Hexagonal LuFeO3 films have been studied using x-ray absorption and optical spectroscopy. The crystal splitting of Fe3+ is extracted as Ee′−Ee″ = 0.7 eV and Ea1′−Ee′ = 0.9 eV, and a 2.0 eV optical bandgap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow Ea1′\u3eEe′\u3eEe″ with slightly underestimated values and a bandgap of 1.35 eV

Cheng, Xuemei

He, Lixin

Keavney, David J.

Shen, Jian

Wang, Hongwei

Wang, Wenbin

Wills, Elizabeth

Wu, Xifan

Xu, Xiaoshan

Xu, Xiaoying

Zhu, Leyi

English

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Bryn Mawr CollegeScholarship, Research, and Creative Work at Bryn MawrCollegePhysics Faculty Research and Scholarship Physics2012Crystal Field Splitting and Optical Bandgap ofHexagonal LuFeO3 FilmsWenbin WangHongwei WangXiaoying XuLeyi ZhuLixin HeSee next page for additional authorsLet us know how access to this document benefits you.Follow this and additional works at: http://repository.brynmawr.edu/physics_pubsPart of the Physics CommonsThis paper is posted at Scholarship, Research, and Creative Work at Bryn Mawr College. http://repository.brynmawr.edu/physics_pubs/52For more information, please contact repository@brynmawr.edu.Custom CitationW. Wang et al., Appl. Phys. Lett. 101, 241907 (2012).AuthorsWenbin Wang, Hongwei Wang, Xiaoying Xu, Leyi Zhu, Lixin He, Elizabeth Wills, Xuemei Cheng, David J.Keavney, Jian Shen, Xifan Wu, and Xiaoshan XuThis article is available at Scholarship, Research, and Creative Work at Bryn Mawr College: http://repository.brynmawr.edu/physics_pubs/52Crystal field splitting and optical bandgap of hexagonal LuFeO3 filmsWenbin Wang,1,2 Hongwei Wang,3,4 Xiaoying Xu,2 Leyi Zhu,5 Lixin He,4 Elizabeth Wills,6Xuemei Cheng,6 David J. Keavney,7 Jian Shen,1,8 Xifan Wu,3,a) and Xiaoshan Xu2,a)1Department of Physics, University of Tennessee, Knoxville, Tennessee 37996, USA2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge,Tennessee 37831, USA3Department of Physics and Institute for Computational Molecular Science, Temple University,Philadelphia, Pennsylvania 19122, USA4Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei,Anhui 230026, China5Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA6Department of Physics, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, USA7Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA8Department of Physics, Fudan University, Shanghai 200433, China(Received 1 October 2012; accepted 27 November 2012; published online 11 December 2012)Hexagonal LuFeO3 films have been studied using x-ray absorption and optical spectroscopy. Thecrystal splitting of Fe3þ is extracted as Ee0  Ee00 ¼ 0:7 eV and Ea01 Ee0 ¼ 0:9 eV, and a 2.0 eVoptical bandgap is determined assuming a direct gap. First-principles calculations confirm theexperiments that the relative energies of crystal field splitting states do follow Ea01> Ee0 > Ee00 withslightly underestimated values and a bandgap of 1.35 eV. VC 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4771601]Multiferroic materials which simultaneously exhibitmore than one type of ferroic order have many advantagesover other existing materials in terms of applications in sen-sor, actuator, and information storage and processing.1,2Hexagonal ferrites (h-RFeO3, R¼Sc, Y, Ho-Lu) wererecently found to be a class of multiferroic materials.3–10 Inparticular, it is suggested by experiments that h-LuFeO3 maybe both ferroelectric and antiferromagnetic at room tempera-ture, indicating important application potential.10 It is fasci-nating that h-RFeO3 exhibit distinct properties such asferrimagnetism, abnormal magneto-dielectric couplings, andstructural instability compare with RMnO3.3,4,6,7 Theseunexpected properties are supposed to have electronic ori-gins because h-RFeO3 and RMnO3 are isomorphic, and theradius of Fe3þ and Mn3þ are almost identical.3,4 Therefore,the information on the electronic structures is crucial inunderstanding the intriguing multiferroicity of h-RFeO3.Hexagonal RFeO3 are not stable in free standing