combinatorial synthesis and evaluation of functional inorganic

IntroductionThe discovery of novel functional mate-

rials can be efficiently accomplished usinghigh-throughput thin-film fabricationtechniques coupled with rapid characteri-zation schemes. Combinatorial thin-film“libraries” have been prepared using dis-crete, sequentially masked depositions,codeposited composition spreads, andcomposition-gradient molecular layerepitaxy. Measurement techniques havebeen adapted for quick evaluation ofthe libraries and composition spreads,mapping a variety of physical charac-teristics including optical, electrical, andmagnetic properties. High-throughputinvestigation of thin-film materials has

already led to the discovery of excitingnew and useful dielectric (Zr0.2Sn0.2Ti0.6O2;Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3) and mag-netic (Co-doped TiO2) materials. It has alsofacilitated investigations of the detailedcomposition dependence of properties insystems that are already known but havebeen incompletely studied. In this article,we review successful discoveries and ap-plications of these combinatorial materialslibraries.

Thin-Film Library SynthesisThe effort to directly transfer the or-

ganic combinatorial synthesis concept toinorganic thin-film materials was started

by a group at the Lawrence Berkeley Na-tional Laboratory.1 In adopting this syn-thetic approach to simultaneously creatinglarge arrays of thin-film samples, the pre-cursor deposition method was used. Thisapproach can truly bring out the combina-torial nature of the synthesis because it en-ables the production of a large number ofdifferent compositions by generating manypermutations of precursors. On a sub-strate as small as 1 cm2, thousands ofdifferent compositions can be integrated,synthesized, and screened for desiredphysical properties in a single experiment.This method takes advantage of the factthat in many materials systems it is pos-sible to grow stoichiometric compoundsby means of controlled thermal diffusionof precursors. For fabricating YBa2Cu3O7,for instance, one can use amorphous lay-ers of Y2O3, BaF2, and CuO2 as precursors.A series of precisely positioned shadowmasks, which allow spatially selectivedepositions, are used to deposit differentcombinations of precursors at differentpositions on a substrate at room tempera-ture in order to generate diverse composi-tional variation across a library. For instance,in the quaternary combinatorial maskingscheme (shown in Figure 1), each mask isused in up to four depositions on a squaresubstrate of the same size with four dif-ferent orientations (rotated 90� from eachother). After the first mask is used, fourquadrants have four different materialsdeposited through the open windows.Going from one mask to the next, the re-gions of openings (windows) continue tobe subdivided into further quadrants. Inthis way, using n masks, after carrying out4 � n depositions, 4n discrete combinationsof precursors can be created on a givenchip. Sputtering, evaporation, and pulsedlaser deposition are some of the physicalvapor deposition techniques that haveproven to be useful for precursor deposi-tion. Following the deposition step, librariesundergo controlled thermal treatments to(1) diffuse and mix the precursors and(2) crystallize compounds (if desired) ateach site.

The utility and effectiveness of combi-natorial synthesis using the precursormethod was first demonstrated withhigh-Tc superconductors.1 Soon after thisfirst proof of principle, a combinatorial li-brary was used to identify a class of cobaltoxide magnetoresistive materials of theform (La1�xSrx)CoO3.2

Because the formation of phases throughpostannealing relies largely on thermo-dynamic equilibrium, the precursor methodcannot be considered a universal solution.The success and integrity of this methoddepend on how well one can map the trend

MRS BULLETIN/APRIL 2002 301

CombinatorialSynthesis andEvaluation ofFunctionalInorganic MaterialsUsing Thin-FilmTechniques

Ichiro Takeuchi, Robert Bruce van Dover,and Hideomi Koinuma

AbstractNovel phases of functional inorganic chemical systems can be efficiently explored

using high-throughput thin-film fabrication techniques coupled with rapid characterizationschemes. High-throughput investigation of thin-film materials has already led to thediscovery of new dielectric and magnetic materials. In this article, we review varioushigh-throughput thin-film synthesis/evaluation techniques and discuss examples ofexciting discoveries and new applications of combinatorial techniques.

Keywords: advanced dielectrics, combinatorial methods, composition spreads,concurrent x-ray diffractometer, thin films, laser molecular-beam epitaxy, scanningevanescent microwave probe, transparent magnetic materials.

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with which the compositional variationchanges the physical properties across thelibrary. Applicability of the precursormethod to a specific materials systemneeds to be determined on a case-by-casebasis, and it depends on the degree towhich defects affect the macroscopicphysical property of interest.

