19. B. S. Meyer, Chondrites and the protoplanetary disk, A. N. Krot,E. R. D. Scott, B. Reipurth, Eds., Astron. Soc. Pac. Conf. Ser.341, 515 (2005).

20. Supplementary text is available for further discussion assupplementary materials on Science Online.

21. A. I. Karakas, D. A. García-Hernández, M. Lugaro, Astrophys. J.751, 8 (2012).

22. M. Lugaro et al., Astrophys. J. 780, 95 (2014).23. M. Lugaro et al., Meteorit. Planet. Sci. 47, 1998–2012

(2012).24. C. L. Doherty, P. Gil-Pons, H. H. B. Lau, J. C. Lattanzio, L. Siess,

Mon. Not. R. Astron. Soc. 437, 195–214 (2014).25. T. Rauscher, A. Heger, R. D. Hoffman, S. E. Woosley, Astrophys. J.

576, 323–348 (2002).26. A. Heger, S. E. Woosley, Astrophys. J. 724, 341–373

(2010).27. V. Bondarenko et al., Nucl. Phys. A. 709, 3–59 (2002).

28. N. Murray, Astrophys. J. 729, 133 (2011).29. M. Gounelle, G. Meynet, Astron. Astrophys. 545, A4

(2012).30. A. Vasileiadis, Å. Nordlund, M. Bizzarro, Astrophys. J. 769, L8

(2013).31. E. D. Young, Earth Planet. Sci. Lett. 392, 16–27 (2014).32. J. C. Holst et al., Proc. Natl. Acad. Sci. U.S.A. 110, 8819–8823

(2013).33. G. A. Brennecka et al., Science 327, 449–451 (2010).34. M. Asplund, N. Grevesse, A. J. Sauval, P. Scott, Annu. Rev.

Astron. Astrophys. 47, 481–522 (2009).

ACKNOWLEDGMENTS

We thank M. Asplund for providing us updated early solar systemabundances, D. Price and C. Federrath for comments, andM. Pignatari for discussion. The data described in the paper arepresented in fig. S2 and table S1. M.L., A.H., and A.I.K. are

Australian Research Council (ARC) Future Fellows on projectsFT100100305, FT120100363, and FT10100475, respectively. Thisresearch was partly supported under ARC’s Discovery Projectsfunding scheme (project nos. DP0877317, DP1095368, andDP120101815). S.G. acknowledges financial support from the FondsNational de la Recherche Scientifique, Belgium. U.O. thanks theMax Planck Institute for Chemistry for use of its IT facilities.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/345/6197/650/suppl/DC1Supplementary TextFigs. S1 and S2Tables S1 and S2References (35–46)

13 March 2014; accepted 16 June 201410.1126/science.1253338

MAGNETIC MICROSCOPY

Real-space imaging of the atomic-scalemagnetic structure of Fe1+yTeMostafa Enayat,1* Zhixiang Sun,1* Udai Raj Singh,1 Ramakrishna Aluru,1

Stefan Schmaus,1 Alexander Yaresko,1 Yong Liu,1† Chengtian Lin,1

Vladimir Tsurkan,2,3 Alois Loidl,2 Joachim Deisenhofer,2 Peter Wahl1,4‡

Spin-polarized scanning tunneling microscopy (SP-STM) has been used extensively tostudy magnetic properties of nanostructures. Using SP-STM to visualize magnetic orderin strongly correlated materials on an atomic scale is highly desirable, but challenging. Weachieved this goal in iron tellurium (Fe1+yTe), the nonsuperconducting parent compound ofthe iron chalcogenides, by using a STM tip with a magnetic cluster at its apex. Our imagesof the magnetic structure reveal that the magnetic order in the monoclinic phase is aunidirectional stripe order; in the orthorhombic phase at higher excess iron concentration(y > 0.12), a transition to a phase with coexisting magnetic orders in both directions isobserved. It may be possible to generalize the technique to other high-temperaturesuperconductor families, such as the cuprates.