bulkform. They can be stabilized by quenching a levitated meltin an aerodynamic levitation furnace or in solvothermal reac-tions.4,11–13 In addition, h-RFeO3 has also been stabilized infilms using pulsed laser deposition (PLD) and metal-organicchemical vapor deposition on yttrium stabilized zirconiumoxides (YSZ) and Al2O3 substrates.3,5–7,9,10 The crystallo-graphic structures of h-RFeO3 is isomorphic to RMnO3 withspace group P63cm (185),3,4 suggesting that h-RFeO3 arealso ferroelectric, which is recently demonstrated.3,4,6,14 Thelarger spin on Fe3þ compared with Mn3þ corresponds tostronger magnetic interactions, allowing for higher magneticordering temperature, which has been indicated by recentexperiments.5,6,10 In stark contrast to the large amount ofwork devoted to RMnO3, detailed studies on the h-RFeO3,and especially of the electronic structures that are crucial forunderstanding the structural, ferroelectric, and magneticproperties are still lacking.9,15–17In this paper, we investigate the electronic properties ofh-LuFeO3 films using x-ray absorption and optical spectros-copy. The analysis of the x-ray absorption spectra (XAS)using crystal field theory reveal a splitting of the Fe 3d levelssignificantly higher than that of Fe3þ in LuFe2O4, indicatingstronger Fe-O interactions. The extracted optical bandgapfrom optical absorption spectra (assuming a direct gap) is2.06 0.1 eV, somewhat smaller than that of perovskite fer-rites.18 The experimental findings have been confirmed byour electronic structure calculations.The XAS was studied on 50 nm h-LuFeO3 films grownon Al2O3 substrates using PLD with 30 nm Pt buffer layer(Fig. 1(a)) to avoid charging effect.19 The XAS was taken atbeam line 4-ID-C at the Advanced Photon Source usingpolarized synchrotron x-rays. A 20 nm thick h-LuFeO3 filmwas grown on a YSZ substrate using PLD (Fig. 1(b)) for op-tical spectroscopy measurements. Part of the substrate wascovered by a mask at growth, so it can be used as a referencein the optical transmittance measurements. Optical spectrawere collected in transmittance mode using a Varian Cary5000 spectrometer.Figure 2(a) shows the XAS corresponding to transitionsfrom a Fe 2p63d5 to a Fe 2p53d6 multiplet. In the spectra,two groups of peaks separated by approximately 12 eV canbe distinguished. Two well-separated peaks (709.1 and710.7 eV) are observed for the s polarization, while addi-tional intensities are observed for p polarization as a peak at709.8 eV, indicating a strong dichroism much more promi-nent than that of YMnO3.20These spectra details are determined by dipole and spinselection rules and a combination of effects from crystal-field, spin-orbit coupling, d-p and d-d interactions and Fe 3da)Authors to whom correspondence should be addressed: Electronicaddresses: xiaoshan.xu@gatech.edu and xifanwu@temple.edu.0003-6951/2012/101(24)/241907/4/$30.00 VC 2012 American Institute of Physics101, 241907-1APPLIED PHYSICS LETTERS 101, 241907 (2012)Downloaded 18 Jan 2013 to 165.106.222.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsO 2p hybridization.21,22 In terms of one-electron energy, theFe 2 p states are split into 2p1=2 and 2p3=2 by the spin-orbitcoupling, which has the energy scale of 15 eV, resulting inthe two groups of excitations L2 ð2p1=2 ! 3dÞ andL3 ð2p3=2 ! 3dÞ in Fig. 2(a).21,23 For the Fe 3d states, theone-electron states are mainly split by crystal fields, which ison the order of one eV.21,23 Here, the trigonal-bipyramidlocal environment of Fe gives rise to a symmetry that can berepresented by the D3h point group as a good approximation,as shown in Fig. 2(c). In this case, Fe 3d states split into irre-ducible representations (IR) e00ðxz; yzÞ; e0ðxx yy; xyÞ anda10ðzzÞ while Fe 2p orbitals can be reduced to states corre-sponding to IR e0ðx; yÞ and a200ðzÞ.24 A recent work onLuFe2O4 in which Fe3þ sites also sit in a trigonal-bipyramidlocal environment has shown that the energies of these crys-tal field states follow Ea01> Ee0 > Ee00 .25 Similar results