This method has been demonstrated toproduce predominantly single-phase epi-taxial films on lattice-matched substratesas determined by x-ray diffraction.3–5 High-resolution cross-sectional transmissionelectron microscopy (TEM) studies havebeen performed on Ba1�xSrxTiO3 filmsmade from BaF2, SrF2, and TiO2 pre-cursors.6 They reveal that the resultingfilms do indeed consist of epitaxial grainswith atomically sharp interfaces with the(100) LaAlO3 substrate.

Codeposition from two or more spa-tially separated sources can be used to ob-tain samples with a natural compositiongradient. The crucial advantage of co-deposition is that the material is inti-mately mixed in the as-deposited state, soit is possible to prepare metastable andlow-temperature phases that are not ac-cessible by any other route. Codepositionschemes can be problematic, as the com-position is not directly controlled andcompositions must be inferred indirectly.However, it is generally possible to modelthe position dependence of the depositionrate from the various sources, and com-

positions accurate to �5 at.% can be cal-culated. Energy-dispersive spectroscopy,wavelength-dispersive spectroscopy, andRutherford backscattering spectroscopyare some of the techniques used for ob-taining compositional mapping.

In view of the fact that every crystallinematerial is composed of molecular layersstacked periodically and that modernelectronic devices have nanoscale layerstructures,7 it is desirable to develop ascheme for the parallel fabrication oflayered structures so that we can quicklyfind the best matching of layered latticesand optimum fabrication conditions. TheKoinuma group implemented the methodsof combinatorial laser molecular-beamepitaxy (CLMBE)8,9 to create artificial oxidelattices and heterojunctions, with each layercomposition, thickness, and sequencecontrolled at the atomic scale. Molecularlayers are deposited from several kinds ofsolid sources through a series of physicalshadow masks placed on a heated sub-strate, and the deposited materials arecrystallized in as-grown states.

The effectiveness of combinatorial latticeintegration using the CLMBE system wasdemonstrated in the concurrent fabrica-tion of a set of SrTiO3/BaTiO3 (STO/BTO)superlattices with equimolar ratios anddifferent periodicities at an oxygen pres-sure of 1.0 � 10�6 Torr and a substratetemperature of 700�C.10,11 The use of chemi-cally etched SrTiO3(001) substrates with

atomically flat surfaces enabled observationof clear reflection high-energy electrondiffraction (RHEED) intensity oscillations,each of which corresponded to the growthof a unit-cell layer (0.4 nm) of each oxide.Synchronization of the RHEED beamsweeping across the substrate surface witha pair of coils and image acquisition werecombined with moving physical masks tosimultaneously control the growth of asmany as 10 stripe-patterned thin films inparallel on a single substrate. RHEEDtraces for the growth of three differentperiodicities are shown in Figure 2. Incor-porating 90� substrate rotation can enablefabrication of as large as a 10 � 10 matrixof superlattices.

Rapid Screening of PhysicalProperties

The scope of the materials systems andproperties that can be addressed is oftenconstrained by the availability and capa-bilities of high-throughput characteriza-tion techniques. These include parallelmeasurement/detection systems or variousscanning probe microscopes, dependingon the specific physical property of inter-est. Many well-established technologiescan be used to map compositions andphysical properties. Here we discuss twoinnovative technologies.

The scanning evanescent microwavemicroscope has proven to be a powerfulcharacterization tool that can providequantitative electrical-impedance mappingof a variety of thin-film libraries.3,4,12–15 Itconsists of a �/4 (quarter-wave) Cu co-axial resonator with a tapered centerconductor. A sharp metal tip mounted onthe center conductor extends beyond anaperture in the center of the end wall(Figure 3). As the resonator-tip assembly isscanned over a sample surface, changes inthe resonant frequency f and the qualityfactor Q (the ratio of the energy-storing ca-pability to energy-dissipating capability)of the resonator caused by the electro-magnetic near-field interaction of thetip with the local sample surface are re-corded to obtain images. Using a quasi-static electric-field-distribution calculation,changes in f and Q can be directly con-verted to real and imaginary parts of thecomplex dielectric constant.13 With itsability to nondestructively measure di-electric constant and sheet resistance,the microscope has been used for study-ing ferroelectric/dielectric materials ingeneral and superconductors.14 Variabletemperature and multifrequency meas-urement features can be added to expandthe capabilities and the scope of investiga-tion using the microscope.15

