TFM(and sample) temperature. The HPA can be read out using anAD25PCI DAQ board and PciDAQ software from AREPOC.For activation, the cryostat is translated into the RT bore ofthe horizontal magnet and cooled down to 100 K, above theTc of YBCO. Next, the applied magnetic field is ramped upto a desired field value followed by a cool down of the TFMto the base temperature. At the base temperature the exter-nal magnetic field is ramped down to zero, leaving the TFMmagnetized. 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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065103-3 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 electricalconnections 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 carriedout at the Tb LIII (7.514 keV) edge with a large resonantenhancement of the magnetic cross section. The monochro-matic beam from a Si (111) double-bounce monochromatorwas vertically focused using a mirror. The horizontal size ofthe incident beam was determined by slits before the sam-ple. A NaI-scintillator point detector was used for recordingscattering intensity of charge Bragg and magnetic superlat-tice peaks. However, no polarization analyses were necessaryto observe magnetic superlattice peaks. TbNi2Ge2 crystalswere 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. Theplate was thinned down to ∼200 μm by polishing one side.XRES measurements were performed in reflection geometryon the unpolished as-grown surface. During these x-ray mea-surements the activation setup was moved inside the experi-mental station. The sample was attached to a TFM mountedon the cryostat (Fig. 2) and used on the diffractometer to per-form scattering measurements. For activation the TFM cryo-stat was removed from the diffractometer without interferingwith cooling. After magnetizing the TFM, the cryostat wasput back on the diffractometer in exactly the previous orien-tation. The sample realignment took about half an hour andthe x-ray measurements continued until a need to activate theTFM to higher fields arose. Note that trapped fields were mon-itored using the HPA intermittently during the x-ray measure-ments in order to minimize heating of the sample.

III. RESULTS AND DISCUSSION

Preliminary measurements of trapped flux were done inLN2 to check for overall quality of the field distribution.These measurements were done soon after activation (typi-cally within half an hour) of TFMs in LN2 using an electro-magnet. 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 awell-defined central peak field characteristic of a single mag-netized TFM without any signs of structural flaws such ascracks, voids, multi-crystalline growth, or pinning center non-uniformity.15 The appearance of a rounded triangular-shapedpeak centered on the TFM implies that trapped fields are ap-proaching the maximum value for this magnet at LN2 temper-ature. For the small square TFMs used, this value is less than0.5 T at LN2 temperatures. The magnet was then installed inthe cryostat for low temperature measurements and x-ray use(see inset in Fig. 2).

We have characterized several TFMs of the same sizeand shape discussed above. Figure 4(a) shows a typical be-havior of magnetic fields as a function of time through theactivation process and beyond. Application of the externalmagnetic field of 3.5 T at 100 K, cool down of the TFMto ∼4 K, followed by a ramp down to zero, and stabilityof the trapped field over time are indicated by arrows inFig. 4(a). After the external field is ramped down to zero,the Hall probe reading corresponds to a central field value atz = 0.8 mm from the TFM surface due to trapped flux. Inthis case the maximum trapped field measured by the ar-ray was ∼2.15 ± 0.05 T, which remained practically un-affected over the entire 16 h of measurements. Figure 4(b)shows the trapped field on an expanded scale, which ex-hibits a well-known logarithmic decay due to flux creep, B(t)= B(t0) − α ln( t

t0), where α is a constant related to flux-flow

rate.28 However, the decay is slow and provides ample timefor 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 scatteringexperiments at a synchrotron facility.

The magnetic field gets larger on decreasing distance tothe TFM surface. The maximum magnetic field on the sur-face, based on our measurements at z = 0.8 mm and the model

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065103-4 Das et al. 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 ofthe degree of uniformity of the magnetic field over the surfaceof the TFM are shown in Fig. 4(c) for different trapped fieldvalues (corresponding to the maximum of each curve). Thetrapped field also decreases with radial distance away fromthe center toward the edge of the TFM. The uniformity de-creases with increasing trapped field value; for a trapped fieldof 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 demag-netized by simply warming above Tc. Since warming reducesthe value of the trapped field, one can use this property asa means of performing lower field measurements followingthose at higher fields. One can warm up to an appropriatetemperature to reduce the field (e.g., 1 T) and cool down tobase temperature and repeat the process to lower fields, al-lowing one to carry out an entire experiment with a singleactivation.

