The magnetism of C-doped SnO$_{2}$ (001) surfaces is studied using
first-principles calculations. It is found that carbon does not induce
magnetism in bulk SnO$_{2}$ when located at the oxygen site, but shows a large
magnetic moment at the SnO$_{2}$ (001) surface. The magnetic moment is mainly
contributed by the carbon atoms due to empty minority spins of $p$ orbitals and
is localized at the surface and subsurface atoms. No magnetism is observed when
the carbon atom is located at the subsurface oxygen sites. The origin of
magnetism is discussed in the context of surface bonding.Comment: 3 pages, 3 figure

García-Suárez, Víctor M.

Rahman, Gul

English

arXiv

García Suárez, Víctor Manuel

Repositorio Institucional de la Universidad de Oviedo

Applied Physics Letters

Surface-induced magnetism in C-doped SnO2

Crossref

3 páginas, 3 figuras.The magnetism of C-doped SnO2 (001) surfaces is studied using first-principles calculations. It is found that carbon does not induce magnetism in bulk SnO2 when located at the oxygen site but shows a large magnetic moment at the SnO2 (001) surface. The magnetic moment is mainly contributed by the carbon atoms due to empty minority spins of p orbitals and is localized at the surface and subsurface atoms. No magnetism is observed when the carbon atom is located at the subsurface oxygen sites. The origin of magnetism is discussed in the context of surface bonding.V.M.G.S. thanks the Spanish Ministerio de Ciencia e
Innovación for a Juan de la Cierva fellowship and the
Marie Curie European ITNs FUNMOLS and NANOCTM
for funding.Peer reviewe