In many strongly correlated high-temperaturesuperconductors (HTSCs), the nonsupercon-ducting parent compound is in an antiferro-magnetically ordered state, which becomessuperconducting upon chemical doping. This

is true formost of the iron-basedHTSCs (1–3) andfor copper oxide–basedmaterials (4). Establishingthe relation between magnetic order and super-conductivity is thought to be key to understand-ing thephysics of thesematerials.Magnetic orderin strongly correlated electronmaterials is usuallyobserved by means of neutron scattering. Real-space imaging of magnetic order is possible inprinciple with spin-polarized scanning tunnelingmicroscopy (SP-STM), which has been used exten-

sively to study magnetic properties of thin films,nanostructures, and magnetic clusters (5–7). Yet,application to strongly correlated electron systemshas remained scarce (7). The experimental chal-lenge lies in identifying a suitable procedure forpreparing a STM tip that yieldsmagnetic contrast.Iron tellurium (Fe1+yTe) is the nonsuperconducting

parent compound of the iron chalcogenide super-conductors, in which superconductivity is inducedby the substitution of Te with Se (8). The parentcompound exhibits a bicollinear stripe magneticorder with a wave vector (1/2, 0, 1/2) [defined inthe two iron unit cell (Fig. 1A)] (9–11). The mag-netic order sets in at a temperature of ~60 to 70K,accompanied by a structural phase transition,with the structure changing from tetragonal tomonoclinic. With increasing concentration y ofexcess iron, the transition temperature is reduced,and the magnetic and structural transitions areseparated (12, 13). At excess iron concentrationsof y > 0.12, the crystal structure becomes ortho-rhombic (13), and the magnetic order becomesincommensuratewith the lattice (11). Themagneticstructure is distinct from the one found in theparent compounds of the iron-pnictide super-conductors. The absence of nesting at the wavevector of the magnetic order suggests that localmoments and their interactions are important.

Density functional theory (DFT) calculations re-produce the magnetic structure at y = 0 (14, 15).Several microscopic Heisenberg models have beenproposed to describe the magnetic order in theFeTe plane and the spin excitations (15–19), butmapping onto a Heisenberg model remains con-troversial (20).Because of their layered structure, electronic

properties and magnetism in iron-pnictides and-chalcogenides are quasi–two-dimensional (3),making them ideally suited for a study by meansof SP-STM. The magnetic interactions in Fe1+yTeare predominantly two-dimensional within theFeTe-plane; hence, we consider in the followingonly the in-plane component of themagnetic order.Here, we report an investigation by means of

SP-STM of the real-space magnetic structure atthe atomic scale of Fe1+yTe. Our STM data wereobtained with a tip that has amagnetic cluster atits apex (Fig. 1B) [(21), section S1B]. In a topo-graphic image of the sample surface obtainedwith a tip that does not yield magnetic contrast(Fig. 1C), the square lattice can be identified fromits lattice constant as the top-layer telluriumatoms.Excess iron atoms at the surface show up as brightprotrusions (22–24). The Fourier transform ofthe topography clearly shows the peaks asso-ciated with the tellurium lattice at the surface.The twononequivalent spots atqaTe ¼ ðT1;0Þ andqbTe ¼ ð0; T1Þ have noticeably different intensities.A topographic image obtained with a tip thatyields magnetic contrast for the same area as inFig. 1C is shown in Fig. 1D. It shows clear stripe-like patterns superimposed to the atomic lattice.The stripes result from spin-polarized tunnelinginto regions with a spin-polarization parallel orantiparallel to that of the tip, imaged higher orlower, respectively. The unidirectional modula-tion has two major components in the Fouriertransform: The first is a pair of distinct peaks atwave vectors qAFM = (T1/2, 0); the second, whichis also seen with a nonmagnetic tip, at qCDW =(T1, 0) coincides with the atomic peak of the tel-lurium lattice. Antiferromagnetic or spin densitywave (SDW) order is expected to be accompaniedby a charge densitywave (CDW) (25, 26) with twicethe wave vector of the magnetic order (qCDW =2qAFM), which is consistent with our data. Asuperposition of a sketch of the magnetic struc-ture with a topographic image is shown in Fig. 1E.

