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Epitaxial integration of the highlyspin-polarized ferromagneticsemiconductor EuO with silicon and GaNANDREAS SCHMEHL1,2, VENU VAITHYANATHAN1, ALEXANDER HERRNBERGER2, STEFAN THIEL2,CHRISTOPH RICHTER2, MARCO LIBERATI3, TASSILO HEEG4, MARTIN ROCKERATH4,LENA FITTING KOURKOUTIS5, SEBASTIAN MUHLBAUER6,7, PETER BONI6, DAVID A. MULLER5,YURI BARASH8, JURGEN SCHUBERT4, YVES IDZERDA3, JOCHEN MANNHART2

AND DARRELL G. SCHLOM1*1Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802-5005, USA2Experimentalphysik VI, Elektronische Korrelationen und Magnetismus, Institut fur Physik, Universitat Augsburg, Augsburg D-86135, Germany3Department of Physics, Montana State University, Bozeman, Montana 59717, USA4Institut fur Bio- und Nanosysteme IBN1-IT, and Center of Nanoelectronic Systems for Information Technology, Forschungszentrum Julich GmbH, Julich 52425, Germany5School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA6Technische Universitat Munchen, Physik Department E21, Garching 85748, Germany7Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Garching 85748, Germany8Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia*e-mail: schlom@ems.psu.edu

Published online: 16 September 2007; doi:10.1038/nmat2012

Doped EuO is an attractive material for the fabrication of proof-of-concept spintronic devices. Yet for decades its use has beenhindered by its instability in air and the difficulty of preparing and patterning high-quality thin films. Here, we establish EuOas the pre-eminent material for the direct integration of a carrier-concentration-matched half-metal with the long-spin-lifetimesemiconductors silicon and GaN, using methods that transcend these difficulties. Andreev reflection measurements reveal thatthe spin polarization in doped epitaxial EuO films exceeds 90%, demonstrating that EuO is a half-metal even when highlydoped. Furthermore, EuO is epitaxially integrated with silicon and GaN. These results demonstrate the high potential of EuO forspintronic devices.

The ongoing miniaturization of semiconductor devices isapproaching length scales at which it becomes possible toexploit the spin degrees of freedom of the conduction electrons,opening up possibilities for new semiconductor-based dataprocessing and memory applications. Key elements for suchdevices are the coherent injection of spin-polarized electronsinto a semiconducting host material, the manipulation of theirpolarization and a read-out scheme for the spin states1. Whereasheterostructures and methods of spin injection, manipulationand read-out have been studied extensively in systems basedon III–V semiconductors1–4, little work concerning silicon-basedsystems has been reported5–7. This is surprising because with itsweak spin–orbit coupling of free electrons, its comparatively longtransverse decoherence time (T2) of bound states8 (∼100 µs), itsspin-decoherence length of several micrometres9–11 and its ubiquityin semiconductor electronics, silicon is a highly promising host forspintronic applications.

The key physical and material needs underlying the design ofactive spintronic devices are typically exemplified by three-terminalsemiconductor spin devices. In particular, the performance of

such spin transistors is characterized by a trade-off betweenmagnetic field sensitivity (given by their magnetocurrent) and theirconventional charge-transfer ratio (collector/emitter current)12.To take two examples, typical hot-carrier spin transistors13 haveexcellent magnetic sensitivity (∼400%) but low transfer ratio (ofthe order of 10−6); spin diffusion transistors6 have transfer ratiosof the order of unity but magnetic sensitivity of only ∼10%.Simultaneously maximizing both parameters for a spintronicdevice requires spin injection into the semiconductor with thehighest possible spin polarization using either (1) tunnel barrierinjection6,7 or (2) ohmic injection (Schottky barrier injection,although highly polarized11, is incompatible with high devicetransfer ratios). However, both methods are beset by materialsintegration problems, method (1) by the difficulty to establish high-quality spin-friendly tunnel barriers on silicon6 and method (2) bythe ‘impedance mismatch’ problem14.