arealso found for Mn3þ in a D3h symmetry.20The transition probability in the XAS depends on thematrix elements jhwijE^ ~rjwf ij2, where the wi and wf are theinitial and final one-electron states, E^ is the direction vectorof the electric field, and ~r is the position vector. Accordingto Hund’s rule, the ground states of Fe3þ are a multiplet 6A01for a 2p43=22p21=23d2e003d2e03d1a01electronic configuration. In theionic model that ignores hybridization between Fe 3d and O2p state, the spin-allowed excited state multiplets and thecorresponding one-electron state populations can be listed asshown in Table I.22,26 Since the ground state multiplet has asymmetry of A01, the dipole-allowed excited states need tocontain E0 or A002 to satisfy the dipole selection rules for a D3hsymmetry.24 The resulting dipole-allowed transitions arelisted in Table I. It is clear that the photon with z polarizationcannot excite an electron from Fe 2p to Fe 3de0 state. Theseselection rules are verified in the L3 part of the XAS: for spolarization, the intensity in the middle is much weaker,which suggests that the three peaks at 709.1, 709.8, and710.7 eV are coming from the effect of crystal field. Asshown in Fig. 2(a) as symbols, the calculated matrix ele-ments qualitatively agree with the dichroism for L3 excita-tion. The less obvious agreement for L2 excitations ispresumably due to a mixed energy splitting from crystal fieldand p-d interactions.21 The peak positions allow for a roughdetermination of the crystal field splitting assuming similar d-dinteractions for different Fe 3d states: Ee0  Ee00 ¼ 0:7 eV andEa01 Ee0 ¼ 0:9 eV, as also shown in Fig. 2(d). The splitting issignificantly larger than that in LuFe2O4 ðEe0  Ee00 ¼ 0:4 eVand Ea01 Ee0 ¼ 0:8 eV for Fe3þ sites).25 The differencesindicate stronger Fe-O interactions, as also suggested by thedifferent Fe-O bond length in h-LuFeO3 and in LuFe2O4.4,27In order to further elucidate the electronic structure ofthe h-LuFeO3, we measured optical absorption spectra of theh-LuFeO3 films. The observed spectra (Fig. 3) shows threepeak-like features at approximately 2.3, 2.9, and 3.9 eV, con-sistent with the recently reported optical properties ofh-RFeO3 films.9 Based upon the overall intensity 107 m1,these peaks correlate to dipole-allowed excitations. Since theFe3þ has a 3d5 configuration, all the on-site excitations arespin forbidden. Therefore, the peak at 2.3 eV is coming fromcharge transfer excitations.18 A 2.06 0.1 eV optical bandgapwas extracted using plots of a2 versus energy (Fig. 3 inset),assuming a direct gap.28First-principles electronic structure calculations can pro-vide insightful picture of crystal field splitting. We deter-mined our projected density of states (PDOS) by the densityfunctional theory (DFT) implemented in the Vienna ab initiosimulations package (VASP).29,30 We adopted the Perdew-Burke-Ernzerhof functional revised for solids (PBEsol)31 inwhich the spin-polarized generalized gradient approximationFIG. 2. Crystal field splitting of Fe sites (300K). (a) The x-ray absorptionspectra corresponding to Fe 2p to Fe 3d excitations. The symbols are the cal-culated matrix elements from the initial to the final one-electron states. (b)Schematics of the experimental setup. (c) Schematics of the local environ-ment of Fe sites. (d) The crystal splitting extracted from the XAS spectra.TABLE I. The spin-allowed excited states and the dipole selection rulesfrom the 6A01ð2p43=22p21=23d2e003d2e03d1a01Þ ground state with a linearly polarizedphoton. Note that both 2p43=22p11=2 and 2p33=22p21=2 multiplets contain IR E0and A002.3d Configuration IR Allowed Polarization3d3e003d2e03d1a01A001 þ A002 þ E0 þ E00 x, y, z3d2e003d3e03d1a01A001 þ A02 þ E0 þ E00 x, y3d2e003d2e03d2a01E0 þ A002 x, y, zFIG. 1. The x-ray diffraction spectra of h-LuFeO3 films grown on (a) Al2O3substrates with Pt buffer layer and (b) YSZ substrates.241907-2 Wang et al. Appl. Phys. Lett. 101, 241907 (2012)Downloaded 18 Jan 2013 to 165.106.222.