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1Max-Planck-Institut für Festkörperforschung,Heisenbergstrasse 1, D-70569 Stuttgart, Germany. 2Centerfor Electronic Correlations and Magnetism, ExperimentalPhysics V, University of Augsburg, D-86159 Augsburg,Germany. 3Institute of Applied Physics, Academy ofSciences of Moldova, MD 2028, Chisinau, RepublicaMoldova. 4Scottish Universities Physics Alliance, Schoolof Physics and Astronomy, University of St. Andrews, NorthHaugh, St. Andrews, Fife KY16 9SS, UK.*These authors contributed equally to this work. †Present address:Division of Materials Sciences and Engineering, Ames Laboratory,U.S. Department of Energy, Ames, IA 50011, USA. ‡Correspondingauthor. E-mail: gpw2@st-andrews.ac.uk

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The modulation caused by the antiferromag-netic order was observed consistently over largesurface areas and was never found to switch di-rection within a domain of the monoclinic distor-tion in samples with low excess iron concentration(y

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0 30 -10 +10Fig. 2. Field dependence of magneticcontrast. (A) Topography of Fe1+yTe at y =0.08 measured at 3.8 K at a magnetic fieldof 5T in a direction close to the surfacenormal. (B) Topography for the same area asin (A) in amagnetic field of–5T (Vb = 80mV,It = 100 pA). Images obtained in zero fieldbefore (A) was measured show the identicalcontrast as theone in (B) (fig.S4A). (C) (Top)Line profile along a [data from the white boxin (A) and (B) were averaged along b] show-ing the change in the modulation with theswitching of the magnetic field. (Bottom)Difference (red) and average (black) of thetwo line cuts shown at top, representingspin polarization and topographic height

along the profile, respectively. The spin polarization is obtained as described in (21), section S2. Vertical dashed lines mark positions of largest positive andnegative spin polarization. (D) Average of the images in (A) and (B) showing the topography as it would be obtained with a nonmagnetic tip. (E) Map of spinpolarization obtained from the difference of the images in (A) and (B). (F) (Top) Line cut through a pair of excess iron defects that have different spin-polarization [taken at light-blue solid line in (A) and (B)]. (Bottom)Profile of spin polarization (red) and topography (black) along the line cut [light blue solid linein (D) and (E)].Vertical dashed linesmark positions of the two excess iron atoms. (G) Model of an excess Fe atom (pink) on an FeTe layerwith the spin-structurededuced from our data.The excess iron atom is marked Fe-II. It resides between four tellurium atoms.

Fig. 3. Temperature dependence of magnetic contrast. (A) Topography of Fe1+yTe at y = 0.08 recorded at 54 K. (Inset) The Fourier transform of thetopography.The unidirectional modulation is not detected at this temperature (Vb = 100mV, It= 800 pA). (B) IntensityyAFM ¼ z̃ðqAFMÞ=z̃ðqTeb Þ at thewavevector of the spin density wave qAFM as a function of temperature. Error bars are obtained from the SD from the mean. Solid red line is a fit of mean field theory

[yðTÞ ¼ y0tanh�π2

ffiffiffiffiTcT

r−–

1

�(35, 36)]. (C) Intensity yCDW ¼ ½z̃ðqCDWÞ−z̃ðqTeb Þ�=z̃ðqTeb Þ analyzed from the same data set as (B) at the wave vector of the charge

density wave qCDW as a function of temperature. For the fit in (C), Tc was fixed at the same value as in (B).

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polarization across the two excess iron atoms(Fig. 2F, bottom) shows that the magneticcontrast found on and near excess iron atoms isenhanced.Our data at low temperature show the same

periodicity of the magnetic order as found inneutron scattering. In order to establish the re-lation of themagneticmodulation to themagneto-structural phase transition observed in the bulk,we studied its temperature dependence. Uponincreasing the temperature, the stripemodulationdisappeared around a temperature T of 45 K andwas not observed at T > 50 K. The Fourier trans-form of a topographic image taken at T = 54 K(Fig. 3A) shows only the peaks associatedwith theTe lattice. To quantify the intensity of the modu-lation, we used the ratio yAFM ¼ z̃ðqAFMÞ=z̃ðqbTeÞ,whose temperature dependence shows a trendconsistent with a mean-field behavior with acritical temperature Tc = 45 K (Fig. 3B). A priori,it is not clear whether the disappearance of themagnetic contrast is related to the transition tothe paramagnetic phase of the sample or to a lossof spin-polarization of the tip. This can be clar-ified by analyzing the CDW modulation, whichaccompanies the antiferromagnetic order andcan be detected with bothmagnetic and nonmag-netic tips. In Fig. 3C, we plot yCDW ¼ ½z̃ðqCDWÞ−z̃ðqbTeÞ�=z̃ðqbTeÞ for the samedata as in Fig. 3B as afunction of temperature. It shows a similar tem-perature dependence as that of yAFM; therefore,the magnetic order at the sample surface sets inat a lower temperature than in the bulk, wherethe transition temperature is 60 to 70 K. A pos-sible explanation is that the surface Fe1+yTe layerhas only one neighboring layer, as opposed totwo in the bulk.The magnetic order in Fe1+yTe becomes more