In the light of this prior art, there is therefore a clear needfor a spintronic injector material with high spin polarizationand the capacity to be epitaxially integrated and conductancematched to the working semiconductor. Here, we describe such

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a material: the half-metallic semiconductor europium monoxide,whose epitaxial integration into silicon and GaN systems may yieldhigh injection-efficiency spin-selective ohmic contacts, therebyenabling the development of proof-of-concept and ultimatelycommercial spintronic devices with both high current transferratios and high magnetic sensitivities.

The ferromagnetic semiconductor EuO (ref. 15) is the onlymagnetic binary oxide that could be thermodynamically stable incontact with silicon16. Stoichiometric EuO is a semiconductor witha bandgap of 1.12 eV at room temperature17. Owing to the 0.6 eVspin splitting of its conduction band18 in the ferromagnetic state(Curie temperature, Tc = 69 K; ref. 19), the spin polarization ofthe mobile electrons is predicted to be nearly 100% for carrierconcentrations below half filling of the conduction band20–22.

The conductivity of EuO can be tuned to match the resistivityof silicon by doping it with oxygen vacancies (EuO1−x) or with rare-earth atoms (for example, gadolinium or lanthanum23,24). Indeed,the carrier densities in lightly doped EuO1−x (∼1019 cm−3) matchthose of doped silicon well25. Such conductivity matching is keyto achieving efficient coherent spin injection into a semiconductor,which requires (1) the spin polarization at the contact between theinjector and the transport medium to be close to 100% and (2) theconductivity of the injector to be of the same order of magnitudeas the conductivity of the semiconductor14,26. As europium oxidecan be epitaxially integrated with silicon27, it might be expectedthat Si/EuO1−x interfaces can be established with negligiblemicrostructural distortions, leading to an almost perfect spin-selective ohmic contact. These unique properties render EuO1−x anoutstanding material for silicon-based spintronic devices.

In addition to its potential for spintronic experiments,europium oxide exhibits giant properties that make it anexceptional material for basic and application-driven science.When oxygen vacancies are introduced into EuO, an insulator-to-metal transition (MIT) is induced near the ferromagnetic transitiontemperature. In single-crystalline EuO1−x, this transition can beaccompanied by resistance changes over 13 orders of magnitude28.With external magnetic fields, the MIT is shifted substantially,causing a colossal magnetoresistive effect (CMR) that is even morepronounced than that of the manganite-based CMR oxides25,29–31.In addition, EuO1−x exhibits a Faraday rotation of 8.5×105 ◦ cm−1

at l = 0.7 µm, H = 20 kOe and T = 10 K, one of the highest ofany material32.

Despite the tremendous potential of EuO1−x for spintronics,experimental studies have been severely limited by the factthat EuO1−x is unstable and reacts to form higher oxides (forexample, Eu2O3) and hydroxides when exposed to air. The coreof our knowledge about this material was established decadesago on single crystals33. Until now this degradation in air hashampered the accessibility of high-quality epitaxial EuO1−x filmsand their patterning.

Here, we report methods that overcome these limitationsand make doped EuO accessible to experimentally establishkey spintronic properties and to fabricate spintronic devicestructures. The magnetic and transport properties of the resultingepitaxial EuO1−x and lanthanum-doped Eu1−yLayO1−x films rivalthose of the best single crystals. We present electrical transportmeasurements providing direct proof that the spin polarizationof the Eu0.995La0.005O1−x conduction electrons exceeds 90%, evenat high charge-carrier doping levels. Finally, we demonstrate theepitaxial integration of these films with silicon and gallium nitride,the most technologically developed semiconductors with longspin lifetimes8,34.