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions(GGA) is made in treating the exchange correlation effect ofelectrons. The resulting PDOS is presented in Fig. 4. Onecan clearly see that our theoretical results are consistent withthe experimental data in which the crystal field states followEa01> Ee0 > Ee00 . An unambiguous assignment of the crystalfield states and energies can be further obtained by generat-ing maximally localized Wannier functions (MLWFs)32based on the ground state electronic structure in the selectedenergy window spanning all the crystal field states underconsideration. The resulting MLWFs for each crystal field stateare shown in Fig. 4 and the resulting energies are Ee0  Ee00¼ 0:41 eV and Ea01 Ee0 ¼ 0:81 eV, which are close to theexperimental values of 0.7 and 0.9 eV, respectively. This quali-tative agreement between experiment and theory is expectedwithin the frame work of DFT. However, we should be awarethat a more proper treatment of electron-hole excitations byGW based Bethe-Salpeter method33,34 can further improve thetheoretical prediction.To overcome the severely underestimated bandgap dueto the delocalization error arising from the incomplete can-cellation of the spurious self-interaction, we used theGGAþU with the effective U value (Ueff ¼ U-J) of4.61 eV.37 This gives a bandgap of Eg ¼ 1:35 eV, which stillunderestimates our experimental value.28 Again, a moreproper treatment of self-energy by GW method will furtherbring the theoretical predictions closer to the experimentalvalue.In conclusion, we have studied h-LuFeO3 films using x-ray and optical spectroscopy. The strong dichroism in XASand large crystal field energy splitting of Fe3þ may be relatedto the stability of hexagonal phase of LuFeO3. A 2.0 eV opti-cal bandgap originated from charge transfer excitations isdetermined from the optical spectra. This important informa-tion of electron structure, confirmed by DFT calculationswill definitely benefit further studies of h-LuFeO3.Research supported by the US DOE, BES MSED(X.S.X.), and partially supported by the Chinese 973 Pro-gram Grant No. 2011CB921801 (J.S.), the US DOE BESGrant DE-SC0002136 (W.B.W.), and the NSF Grant No.1053854 (X.M.C.). Use of the APS supported by the USDOE, BES Contract No. DE-AC02-06CH11357.1D. Khomskii, Physics 2, 20 (2009).2N. A. Spaldin, S.-W. Cheong, and R. Ramesh, Phys. Today 63, 38 (2010).3A. A. Bossak, I. E. Graboy, O. Y. Gorbenko, A. R. Kaul, M. S. Kartavt-seva, V. L. Svetchnikov, and H. W. Zandbergen, Chem. Mater. 16, 1751(2004).4E. Magome, C. Moriyoshi, Y. Kuroiwa, A. Masuno, and H. Inoue, Jpn. J.Appl. Phys., 49, 09ME06 (2010).5A. R. Akbashev, A. S. Semisalova, N. S. Perov, and A. R. Kaul, Appl.Phys. Lett. 99, 122502 (2011).6Y. K. Jeong, J.-H. Lee, S.-J. Ahn, S.-W. Song, H. M. Jang, H. Choi, and J.F. Scott, J. Am. Chem. Soc. 134, 1450 (2012).7H. Iida, T. Koizumi, Y. Uesu, K. Kohn, N. Ikeda, S. Mori, R. Haumont, R.Janolin, P. E. Kiat, J. M. Fukunaga, and Y. Noda. J. Phys. Soc. Jpn. 81,024719 (2012).8L. J. Downiea, R. J. Goffa, W. Kockelmannb, S. D. Forderc, J. E. Parkerd,F. D. Morrisona, and P. Lightfoota, J. Solid State Chem. 190, 52 (2012).9V. V. Pavlov, A. R. Akbashev, A. M. Kalashnikova, V. A. Rusakov, A. R.Kaul, M. Bayer, and R. V. Pisarev, J. Appl. Phys. 111, 056105 (2012).10W. B. Wang, J. Zhao, Z. Gai, N. Balke, M. Chi, H. N. Lee, W. Tian, L.Zhu, X. Cheng, D. J. Keavney, J. Yi, T. Z. Ward, P. C. Snijders, H. M.Christen, J. Shen, and X. S. Xu, e-print arXiv 1209.3293 (2012).11K. Kuribayashi, K. Nagashio, K. Niwata, M. S. V. Kumar, and T. Hibiya,Rev. Adv. Mater. Sci. 18, 444 (2008).12M. S. V. Kumar, K. Kuribayashi, and K. Kitazono, J. Am. Ceram. Soc. 92,903 (2009).13M. S. V. Kumar, K. Nagashio, T. Hibiya, and K. Kuribayashi, J. Am.Ceram. Soc. 91, 806 (2008).14C. J. Fennie and K. M. Rabe, Phys. Rev. B 72, 100103 (2005).15G. J. McCarthy, P. V. Gallagher, and C. Sipe, Mater. Res. Bull. 8, 1277–1284 (1973).16I. E. Chupis and G. A. Smolenskii, Sov. Phys. Usp. 25, 475 (1982).17R. Thomas, J. F. Scott, D. N. Bose, and R. S. Katiyar, J. Phys: Condens.Matter 22, 423201 (2010).18X. S. Xu, T. V. Brinzari, S. Lee, Y. H. Chu, L. W. Martin, A. Kumar, S.McGill, R. C. Rai, R. Ramesh, V. Gopalan, S.