Digital.CSIC

Surface-induced magnetism in C-doped SnO2Gul Rahman and Víctor M. García-Suárez  Citation: Appl. Phys. Lett. 96, 052508 (2010); doi: 10.1063/1.3302468 View online: http://dx.doi.org/10.1063/1.3302468 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v96/i5 Published by the American Institute of Physics.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 18 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsSurface-induced magnetism in C-doped SnO2Gul Rahman1,a and Víctor M. García-Suárez2,b1Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-834,Republic of Korea2Departamento de Física, Universidad de Oviedo–CINN, 33007 Oviedo, Spain and Department of Physics,Lancaster University, Lancaster LA1 4YB, United KingdomReceived 16 November 2009; accepted 11 January 2010; published online 4 February 2010The magnetism of C-doped SnO2 001 surfaces is studied using first-principles calculations. It isfound that carbon does not induce magnetism in bulk SnO2 when located at the oxygen site butshows a large magnetic moment at the SnO2 001 surface. The magnetic moment is mainlycontributed by the carbon atoms due to empty minority spins of p orbitals and is localized at thesurface and subsurface atoms. No magnetism is observed when the carbon atom is located at thesubsurface oxygen sites. The origin of magnetism is discussed in the context of surface bonding.© 2010 American Institute of Physics. doi:10.1063/1.3302468In search of spintronic materials, many materials, whichare nonmagnetic NM but can show magnetism withtransition-metal TM doping,1,2 have been discovered. Thebest example is GaAs, which is a semiconductor, but exhibitsinteresting magnetic properties when doped with Mn.3 Thesearch for spintronic materials has also produced magneticcompounds with transition temperatures TC higher thanroom temperature. Among these high TC compounds, apromising but less studied material is TM doped-SnO2,which evoked a lot of attention when S. B. Ogale et al.4found a giant magnetic moment GMM in Co-doped SnO2.Following this discovery, TM doped-SnO2 has been exten-sively studied experimentally5–7 and theoretically.8–11Very recently,12 we investigated the origin of GMM indoped SnO2. Our calculations showed that Sn vacancies areresponsible for magnetism in this material. This allowed usto classify SnO2 into a class of magnetic materials wheremagnetism is made possible without magnetic impurities.Therefore, intrinsic defects in SnO2 can play an importantrole to explain magnetism or even optical conductivity.13 Re-cent theoretical calculations further showed that magnetismcan be induced with NM impurities.14,15 A good example ofNM impurity is carbon, which has been recently predicted toinduce magnetism in bulk materials.16–18All previous investigations focused mainly on bulk mag-netism. However, there are no reports on the surface magne-tism of SnO2. The questions that will be addressed here areas follows: what is the role played by the surface and is itpossible to induce magnetism at the surface without TM im-purities? We, therefore, go beyond bulk magnetism and showthat a surface can also induce magnetism in SnO2 withoutTM impurities, which proves that impurities that are NM inbulk can become magnetic on the surface. This is not a gen-eral rule because the electronic and magnetic properties de-pend strongly on the combination of host and impurity18–20e.g., other sp elements are magnetic in bulk SnO2 14,15,20but demonstrates the feasibility of such effect.Bulk SnO2 has a tetragonal unit cell with two SnO2 unitslayers per cell. Each Sn atom is coordinated to six O atomssituated at the vortices of a distorted octahedron. Each Oatom is coordinated to three Sn atoms labeled Sn2, Sn3,and Sn4 in Fig. 1 and all three O–Sn bonds lie on the sameplane.To study the bulk properties of C-doped SnO2, a 222 supercell was considered and a C atom was located atthe O site. For surface magnetism, we considered a 001surface of SnO2 and used different numbers of layers. WeaElectronic mail: grnphysics@yahoo.com.bElectronic mail: v.garcia-suarez@lancaster.ac.uk.1234IIIIIIIVFIG. 1. Stoichiometric supercell of SnO2 used in the calculations for the001 surface. Big and small balls represent tin and oxygen atoms, respec-tively. Surface oxygen and tin atoms are represented by OI and Sn1,respectively. The immediate subsurface atoms are represented by OII andSn2. Note that OII is surrounded by three Sn atoms, Sn2, Sn3, andSn4. The surface OI is surrounded only by two Sn atoms, Sn1 andSn2.APPLIED PHYSICS LETTERS 96, 052508 20100003-6951/2010/965/052508/3/$30.00 © 2010 American Institute of Physics96, 052508-1Downloaded 18 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsconstructed free stoichiometric slabs with total compositionsof Sn6O12 six layers, Sn7O14 seven layers, see Fig. 1, andSn10O20 ten layers separated by a vacuum region of 10 Å.Note that six layers are already enough to converge the SnO2surface.21,22 To include the C atom different O sites, i.e.,OI, OII, OIII, and OIV, were considered for all layers.We will only focus on two cases, OI and OII, becauseOIII and OIV are similar to OII.First-principles calculations based on density functionaltheory within the local spin density approximation23 wereperformed using the SIESTA code24 with a double-zeta polar-ized basis set. The Hamiltonian matrix elements were calcu-lated on a real space grid defined with a plane-wave energycutoff of 400 Ry. A 551 Monkhorst–Pack mesh wasused for k-point sampling. All atomic positions were fullyrelaxed until all atomic forces were smaller than 0.05 eV/Å.For all calculations, the optimized lattice parameters12 wereused.Nonmagnetic NM and spin-polarized SP calculationswere performed for bulk C-doped SnO2. The calculated atomprojected density of states PDOS of C and O are shown inpanel c of Fig. 2. The bulk PDOS shows no exchange split-ting, which signals that C does not induce magnetism in bulkSnO2. We found however that N can induce magnetism inbulk SnO2, in agreement with previous studies.20 It was alsoconfirmed that