With the aim of making epitaxial EuO1−x films accessible asa source for highly spin-polarized electrons, we systematicallyoptimized their structural and magnetotransport properties before

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films exhibit magnetic hystereses with saturation magnetizations of∼6.7 µB per Eu.

integrating them with silicon and gallium nitride. As a first step,we grew epitaxial EuO1−x and lanthanum-doped Eu1−yLayO1−x

films on (110) YAlO3. This highly insulating substrate (7.5 eVbandgap35) enables precise measurements of the electronictransport properties of the highly resistive films without beingshunted by a conductive substrate. Transport measurements were

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Figure 2 Electronic transport properties of 130-nm-thick Eu1−yLayO1−x films with y = 0 and 0.005 grown on (110) YAlO3. a, The EuO1−x sample exhibits aninsulator-to-metal transition, which results in a resistance change exceeding eight orders of magnitude. b, Application of out-of-plane magnetic fields of 8 T suppresses thetransition by up to five orders of magnitude and shifts Tc to ∼140 K. For the Eu0.995La0.005O1−x sample, both the resistance and the MIT are substantially suppressed.

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Figure 3 Andreev reflection measurements on a Eu0.995La0.005O1−x/Nb contact. a, A micrograph of the configuration that allows the independent measurement of thetransport properties of Nb (VNb), EuO (VEuO) and the Andreev reflection contact (VAR) each in a four-point configuration. b, Conductance versus voltage characteristics of theAndreev reflection contact measured at different temperatures (raw data). c, The onset of the Andreev reflection is in good agreement with the Tc = 8.5 K measuredindependently on the niobium part of the bridge. d, Fit (open blue circles) to the normalized conductance data (red circles) of the Andreev reflection contact measured atT= 1.7 K. The respective fitting parameters are: ∆= 0.88mV, Z= 0.15, Rs = 2.53 a.u. and P= 91%.

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Figure 4 X-ray diffraction scans of Si-capped EuO1−x films (typical thickness 130 nm) grown on (001) Si and (0001) GaN. a,b, θ–2θ XRD scan (a) and φ-scan (b) ofthe 202 EuO reflections (φ-FWHM= 0.8◦ ) of a Si/EuO1−x/SrO/Si heterostructure. The substrate peak is labelled with an asterisk. c,d, θ–2θ XRD scan (c) and φ-scan (d)of the 200 EuO reflections (φ-FWHM= 1.8◦ ) of a Si/EuO1−x/GaN/Al2O3 heterostructure. The Al2O3 peaks are labelled with asterisks. These plots demonstrate the epitaxialgrowth of EuO1−x films with high structural quality and the absence of second phases within the resolution limits of the XRD.

used to identify conditions yielding EuO1−x films with highmagnetization, magnetoresistance and MIT. These conditions werethen used for the growth on (001) Si and (0001) GaN. Thefilms were deposited by reactive molecular-beam epitaxy (see theSupplementary Information for details on growth). To prevent thedegradation of the EuO1−x when exposed to air, the films wereprotected in situ by 10–20-nm-thick capping layers (Al2O3, Ti orSi, see the Supplementary Information for details on capping).

The in-plane magnetic properties were determined bysuperconducting quantum interference device (SQUID)magnetometry. Figure 1a shows the temperature-dependentmagnetization, M(T), of Eu1−yLayO1−x films with y = 0, 0.005 and0.01 grown on (110) YAlO3 that were cooled in zero field. The filmsexhibit a ferromagnetic onset at 69 K for y =0 (equal to that of bulkEuO single crystals19), 105 K for y = 0.005 and 118 K for y = 0.01.Further increase of the lanthanum content does not further increaseTc. In-plane hysteresis loops measured at 5 K on the EuO1−x filmsshow a saturation magnetization of ∼6.7 µB per Eu (Fig. 1b,c).This value rivals that of the best EuO1−x single crystals and it is alsoclose to the theoretical prediction of ∼7 µB per Eu (ref. 15), arisingfrom the 4f 7 electron configuration of Eu2+.