According to the results presented in Fig. 4 the trappedmagnetic field is stable over time and centrally uniform overmillimeter-length scales parallel to the TFM surface. We notethat typical x-ray beam sizes of ∼100 × 100 μm2 are rou-tinely available on undulator beamlines of third-generationsynchrotron facilities. Furthermore, for resonant scatteringsuch as XRES and many high-resolution diffraction stud-ies, x-ray penetration depths are of the order of μms (forenergies 3–15 keV) for numerous materials. Therefore, themagnetic field of a TFM in the scattering volume of suchx-ray measurements is practically constant. Note that al-though in reflection geometry the scattering volume is nearthe top surface of the crystal, for thin samples the probedvolume remains very close to the TFM surface. Magneticfields quoted for x-ray data below are those for the scatteringvolume.

In order to illustrate the use of TFM for synchrotron ex-periments we have performed XRES studies on TbNi2Ge2,

which is perhaps the most interesting member of the RNi2Ge2

family of compounds.27, 29 Magnetic properties in this mate-rial arise from the Tb 4f moments located at the corners and atthe body center of the tetragonal crystal structure (I4/mmm)with an easy axis of magnetization along the c direction. Oncooling below TN ∼ 17 K the material becomes an incom-mensurate (IC) amplitude modulated (AM) anti-ferromagnet(AFM), characterized by magnetic scattering at τ 1 = (0, 0,τ z). TbNi2Ge2 undergoes a second transition below Tt ∼ 9 Kinto a commensurate structure described by a set of threewave vectors (equal-moment, EM, phase). For both phases,ordered magnetic moments are strictly along the c axis. Inapplied magnetic field along the c axis at low temperaturesa rich phase diagram (Fig. 5) with clearly defined bound-aries and the possible existence of several critical points havebeen determined by thermodynamic measurements. However,in order to access these “meta-magnetic” phases (MPs) oneneeds to apply field along c and also have optical access forx-ray magnetic scattering measurements along c* = [0, 0, 1],which is the direction of the primary ordering vector τ 1. Ourinstrument, with the c axis of TbNi2Ge2 orthogonal to theTFM surface (i.e., B||c), offers an ideal geometry for suchmeasurements.

We performed the measurements at 0 T and 2.0 T mag-netic fields at 4.5 K (below Tt). Note that at 4.5 K, 2 Tshould be sufficient to access meta-magnetic phase MPII,which exists over a large field and temperature range. Rele-vant reciprocal-lattice scans are shown in Figs. 5(b) and 5(c).As expected, in zero field the low-temperature EM phaseis characterized by τ 1 = (0, 0, 3

4 ), τ 2 = ( 12 , 1

2 , 0), and τ 3

= ( 12 , 1

2 , 12 ) wave vectors, as observed in [0, 0, L] and [ 1

2 , 12 , L]

scans. We have increased fields up to ∼1.5 T and observedno changes in the EM phase, consistent with the phase dia-gram. However, at 2 T we were able to access a field-inducedmeta-magnetic phase. At this field the same scans now

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065103-5 Das et al. Rev. Sci. Instrum. 83, 065103 (2012)

1 10 100

2.05

2.06

2.07

2.08

B(t) = B(t0) - ln(t/t

0)

t0 = 1 hour

B(t0) = 2.09 T

= 0.00853 T

Tra

pped

Fie

ld (

T @

z =

0.8

mm

)

t (hour)

b))

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 bythe 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) Trappedmagnetic field as a function of radial distance from the TFM center. Different colored lines are for FC activation to different maximum values of trapped fieldsat 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 warmingabove 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 TbNi2Ge2 with magnetic field applied along the c axis (adapted from Ref. 27). The dashed–dotted line shows the H-T pathfollowed 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, 3

4 ). At 2 T the orderingvector is IC (0, 0, ∼0.765) with a weaker satellite at (0, 0, ∼0.796), as indicated by arrows. (c) ( 1

2 , 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 , 1

2 , 0) and ( 12 , 1

2 , 12 ) give way to higher order satellites as indicated by arrows.