complex with increased excess iron content y(11, 28). A topographic image obtained on asample with y = 0.15 (Fig. 4) reveals stripelikepatterns in bothdirections, at (T1/2, 0) and (0, T1/2)in Fourier space (Fig. 4A, inset), coexisting in the

same domain of the orthorhombic distortion.The image reveals nanoscale domains of pre-dominantly unidirectional stripes coexistingwith regions of bidirectional patterns. Thesetwo-dimensional patterns could be caused by asuperposition between the two unidirectionalmodulations or by a transition toward a pla-quette order, which has been theoretically pre-dicted to become more important as the samplebecomes orthorhombic (19). The magnetic struc-ture appears still locally commensurate with thelattice, but there are phase slips between dif-ferent nanoscale domains. This can bemore clearlyseen from a Fourier-filtered image in which onlythe Fourier components associated with the mag-netic contrast are shown (Fig. 4B).Our observation of a unidirectional stripelike

magnetic order in the monoclinic phase indi-cates that the monoclinic distortion suppressesbidirectional or plaquette magnetic order at lowexcess iron concentrations, strongly favoring uni-directional stripe order. As the lattice constantsin the a- and b-direction approach each otherwith increasing excess Fe concentration (11, 29),we observe both directions of themagnetic ordernear (T1/2, 0) and (0,T1/2) coexisting in the samedomain of the sample, with patches reminiscentof plaquette order. Further, the magnetic orderbecomes incommensurate. Noncommensuratemagnetic order has also been detected in neutronscattering at high excess iron concentrations,showing a shortening of the wave vector asso-ciated with the magnetic order (11, 28). In ourcase, the peak associated with the magnetic orderspreads away from the high-symmetry direction.Because the material still has an orthorhombicdistortion, the two directions will not have thesame energetics, which raises interesting ques-tions as to how the magnetic order sets in as afunction of temperature. For high excess ironconcentrations y ≳ 0.12, evidence for an addi-tional magnetically ordered phase has been re-ported (13, 28, 29).

Our work brings into reach the possibility toobtain real-space images of stripe order in cup-rates (30–32) and search for magnetic order ac-companying the spatially modulated electronicstates found in the pseudogap phase (33, 34).

REFERENCES AND NOTES

1. K. Ishida, Y. Nakai, H. Hosono, J. Phys. Soc. Jpn. 78, 062001(2009).

2. J. Paglione, R. Greene, Nat. Phys. 6, 645–658 (2010).3. M. D. Lumsden, A. D. Christianson, J. Phys. Condens. Matter

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R. Ruf, Phys. Rev. Lett. 65, 247–250 (1990).6. A. Smith, J. Scan. Probe Micro. 1, 3–20 (2006).7. R. Wiesendanger, Rev. Mod. Phys. 81, 1495–1550 (2009).8. Y. Mizuguchi, Y. Takano, J. Phys. Soc. Jpn. 79, 102001 (2010).9. D. Fruchart et al., Mater. Res. Bull. 10, 169–174 (1975).10. S. Li et al., Phys. Rev. B 79, 054503 (2009).11. W. Bao et al., Phys. Rev. Lett. 102, 247001 (2009).12. A. Martinelli et al., Phys. Rev. B 81, 094115 (2010).13. S. Rößler et al., Phys. Rev. B 84, 174506 (2011).14. M. Johannes, I. Mazin, Phys. Rev. B 79, 220510 (2009).15. F. Ma, W. Ji, J. Hu, Z. Y. Lu, T. Xiang, Phys. Rev. Lett.

102, 177003 (2009).16. C. Fang, B. Bernevig, J. Hu, Europhys. Lett. 86, 67005

(2009).17. A. Turner, F. Wang, A. Vishwanath, Phys. Rev. B 80, 224504

(2009).18. I. A. Zaliznyak et al., Phys. Rev. Lett. 107, 216403 (2011).19. S. Ducatman, N. B. Perkins, A. Chubukov, Phys. Rev. Lett.