To assess the resistance and magnetoresistance properties ofthe samples as a function of temperature and lanthanum doping,we carried out transport measurements using contacts in a four-point configuration that were patterned into the films (see the

Supplementary Information for details on patterning). Figure 2ashows resistance versus temperature (R(T)) characteristics of∼130-nm-thick Eu1−yLayO1−x films with y = 0 and 0.005 grownon (110) YAlO3. On cooling, the samples exhibit semiconductor-like behaviour with rising resistivities that reach their maximumat ∼69 K (y = 0) and ∼98 K (y = 0.005). With the onset of theferromagnetic transition, the EuO1−x film undergoes a pronouncedinsulator-to-metal transition with a reduction of its resistivity byabout eight orders of magnitude. This is the largest MIT everreported for EuO1−x films. Because the MIT requires the presence ofoxygen vacancies, these data reveal that our EuO1−x films are indeedoxygen deficient (x < 0.1%; ref. 36). Replacing 0.5% europium bylanthanum leads to a pronounced increase in the film conductivity,with a suppression of the peak resistance by nearly nine orders ofmagnitude and about two orders of magnitude at low temperatures.The resistance change of the MIT is thus reduced to about oneorder of magnitude. This demonstrates the large range of sampleresistivity that is accessible in the EuO system by chemical doping.

By applying 8 T out-of-plane magnetic fields, the resistivity ofthe EuO1−x films is substantially reduced and the R(T) resistancepeak temperatures increase to ∼140 K. As Fig. 2b shows, theresistivity suppression reaches a maximum amplitude of about fiveorders of magnitude near the zero-field Tc (=69 K). This is the mostpronounced CMR effect reported in EuO1−x films. Lanthanumdoping progressively suppresses the CMR effect; with increasing

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doping, for example, for y =0.005, the resistance at Tc only changesby a factor of four.

To directly measure the spin polarization of the epitaxialEu1−yLayO1−x films, we have used the Andreev reflectiontechnique37–39, which was successfully used by others to establish>90% spin polarization of another oxide that had been predictedto be a half-metal—CrO2 (ref. 40). This technique requires a high-quality interface between Eu1−yLayO1−x and a superconductor.Niobium was chosen as the superconductor, as it is well understoodand chemically robust. The Andreev reflection contact wasmade as a ramp-type junction with voltage and current leadson both sides of the interface to allow the conductance ofthe Andreev reflection junction to be precisely measured (seethe Supplementary Information for details on patterning). Thefabrication of this contact geometry requires extensive patterningof the samples. As the Eu1−yLayO1−x films are not stable in air,standard ex situ structuring methods are not applicable. Therefore,we have developed a patterning method that combines standardex situ photolithography with an in situ combination of ionetching and sputtering (see the Supplementary Information fordetails on patterning). This technique makes it possible to definecomplex device structures while protecting the Eu1−yLayO1−x,allowing the ex situ manipulation and measurement of thepatterned devices.

Figure 3a shows a micrograph of the structure that was used tomeasure the Andreev reflection on a Eu0.995La0.005O1−x/Nb contact.The lanthanum doping was set to 0.5% as it (1) leads to theformation of an ohmic contact to the niobium superconductor,(2) keeps the device resistivity at a level that does not mask theAndreev reflections and (3) allows the spin-polarization valuesto be assessed at high doping levels. The patterned contactstructure enables the independent determination of the transportproperties of the Andreev reflection contact as well as the Nband Eu0.995La0.005O1−x sections of the same bridge that forms theAndreev reflection contact. This structure was used to measurethe current, I , and the conductance, G = dI/dV , across theEu0.995La0.005O1−x/Nb contact at different temperatures (Fig. 3b).The onset of the Andreev-reflection-induced conductancesuppression was found at ∼8.4 K, in good agreement with themeasured superconducting transition temperature of the niobiumbridge (Tc = 8.5 K, Fig. 3c).

Only the contribution of the Andreev reflection contactto the measured total differential resistance is relevant forthe determination of the spin-polarization values. As theEu0.995La0.005O1−x/Nb contact is a combination of two materialswith very different band structures and charge-carrier densities, anon-negligible interface resistance is to be expected even withoutthe presence of Andreev reflections. Furthermore, the seriesresistance of the Eu0.995La0.005O1−x part of the bridge also influencesthe measured conductance. The contribution of both of theseresistances can be accounted for by introducing a combined seriesresistance, Rc. As the I(V ) characteristics of the Andreev reflectioncontact are linear above the superconducting Tc of niobium, thiscontact resistance Rc is voltage independent. Unfortunately Rc