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065103-6 Das et al. Rev. Sci. Instrum. 83, 065103 (2012)

revealed profound changes to the wave vectors and their inten-sity. The EM structure τ 1 became IC (0, 0, ∼0.765) with theappearance of a weaker τ ′

1 = (0, 0,∼ 0.796) satellite,while the half-order peaks characteristic of AFM planes werestrongly suppressed. The presence of a weak τ 2 peak andthe absence of τ 3 peaks indicate that this state is distinct fromthe EM or the AM phases observed in zero field. Note thatsince all the MP phases have net FM components, it is ex-pected that the AFM superlattice peaks in general will de-crease in intensity. Interestingly, the width of the stronger ICpeak was larger at 2 T, indicating that the correlation lengthof the field-induced state also decreased compared to that ofthe EM phase. This loss of correlation is in contrast to anincrease in correlation in the EM-to-AM phase transition atTt in zero field. Furthermore, weak peaks due to combina-tions of IC and τ 2 peaks appear, indicating a more complexmagnetic structure than the EM phase. We note that since wecooled the sample in applied field, the zero-field IC phase maypersist down to lower temperatures (“supercooled”) than thephase diagram in Fig. 5 suggests, effectively shifting the MPboundaries. So, it is possible that two magnetic phases coex-ist, which appear to be distinct from EM and AM structures inzero fields. The exact relationship of these two MPs observedwith XRES to MPI and MPII as identified using magnetiza-tion measurements remains to be clarified.

IV. CONCLUDING REMARKS

We have developed a paradigm-shifting approach to x-ray scattering studies in magnetic fields using trapped flux intype-II superconductors. This novel method utilizing TFMsuniquely facilitates photon-in and photon-out scattering ex-periments with unrestricted optical access over the entirehemisphere and with a capability of applying magnetic fieldsalong the x-ray momentum transfer, respectively. It is eas-ily portable due to the compact and lightweight design of theTFM cryostat. These capabilities have been demonstrated byXRES measurements of a field-induced meta-magnetic statein TbNi2Ge2.

We note that our current instrument has a base tempera-ture of ∼4 K with highest available magnetic field of ∼3 Tfrom a small TFM. However, the potential for larger fieldsand further innovative applications is evident. With the useof 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 betweenor a TFM with an axial hole can offer conventional “split-pair” and “solenoid” geometries, should that be desirable. Itis possible to activate TFMs to perform higher field measure-ments first, followed by successive warming to appropriatetemperatures to reduce fields for lower field measurements.Although one needs to keep track of field-temperature historyof the sample, this method offers a wide field range with asingle activation of the TFM. Finally, it is possible to inde-pendently cool the TFM and sample by placing them on twoseparate thermal stages of a single cryostat with the samplemounted on a linear positioner to adjust its distance from theTFM surface. Such a scheme is being implemented, whichwill enable one to keep the sample at any desired temperature

and change magnetic field simply by varying sample distancewhile the TFM remains magnetized during the course of theexperiment.

ACKNOWLEDGMENTS

We appreciate having fruitful discussions with G.Shenoy, U. Welp, B. W. Veal, and D. Robinson of ArgonneNational Laboratory, and with D. Parks of University ofHouston (UH). The work performed at the Advanced Pho-ton Source was supported by the U.S. Department of Energy(DOE), Office of Science, Office of Basic Energy Sciencesunder Contract No. DE-AC02-06CH11357. Work at UH onTFM development is supported by The Welch Foundation(Grant No. E-1380), the U.S. Army Research Office (US-ARO), and the State of Texas via the Texas Center for Su-perconductivity at UH. Work at Ames Laboratory was sup-ported by the U.S. Department of Energy, Office of Basic En-ergy Science, Division of Materials Sciences and Engineer-ing. Ames Laboratory is operated for the U.S. Departmentof Energy by Iowa State University under Contract No. DE-AC02-07CH11358.

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