302 MRS BULLETIN/APRIL 2002

Combinatorial Synthesis and Evaluation of Inorganic Materials Using Thin-Film Techniques

Figure 1. (a)–(e) Shadow mask patterns used for quaternary design of combinatoriallibraries. Each mask is used in up to four depositions on a square substrate of the samesize with four different orientations rotated 90� from each other. After the first mask is used,four quadrants have four different materials deposited through the open windows. Movingfrom one mask to the next, the regions of openings (windows) continue to be subdividedinto further quadrants. In this way, using n masks, after carrying out 4 � n depositions,4n discrete combinations of precursors can be created on a given chip.

Structural characterization of librariesand spreads by x-ray diffraction is an im-portant part of combinatorial experiments.One way to perform rapid x-ray charac-terization is to use a focused synchrotronx-ray microbeam. This has been success-fully used for rapid phase mapping andcomposition mapping (using fluorescence)of combinatorial libraries.16,17 For in-houseapplications, a concurrent x-ray diffrac-tometer has been developed,18 which con-sists of a convergent x-ray beam source(Cu K�1) with a dispersion of 2� formed bya rotating anode x-ray source and properx-ray optics (including a cylindrical, Johann-type mirror for converging x-rays), a con-ventional powder diffraction goniometer,and a two-dimensional detector (imagingplate or charge-coupled device, Figure 4a).

Figures 4b and 4c are from a concurrentx-ray diffractometer measurement, wherethe beam was focused into a 10 mm � 1 mmline on an integrated superlattice chip togenerate a two-dimensional-intensity imageon the detector. On this chip, there were10 superlattices: [(SrTiO3)m/(BaTiO3)n]30(m � n � 12, 14, 16, . . . , 28, and 30) (Fig-ure 4d). In addition to the specularly re-flected band near 0.70� and the reflectedSrTiO3(001) band near 22.75�, one clearlyobserves reflection lines at the angles



Figure 2. Fabrication of an atomically controlled materials library. (a) Schematic setup forparallel growth of epitaxial superlattices. (b) Simultaneously obtained reflection high-energyelectron diffraction (RHEED) intensities versus time for three SrTiO3/BaTiO3 superlatticesfabricated on a single atomically controlled materials library chip.The deposition sequenceis shown below the graph.

Figure 3. Schematic illustration of ascanning evanescent microwavemicroscope consisting of a Cu coaxialresonator with a tapered centerconductor. A sharp metal tip mountedon the center conductor extendsbeyond an aperture in the center of theend wall. As the resonator-tip assemblyis scanned over a sample surface,changes in the resonant frequency fand the quality factor Q of the resonatorcaused by the electromagneticnear-field interaction of the tip with thelocal sample surface are recorded toobtain images.

corresponding to the designed perio-dicities of the individual superlattices.The line profiles extracted from the[(SrTiO3)12/(BaTiO3)12]30 diffraction pat-terns are also shown in Figures 4e and 4f.

A similar microdiffractometer with apoint x-ray source (diameter of down to50 �m) has also been used in two-dimensional phase mapping of librariesand spreads.15

New Compositions of Ferroelectricand Dielectric Materials

Thin films of (Ba, Sr)TiO3 are widelypursued for embedded dynamic random-access memory (DRAM) capacitors as wellas for various tunable microwave deviceapplications. The precursor method usingthe quaternary masking scheme was suc-cessfully implemented for fabricating li-braries with discrete patterns for systematicdoping studies into (Ba, Sr)TiO3; W wasidentified as a metallic dopant that helpsreduce the dielectric loss at microwavefrequencies.3 Compared with undopedBaTiO3, W doping decreases microwaveloss by a factor of �4 at 1 GHz. The effectof W doping has since been confirmed byother groups who have fabricated doped(Ba, Sr)TiO3 films using conventional thin-film deposition techniques.19 In a relatedstudy, capacitor device libraries incorpo-rating all epitaxial multilayer structuresinto a compositionally varying back-ground were fabricated, and W dopingwas also found to reduce the dc leakagecurrent in capacitors.5 It is believed that asmall trace of W (up to �2 mol%) intro-duced in the host substitutes on the Ti siteand reduces the quantity of oxygen va-cancies formed in the crystallization proc-ess of (Ba, Sr)TiO3, thus reducing theoverall loss of the compound.