-W. Cheong, and J. L. Mus-feldt, Phys. Rev. B 79, 134425 (2009).19The lattice constants of h-LuFeO3 films on YSZ and on pt buffered Al2O3are the same within the experimental uncertainty, due to the fast structuralrelaxation (see Ref. 3). Our measurements show similar XAS spectra forthe two kinds of films, while those of h-LuFeO3=YSZ have worse statisticsdue to the charging problem.20D.-Y. Cho, J.-Y. Kim, B.-G. Park, K.-J. Rho, J.-H. Park, H.-J. Noh, B. J.Kim, S.-J. Oh, H.-M. Park, J.-S. Ahn, H. Ishibashi, S.-W. Cheong, J. H.Lee, P. Murugavel, T. W. Noh, A. Tanaka, and T. Jo, Phys. Rev. Lett. 98,217601 (2007).21J. Stohr and H. C. Siegmann, Magnetism from Fundamentals to NanoscaleDynamics (Springer, Berlin, 2006).FIG. 4. Projected density of states for conduction band of h-LuFeO3 withtheoretical ground state structure (space group P63cm) and MLWFs (byWIEN2K software package (Refs. 35 and 36)) with crystal field splitting statescharacters.FIG. 3. Optical absorption coefficient a as a function of photon energy.Inset: a2 as a function of photon energy, which indicates an optical bandgapof 2.0 eV.241907-3 Wang et al. Appl. Phys. Lett. 101, 241907 (2012)Downloaded 18 Jan 2013 to 165.106.222.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions22G. van der Laan, J. Zaanen, G. A. Sawatzky, R. Karnatak, and J.-M.Esteva, Phys. Rev. B 33, 4253 (1986).23H. Wadati, D. Kobayashi, H. Kumigashira, K. Okazaki, T. Mizokawa, A.Fujimori, K. Horiba, M. Oshima, N. Hamada, M. Lippmaa, M. Kawasaki,and H. Koinuma. Phys. Rev. B 71, 035108 (2005).24F. A. Cotton, Chemical Applications of Group Theory (Wiley, New York,1990).25K.-T. Ko, H.-J. Noh, J.-Y. Kim, B.-G. Park, J.-H. Park, A. Tanaka, S. B.Kim, C. L. Zhang, and S.-W. Cheong, Phys. Rev. Lett. 103, 207202 (2009).26These multiplets are divided into groups due to the p-d interactions, whichhave an energy scale of 5 eV (Ref. 21).27M. Isobe, N. Kimizuka, J. Iida, and S. Takekawa, Acta Crystallogr., Sect C.46, 1917 (1990).28Our first principle calculations show a fairly flat top for the valence band,indicating a mixture of direct and indirect nature of the bandgap. Here, wemade the assumption that the part of optical absorption spectrum shown inFig. 3 inset is related to a direct bandgap according to the strong absorp-tion coefficient (107 m1) above the gap. The tail below the gap in Fig. 3is consistent with a typical experimental artifact on film samples due to thereflections.29G. Kresse and J. Hafner, Phys. Rev. B 47, R558 (1993); G. Kresse and J.Furthmuller, Phys. Rev. B 54, 11169 (1996).30A 500 eV planewave cutoff and 6 6 3 k points mesh are used.31J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria,L. A. Constantin, X. L. Zhou, and K. Burke, Phys. Rev. Lett. 100, 136406(2008).32N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997).33G. Onida, L. Reining, and A. Rubio, Rev. Mod. Phys. 74, 601 (2002).34W. Chen, X. Wu, and R. Car, Phys. Rev. Lett. 105, 017802 (2010).35K. Schwartz and P. Blaha, Comput. Mater. Sci. 28, 259 (2003).36The energy threshold separating the localized and nonlocalized electronicstates is chosen to be 7Ry. The muffin-tin radii are 2.35 Bohr for Lu,1.98 Bohr for Fe, and 1.65 Bohr for O. The criterion for the number ofplane waves is chosen to be RminMT  Kmax ¼ 7, and the number of k points is8 8 2 for A-type antiferromagnetic calculation.37H. J. Xiang and M.-H. Whangbo, Phys. Rev. Lett. 98, 246403 (2007).241907-4 Wang et al. Appl. Phys. Lett. 101, 241907 (2012)Downloaded 18 Jan 2013 to 165.106.222.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Crystal Field Splitting and Optical Bandgap of Hexagonal LuFeO3 Films

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Crystal Field Splitting and Optical Bandgap of Hexagonal LuFeO3 Films

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