cannot be measured directly, because it partly originates fromthe contact itself. As it, nevertheless, influences the shapes ofthe measured conductance curves, we modified the model ofref. 41 to account for this series resistance (see the SupplementaryInformation for details on the modified model). This modifiedmodel, in which Rc is used as a fit parameter, was then appliedto fit the measured data and to extract the spin polarization,P. In Fig. 3d, one of the curves from the data set shown inFig. 3b has been fitted using this model, resulting in a value ofthe spin polarization of P = 91%. Polarization values exceeding80% were extracted from fits to the measured conductance curves

in the temperature range from 1.7 to 7.4 K (see SupplementaryInformation, Fig. S9). Our results reveal a very high (∼90%)value of the spin polarization, strongly indicating that EuO1−x isa half-metal even at high carrier-concentration levels.

The reason that our measurements do not show thetheoretically expected spin polarization of 100% (refs 20–22) mightbe due to a non-perfect Eu0.995La0.005O1−x/Nb interface, caused,for example, by microstructural defects at the interface inducedby the ion etching. In addition, the wider rocking curves of thelanthanum-doped samples with respect to the undoped ones,indicate the presence of defects in the Eu0.995La0.005O1−x films, whichmight act as scattering centres.

Having fulfilled the benchmarks for high-quality Eu1−yLayO1−x

films on YAlO3 substrates, the optimized deposition conditionswere then implemented on silicon and GaN. The films wereepitaxially grown on thermally cleaned 3-inch-diameter (001)Si wafers with or without an intermediate 1.3-nm-thick SrObuffer layer27 and on Ga-face (0001) GaN/(0001) Al2O3 wafers,respectively. X-ray diffractometer (XRD) measurements indicatethat the films on silicon are single phase, epitaxial and ofhigh crystalline quality (Fig. 4a). With rocking-curve full-widthat half-maximums (FWHMs) of 0.2◦–0.4◦ in ω and in-planeorientations φ-FWHM of 0.8◦ (Fig. 4b), the films on silicon arestructurally comparable to the films grown on (110) YAlO3. SQUIDmeasurements show ferromagnetic onsets of the EuO1−x films atTc = 69 K, again matching those of the best single crystals. Neutronreflectometry measurements on a EuO1−x film grown on (001)Si reveal a magnetization value of 6.6 µB per Eu, matching thaton YAlO3. The XRD measurements of the EuO1−x films on GaNindicate that the films are single phase and of high structuralquality (Fig. 4c) with rocking-curve FWHMs of 0.4◦–0.5◦ in ω. In-plane orientation measurements corroborate the epitaxy and theexistence of 180◦ rotational twins (Fig. 4d). SQUID measurementsyield ferromagnetic onsets of the EuO1−x films on GaN at Tc =69 K.

In summary, we have demonstrated that doped EuO hasa high spin polarization and we epitaxially integrated it withsilicon and GaN. This combination of a conductance-matchedhalf-metal with versatile semiconductors characterized by longspin-decoherence lengths enables fundamental spintronic studiesand proof-of-concept devices. Because the Tc of EuO1−x can besubstantially enhanced by strain42–44 and chemical doping23,24, thishighly versatile spintronic system is also a candidate for practicalspintronic devices.

Received 14 May 2007; accepted 14 August 2007; published 16 September 2007.

References1. von Molnar, S. & Reed, D. New materials for semiconductor spin-electronics. Proc. IEEE 91,

715–726 (2003).2. Fiederling, R. et al. Injection and detection of a spin-polarized current in a light-emitting diode.

Nature 402, 787–790 (1999).3. Ohno, Y. et al. Electrical spin injection in a ferromagnetic semiconductor heterostructure. Nature

402, 790–792 (1999).4. Crooker, S. A. et al. Imaging spin transport in lateral ferromagnet/semiconductor structures. Science

309, 2191–2195 (2005).5. Xiao, M., Martin, L., Yablonovitch, E. & Jiang, H. W. Electrical detection of the spin resonance of a

single electron in a silicon field-effect transistor. Nature 430, 435–439 (2004).6. Dennis, C. L., Sirisathitkul, C., Ensell, G. J., Gregg, J. F. & Thompson, S. M. High current gain

silicon-based spin transistor. J. Phys. D 36, 81–87 (2003).7. Jonker, B. T. et al. Electrical spin-injection into silicon from a ferromagnetic metal/tunnel barrier

contact. Nature Phys. 3, 542–546 (2007).8. Gordon, J. P. & Bowers, K. D. Microwave spin echoes from donor electrons in silicon. Phys. Rev. Lett.