An automated gliding shutter was usedto make a continuous compositional spreadfabricated from precursors, where the(Ba,Sr,Ca)TiO3 ternary system was mappedon a triangular chip of LaAlO3(100)(Figure 5).4 In order to lay out theternary phase diagram, three gradientdepositions were performed in three dif-ferent directions for three precursors,BaCO3, CaCO3, and SrCO3. A large frac-tion of this phase diagram had neverbeen produced in either bulk or thinfilms prior to this study. The imagesshown in Figure 5 are dielectric-constantimages obtained as a result of scanningevanescent microwave microscopy per-formed on the chip. Figure 5a is an � plot,and Figure 5b is a tan plot. From thisstudy, a new low-microwave-loss region,Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3, was suc-cessfully identified without compromis-ing the value of �.

A recent codeposition study20 employedthree magnetron sputtering guns in the90� off-axis configuration to form a two-dimensional pseudoternary spread ofoxide dielectric materials (Figure 6). Over30 chemical systems were explored in thisstudy, resulting in the identification ofamorphous Zr0.2Sn0.2Ti0.6O2 as a particu-larly promising material. For each chemi-cal system, a codeposited compositionspread was prepared on a 7 cm � 7 cm



Figure 4. Characterization of atomically controlled combinatorial libraries using a concurrentx-ray diffractometer. (a) Schematic illustration of the setup. (b), (c) Concurrent x-raydiffraction data from a [(BaTiO3)m /(SrTiO3)n ] superlattice library for two ranges of 2.(d) Cross-sectional diagram of the superlattices. On this chip, there were 10 superlattices:[(SrTiO3)n /(BaTiO3)n ]30 (n � 12, 14, 16, . . . , 28, and 30). (e), (f) Line profiles extracted fromthe diffraction patterns of [(SrTiO3)12/(BaTiO3)12]30.

substrate coated with a metallic base elec-trode. The deposition was performedusing metal targets and a reactive Ar-O2atmosphere. The capacitance and current–voltage characteristics of the resulting filmswere measured using a scanning Hg probecounter electrode, and a virtual array ofabout 4000 measurements was constructedby making measurements of the film on1-mm centers. The product of the capaci-tance and breakdown voltage, normalizedto the area of the Hg counter electrode, isan important figure of merit for capacitormaterials, as it represents the maximumcharge per unit area that can be stored inthe capacitor. This product is plotted inFigure 7a for a sample made in the Zr-Sn-

Ti-O system, and the data are replotted ina standard ternary chart in Figure 7b. It isimmediately obvious that compositions inthe vicinity of Zr0.2Sn0.2Ti0.6O2 have optimalproperties, with a maximum stored chargeof about 25 �C/cm2. This value comparesfavorably with the value obtained for de-posited films of SiO2 (3–5 �C/cm2), Al2O3(6–10 �C/cm2), and Ta2O5 (10–12 �C/cm2).Films made by conventional single-composition on-axis sputtering were sub-sequently shown to yield identical results,21

raising the prospect that this material mightprove useful as an alternative dielectric forDRAM capacitors, although the superiorproperties have not yet been demonstratedin films synthesized by chemical vapordeposition, the synthetic technique ofchoice in the Si integrated-circuit industry.

Automated evaluation techniques canalso improve the throughput of studies ofthe composition dependence of materialsproperties. Here the fine granularity withwhich we can delineate the compositiondependence can sometimes provide uniqueinsights. For example, in the van Dovergroup, the behavior of the dielectric con-stant in the pseudobinary system Zr-Al-O,22

a system that might prove useful as an al-ternative gate dielectric for advanced Siintegrated circuits, was studied.23 For thisapplication, a dielectric is sought that isamorphous, thermodynamically stable incontact with Si,24 and has a dielectric con-stant of 12 � � � 50, among other criteria.Compositions in the Zr-Al-O system areexpected to meet these criteria. One mightexpect that the dielectric constant wouldbe a slowly varying function of composi-tion in an amorphous system, so when thecomposition dependence of the dielectricconstant was evaluated (Figure 8), the



Figure 5. Scanned images of a (Ba,Sr,Ca)TiO3 composition spread fabricated on atriangular LaAlO3 substrate. (a) An � plot (�r is the relative dielectric constant), and (b) a tan plot (ratio between the imaginary and the real part of the dielectric constant), both obtainedwith the scanning evanescent microwave microscope. A new low-microwave-loss region,Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3, was identified. (From Reference 4.)