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(2014).21. Materials and methods are available as supplementary

materials on Science Online.22. X. He et al., Phys. Rev. B 83, 220502 (2011).23. T. Machida et al., J. Phys. Soc. Jpn. 81, 074714 (2012).24. T. Machida et al., Phys. Rev. B 87, 214508 (2013).25. O. Zachar, S. Kivelson, V. Emery, Phys. Rev. B 57, 1422–1426

(1998).26. A. Balatsky, D. Basov, J. Zhu, Phys. Rev. B 82, 144522

(2010).27. M. Bode, M. Getzlaff, R. Wiesendanger, Phys. Rev. Lett. 81,

4256–4259 (1998).28. E. Rodriguez et al., Phys. Rev. B 84, 064403 (2011).29. C. Koz, S. Rößler, A. Tsirlin, S. Wirth, U. Schwarz, Phys. Rev. B

88, 094509 (2013).30. J. Zaanen, O. Gunnarsson, Phys. Rev. B 40, 7391–7394

(1989).31. J. Tranquada, B. Sternlieb, J. Axe, Y. Nakamura, S. Uchida,

Nature 375, 561–563 (1995).32. S. Kivelson, E. Fradkin, V. Emery, Nature 393, 550–553

(1998).33. M. Vershinin et al., Science 303, 1995–1998 (2004).34. Y. Kohsaka et al., Science 315, 1380–1385 (2007).35. F. Gross et al., Z. Phys. B 64, 175–188 (1986).36. D. S. Inosov et al., Phys. Rev. Lett. 104, 187001 (2010).

ACKNOWLEDGMENTS

We acknowledge discussions with S. Chi, M. Etzkorn, F. Kruger,A.P. Mackenzie, S. Rößler, U. Rößler, and M. Ternes. Z.S.acknowledges financial support from Rubicon grant 680.50.1119(Nederlandse Organisatie voor Wetenschappelijk Onderzoek).V.T., A.L., and J.D. acknowledge financial support from theDeutsche Forschungsgemeinschaft Transregional ResearchCenter TRR80 (Augsburg, Munich, Stuttgart). P.W. acknowledgesfunding from the Engineering and Physical Sciences ResearchCouncil and Max Planck Society.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/345/6197/653/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S5References (37–43)

3 February 2014; accepted 18 June 2014Published online 31 July 2014;10.1126/science.1251682

656 8 AUGUST 2014 • VOL 345 ISSUE 6197 sciencemag.org SCIENCE

Fig. 4. Magnetic structure at higher excess iron concentration (y > 0.12). (A) Topography obtainedfrom a sample with high excess iron concentration (y = 0.15), showing stripe modulation in two directionssuperimposed (Vb = 100mV, It = 100 pA, T = 1.8 K). A large part of excess iron has been picked up by the tip,leaving an almost clean Te-terminated surface. (Inset) Fourier transform of the topography showing peaksassociated with magnetic contrast around (T1/2, 0) and (0, T1/2). (B) Filtered image showing the compo-nents associated with magnetic contrast in different colors in order to visualize the bidirectional stripe order.

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originally published online July 31, 2014 (6197), 653-656. [doi: 10.1126/science.1251682]345Science

Wahl (July 31, 2014) Vladimir Tsurkan, Alois Loidl, Joachim Deisenhofer and PeterStefan Schmaus, Alexander Yaresko, Yong Liu, Chengtian Lin, Mostafa Enayat, Zhixiang Sun, Udai Raj Singh, Ramakrishna Aluru,

Tey1+Real-space imaging of the atomic-scale magnetic structure of Fe

Editor's Summary

, this issue p. 653Sciencesurface of the material.substituting Te with Se atoms. The researchers prepared the tip by simply picking up atoms from the

Te, which becomes superconducting by1+yreveal patterns of magnetic ordering in the material Fe used a tip with a magnetic cluster on its apex toet al.magnetic order in more exotic materials. Enayat

on simple nanostructures, they've had trouble preparing the tip in just the right way to visualize theof a material can reveal the material's magnetic structure. Although researchers have used the technique

Electrons tunneling from the magnetized tip of a scanning tunneling microscope into the surfaceSeeing magnetism on an atomic level

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