1, 368–370 (1958).9. Gregg, J. F. et al. The art of spin electronics. J. Magn. Magn. Mater. 175, 1–9 (1997).10. Gregg, J. F., Petej, I., Jouguelet, E. & Dennis, C. Spin electronics—a review. J. Phys. D 35,

R121–R155 (2002).11. Appelbaum, I., Huang, B. & Monsma, D. J. Electronic measurement and control of spin transport in

silicon. Nature 447, 295–298 (2007).12. Sellmyer, D. & Skomsky, R. Advanced Magnetic Nanostructures Ch. 14, 442–453 (Springer,

Berlin, 2005).13. Monsma, D. J., Lodder, J. C., Popma, T. J. A. & Dieny, B. Perpendicular hot electron spin-valve effect

in a new magnetic field sensor: The spin-valve transistor. Phys. Rev. Lett. 74, 5260–5263 (1995).

886 nature materials VOL 6 NOVEMBER 2007 www.nature.com/naturematerials

© 2007 Nature Publishing Group

ARTICLES

14. Schmidt, G., Ferrand, D., Molenkamp, L. W., Filip, A. T. & van Wees, B. J. Fundamental obstacle forelectrical spin injection from a ferromagnetic metal into a diffusive semiconductor. Phys. Rev. B 62,R4790–R4793 (2000).

15. Matthias, B. T., Bozorth, R. M. & Van Fleck, J. H. Ferromagnetic interaction in EuO. Phys. Rev. Lett.7, 160–161 (1961).

16. Hubbard, K. J. & Schlom, D. G. Thermodynamic stability of binary oxides in contact with silicon.J. Mater. Res. 11, 2757–2776 (1996).

17. Hellwege, K.-H. & Madelung, O. (eds) Landolt-Bornstein: Numerical Data and FunctionalRelationships in Science and Technology 321 (New Series—Group III, Vol. 17, Springer, Berlin, 1984).

18. Schoenes, J. & Wachter, P. Exchange optics in Gd-doped EuO. Phys. Rev. B 9, 3097–3105 (1974).19. McGuire, T. R. & Shafer, M. W. Ferromagnetic europium compounds. J. Appl. Phys. 35,

984–988 (1964).20. von Molnar, S. Transport properties of the europium chalcogenides. IBM J. Res. Develop. 14,

269–275 (1970).21. Sattler, K. & Siegmann, H. C. Paramagnetic sheet at the surface of the Heisenberg ferromagnet EuO.

Phys. Rev. Lett. 29, 1565–1567 (1972).22. Steeneken, P. G. et al. Exchange splitting and charge carrier spin polarization in EuO. Phys. Rev. Lett.

88, 047201 (2002).23. Holtzberg, F., McGuire, T. R., Methfessel, S. & Suits, J. C. Effect of electron concentration on

magnetic exchange interactions in rare earth chalcogenides. Phys. Rev. Lett. 13, 18–21 (1964).24. Mauger, A., Escorne, M., Godart, C., Desfours, J. P. & Archard, J. C. Magnetic properties of Gd doped

EuO single crystals. J. Phys. Colloq. 41, C5-263 (1980).25. Shapira, Y., Foner, S. & Reed, T. B. EuO. I. Resistivity and Hall effect in fields up to 150 kOe. Phys.

Rev. B 8, 2299–2315 (1973).26. Zutic, I., Fabian, J. & Das Sarma, S. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 76,

323–410 (2004).27. Lettieri, J. et al. Epitaxial growth and magnetic properties of EuO on (001) Si by molecular-beam

epitaxy. Appl. Phys. Lett. 83, 975–977 (2003).28. Petrich, G., von Molnar, S. & Penney, T. Exchange-induced autoionization in Eu-rich EuO. Phys. Rev.