Figure 6. Schematic diagram of anoff-axis sputtering system used for thepreparation of two-dimensionalpseudoternary oxide dielectrics.

Figure 7. (a) Figure of merit (maximumstored charge per unit area, obtainedfrom the product of capacitance andbreakdown voltage, normalized tocapacitor area) for an amorphousZr-Sn-Ti-O composition spread.(b) Data in (a) plotted as a function ofcomposition in a conventional ternaryplot.The experiment, from chamberpumpdown to printout of the ternarydiagram, was completed in under 24 h.(From Reference 20.)

Figure 8. Dielectric constant as afunction of composition in the Zr-Al-Osystem proposed as an alternative gatedielectric in advanced field-effecttransistors.

results were surprising. The dense dataclearly delineate an abrupt transition inthe dielectric constant. Subsequent evalua-tion of the composition spread using high-sensitivity x-ray diffraction verified thehypothesis that the ZrO2-rich composi-tions, with their high dielectric constant,were in fact nanocrystalline. Less densecoverage of the composition dependencemight have missed the transition in � andtherefore would not have motivated thecareful x-ray study.

Intematix Corp. recently applied thecombinatorial thin-film library techniqueto discover advanced microwave tunabledielectrics, as discussed in the accompany-ing sidebar article.

Discovery of Optically TransparentFerromagnets

Another good example of serendipitousdiscovery brought about by the com-binatorial approach is the unexpected find-ing of optically transparent magnetismin TiO2 anatase films doped with Co.25

Ferromagnetic semiconductors formed bydoping magnetic impurities into hostsemiconductors are key materials for spinelectronics, where the correlation betweenthe spin and the charge of electrons givesrise to spin-dependent functionalitiessuch as giant magnetoresistance and spinfield-effect transistors. All known mag-netic semiconductors until now had beenbased on nonoxides such as GaAs and

ZnSe, and their highest Curie temperaturewas �100 K. There was a theoreticalprediction that ZnO would become ferro-magnetic when doped with certain 3dtransition elements, but to date no signof ferromagnetism has been observed insuch systems.

Titanium dioxide (TiO2) is commonlyused for photocatalysis for water cleavageand is well known for its unique proper-ties including high refractive index, excel-lent optical transmittance in the visibleand infrared regions, and high dielectricconstant. Libraries of 3d transition-metal-doped TiO2 were originally fabricated byCLMBE in a search for photocatalysts.When their magnetic properties were



A combinatorial materials science ap-proach was recently applied by theIntematix Corp. of Moraga, Calif., tooptimize a tunable barium strontiumtitanate system with the objective of low-

ering its dielectric loss (tan �) while re-taining its high dielectric tunability.(Ba0.6Sr0.4)TiO3 (BST) was systematicallystudied with transition-metal ions M assubstitutions for Ti. The generic formula

is (Ba0.6Sr0.4)Ti1�xMxO3. A 1-in.2 LaAlO3substrate was used, and three libraries of(Ba0.6Sr0.4) (Ti1�xMx)O3 with M � A and B,and x continuously changing from 0 to 0.3,were synthesized by the combinatorialion-beam deposition process.

A recently developed microbeam x-raydiffractometer/x-ray fluorescence spec-trometer with a beam size of 50 �m (beamsizes from 20 �m to 100 �m can be se-lected) and sample scanning capabilitywas used for the high-throughput screen-ing of material composition and crystalstructures. Figure 1 shows an x-ray dif-fraction (002) profile as a function of com-position for BST-A and BST-B libraries. Itis clearly observed in Figure 1a that thelattice constant of the highly orientedBST-A library film decreases (diffractionangle increases) with A concentration,

Combinatorial Synthesisof Advanced MicrowaveTunable Dielectrics

Yi-Qun Li

Figure 1. X-ray diffraction (002) profile as a function of composition for (a) BST-A (0–30%) and (b) BST-B (0–30%) libraries, which aresynthesized by ion-beam sputtering and postannealing on single-crystal LaAlO3 substrates with linear composition x ranging from 0% to30% for M.

checked with a scanning superconductingquantum interference device (SQUID)microscope, Co-doped TiO2 was found,surprisingly, to exhibit ferromagnetism atroom temperature while maintaining itstransparency and single phase up to acomposition of 8% Co. In order to system-atically study the changes of film proper-ties as a function of doping level, a librarycontaining nine Ti1�xCoxO2 films with dif-ferent x values was fabricated. TEM of thefilms has indicated no sign of segregationof impurity phases in the Co/Ti composi-tional range of x � 0.08.

Figure 9 shows a series of images takenwith the SQUID microscope at 3 K foranatase films with different Co contents

(x � 0–0.06) on a combinatorial chip. In allof the Co-doped films, magnetic-domainstructures of around 20 �m are observed.With increasing Co content in the film, themagnitude of the magnetic field is system-atically enhanced as a result of increasedspontaneous magnetization. This observa-tion established the presence of long-rangeorder in the Co-doped TiO2 anatase phase.Further magnetic characterization indi-cated that the films are ferromagnetic evenat room temperature. The spontaneousmagnetic moment per Co atom was de-duced to be 0.32 �B from M–H curves.From a temperature-dependent magneti-zation measurement, Tc was estimated tobe higher than 400 K, significantly higher

than that for known nonoxide ferromag-netic semiconductors. At present, the exactnature of the magnetic long-range order inCo-doped TiO2 is not understood. Recently,Co-doped TiO2 rutile films were also foundto exhibit robust ferromagnetism at roomtemperature.26 The films were also foundto display excellent optical transmissionwith a bandgap of 400 nm (3.1 eV). Trans-parent ferromagnets have a large potentialfor serving the integration of electroniccircuits and magnetic storage with theuser interface in a single flat-panel display.

AcknowledgmentsWe are grateful to many colleagues for

extensive collaboration and many insightful



and the solubility of A in the BST phase isabout 18%. Figure 1b shows a high solu-bility of B in BST of up to 30% with littlechange in lattice parameter as B concen-tration increases.

Dielectric characterization of the li-braries was carried out at 1 GHz by ascanning evanescent microwave probe(SEMP) as a function of A and B concen-tration in the BST phase (see Figure 2).The dielectric constant and tangent lossdecrease in both libraries as A and B con-centrations increase. The minimum tan-gent loss is about 0.05 at 1 GHz for the

BST-A libraries, while loss in the BST-Blibrary decreases sharply to below 0.001(which is the limitation of SEMP) as Bconcentration increases.

Tunability of the BST-B thin film wasmeasured at 1 MHz by an HP 4280A C–Vplotter on an interdigital electrode pat-terned at the location corresponding toabout 22% B. The results for dielectricconstant as a function of electrical fieldare shown in Figure 3. The tunability ofBST-22%B is 15% in an electrical field of4 V/�m, and the tangent loss is again inthe noise range of the instrument. A se-

ries of ceramic bulk materials with differ-ent dielectric constants and tunabilitieswere further developed based on theBST-B composition. These materials havea dielectric constant ranging from 200 to1500 and tunability from 20% to 60% in afield of 8 V/�m with tangent loss of theorder of 0.001 at 1 GHz. With dielectrictunability comparable to (BaSr)TiO3 anda much lower tangent loss, these mate-rials have great potential for applicationsin wireless communications, for instance,as tunable filters and beam-steeringantennas.

Figure 2. Plots of dielectric constant � and tangent loss tan at 1 GHzas a function of (a) A phase concentration in a BST-A library and(b) B phase concentration in a BST-B library.

Figure 3.Tunability of (a) dielectric constant � and(b) tangent loss tan for Ba0.6Sr0.4Ti0.78B0.22O3.

discussions. We would particularly like toacknowledge the key contributions by ourclosest co-workers, including H. Chang,T. Chikyow, F. Duewer, T. Fukumura,C. Gao, T. Hasegawa, M. Kawasaki,Y. Matsumoto, T. Ohnishi, ‘P.G. Schultz,L.F. Schneemeyer, T. Wei, and X.-D. Xiang.

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Figure 9. A series of scanning superconducting quantum interference device (SQUID) microscope images (200 �m � 200 �m) taken at 3 K foranatase thin films with different Co contents (Ti1�xCoxO2) on a combinatorial chip. Bz stands for the magnetic field in the z direction. (a) x � 0,(b) x � 0.02, (c) x � 0.03, and (d) x � 0.06. Magnetic-domain structures were observed in all films, suggesting the presence of long-rangeordering. (From Reference 25.)

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