Lett. 26, 885–888 (1971).29. Oliver, M. R., Dimmock, J. O., McWorther, A. L. & Reed, T. B. Conductivity studies in europium

oxide. Phys. Rev. B 5, 1078–1098 (1972).30. von Helmolt, R., Wecker, J., Holzapfel, B., Schultz, L. & Samwer, K. Giant negative magnetoresistance

in perovskitelike La2/3Ba1/3MnOx ferromagnetic films. Phys. Rev. Lett. 71, 2331–2333 (1993).31. Jin, S. et al. Thousandfold change in resistivity in magnetoresistive La–Ca–Mn–O films. Science 264,

413–415 (1994).32. Ahn, K. Y. & Shafer, M. W. Relationship between stoichiometry and properties of EuO films. J. Appl.

Phys. 41, 1260–1262 (1970).

33. Mauger, A. & Godart, C. The magnetic, optical, and transport properties of representatives of a classof magnetic semiconductors: The europium chalcogenides. Phys. Rep. 141, 51–176 (1986).

34. Beschoten, B. et al. Spin coherence and dephasing in GaN. Phys. Rev. B 63, 121202 (2001).35. Abramov, V. N. & Kuznetsov, A. I. Fundamental absorption of Y2O3 and YAlO3 . Fiz. Tverd. Tela 20,

689–694 (1978).36. Sinjukow, P. & Nolting, W. Metal-insulator transition in EuO. Phys. Rev. B 68, 125107 (2003).37. Andreev, A. F. The thermal conductivity of the intermediate state in superconductors. Sov. Phys. JETP

19, 1228–1231 (1964).38. Soulen, R. J. Measuring the spin polarization of a metal with a superconducting point contact. Science

282, 85–88 (1998).39. Upadhyay, S. K., Palanisami, A., Louie, R. N. & Buhrman, R. A. Probing ferromagnets with Andreev

reflection. Phys. Rev. Lett. 81, 3247–3250 (1998).40. Anguelouch, A. et al. Properties of epitaxial chromium dioxide films grown by chemical vapor

deposition using a liquid precursor. J. Appl. Phys. 91, 7140–7142 (2002).41. Mazin, I. I., Golubov, A. A. & Nadgorny, B. Probing spin polarization with Andreev reflection: A

theoretical basis. J. Appl. Phys. 89, 7576–7578 (2001).42. McWhan, D. B., Souers, P. C. & Jura, G. Magnetic and structural properties of europium metal and

europium monoxide at high pressure. Phys. Rev. 143, 385–389 (1965).43. Zimmer, H. G., Takemura, K., Sayassen, K. & Fischer, K. Insulator-metal transition and valence

instability in EuO near 130 kbar. Phys. Rev. B 29, 2350–2352 (1984).44. DiMarzio, D., Croft, M., Sakai, N. & Shafer, M. W. Effect of pressure on the electrical resistance of

EuO. Phys. Rev. B 35, 8891–8893 (1987).

AcknowledgementsA.S. thanks the Alexander von Humboldt Foundation for a research fellowship. The work at Penn Statewas supported by the Office of Naval Research (ONR) through grants N00014-03-1-0721 andN00014-04-1-0426 monitored by Colin Wood. The work at the University of Augsburg was supportedby the BMBF (13N6918), the EU (Nanoxide), the DFG (SFB484) and the ESF (THIOX). The work atMontana State was supported by NSF EEC-0303774 and ONR through contract N00014-03-1-0692.Y.B. acknowledges support from the Russian Foundation for Basic Research through grant05-02-17175. L.F.K. and D.A.M. acknowledge support under the ONR EMMA MURI monitored byColin Wood and by the Cornell Center for Materials Research (NSF DMR–0520404 andIMR-0417392). L.F.K. acknowledges financial support by Applied Materials. The Advanced LightSource is supported by the Department of Energy.Correspondence and requests for materials should be addressed to D.G.S.Supplementary Information accompanies this paper on www.nature.com/naturematerials.

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