the O vacancy at the surface25 and in bulk12does not show any magnetism.Figure 2a shows the PDOS on the orbitals of the sur-face atoms when the C atom is located at the surface Oposition, i.e., at OI. Clearly, the C atom induces magnetismat the 001 surface of SnO2. The low lying s orbitals of Care SP and strongly hybridized with the surface sp orbitals ofSn. The Fermi energy EF is mainly dominated by the porbitals of C, which indicates that magnetism is mainly in-duced by the p orbitals and localized at the C atom. Majorityp spin states of the C atom are completely occupied andminority spin states are nearly empty, leading to a significantspin splitting. The impurity bands introduced by the C atomhave then a large weight and are rather localized below andabove the valence band maximum so that EF is just betweenthe majority and minority spin states Fig. 2a. Similarbehavior was also observed in other C-doped bulksystems.16,18,26 Surface states near EF at the Sn and O atomswhich have strong bonding with the C surface atom can alsobe seen.The calculated magnetic moment is 2.00 B /cell. The Catom contributes about 1.75 B, according to the Mullikenpopulations given by SIESTA. The induced magnetic momentsat the surface Sn1 and OI atoms are 0.002 and 0.077 B,respectively. The magnetic moments of the atoms in the slabhave the same direction, indicating that there is a FM cou-pling between the C atom and the neighboring surface atoms.The PDOS on the subsurface atoms is shown in Fig. 2b.The subsurface atoms OII, Sn2 are also polarized andcontribute partially to magnetism. The induced magnetic mo-ments at the subsurface Sn2 and both the OII atoms are0.002 and 0.068 B, respectively. In both cases surface andsubsurface, the O atoms have larger magnetic moments thanthe Sn atoms because their p orbitals have larger weight nearEF, where the C spin-split p orbitals are localized.We discuss now the case where the C atom is located atthe subsurface O site, i.e., at OII; this configuration re-sembles the case where the C atom is covered by layers ofSnO2, which is NM. Now the surface layer SnO2 screens theC atom and therefore the system is expected to behave like abulk C-doped SnO2. This case does not produce magnetismFig. 3 since the exchange splitting in the s as well as in thep orbitals of the C atom is absent. The PDOS shows that the001 surface has small surface states just below EF, which isin agreement with previous theoretical calculations.21 Asmall peak, which is induced by the subsurface C p orbitalsjust above EF, can also be seen. The PDOS also shows hy-bridization between the p orbitals of C, Sn, and O atoms,particularly near EF. We can then conclude that C does notinduce magnetism when located at the subsurface OII po-sitions due to the lack of empty minority p spin states, asopposed to the surface OI positions.Figure 1 illustrates that each O atom is surrounded bythree Sn atoms and each atomic layer consists of O–Sn–Ounits which have a total of 16 valence electrons. A simpleelectron counting suggests that when an O atom is replaced-10 -5 0 5-101PDOS(a.u.)-10 -5 0 5-101-10 -5 0 5-0.300.3PDOS(a.u.)-10 -5 0 5-0.300.3spd-10 -5 0 5E - EF(eV)-101PDOS(a.u.)-10 -5 0 5E - EF(eV)-101COO OSn SnC(c)(a) (b)FIG. 2. Color online PDOS on the surface a and subsurface b atomswhen the C atom is located on the surface. Solid, dashed and dashed-dottedlines represent s, p, and d orbitals, respectively. The positive negativePDOS shows majority minority spin states. Panel c shows the PDOS ofC and the nearest O in bulk C-doped SnO2.-10 -5 0 5-101PDOS(a.u.)-10 -5 0 5-101-10 -5 0 5E - EF(eV)-0.300.3PDOS(a.u.)-10 -5 0 5-0.300.3spd-10 -5 0 5E - EF(eV)-101PDOS(a.u.)O OSn SnC(a) (b)FIG. 3. Color online PDOS on the surface a and subsurface b atomswhen the C atom is located below the surface. Solid, dashed, and dashed-dotted dotted lines represent s, p, and d orbitals, respectively. The positivenegative PDOS shows majority minority spin states.052508-2 G. Rahman and V. M. García-Suárez Appl. Phys. Lett. 96, 052508 2010Downloaded 18 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsby a C atom, the total number of valence electron changes to14 and hence induces holes in the system. At the 001 sur-face, the O atoms are not surrounded by three Sn atoms butby two Sn atoms. Hence, magnetism at SnO2 001 in thiscase is driven simultaneously by a surface effect low coor-dination and unpaired electrons. Calculations with two Clocated at the O sites show that the magnetic moment in-creases, suggesting that hole doping enhances magnetism. Inbulk C-doped SnO2 the C atom bonds directly to the three Snatoms, which allow the six valence electrons to delocalize onthe states associated to the bonds and reduce their Coulombrepulsion, leaving empty the localized state of the C atom, ascan be seen in Figs. 2c and 3b. In the surface, however,the number of bonds is reduced to two, which leaves twolocalized states, as can be seen in Fig. 2a. The two elec-trons which are not associated to the bonds follow thenHund’s rules to reduce again their Coulomb repulsion andoccupy each state with parallel spins, which gives a totalmagnetic moment of 2.00 B /C.Finally, similar calculations were also performed for dif-ferent layers of SnO2 and we always found that the C atomcan only induce magnetism at the surface layers and themagnetic moment is only localized at the surface and sub-surface atoms.In summary, first-principles calculations showed that Cinduces a large magnetic moment at the SnO2 001 surface.The magnetic moment, which is localized at the surface andsubsurface atoms, was mainly contributed by the C atom andpartially by the O atoms. When the C atom was located at thesubsurface O sites it did not show magnetism. The possibleorigin of magnetism was discussed in terms of surface bond-ing. Further work would be necessary to investigate similarphenomena at other semiconductor surfaces. We hope ourresults stimulate further experimental verification.V.M.G.S. thanks the Spanish Ministerio de Ciencia eInnovación for a Juan de la Cierva fellowship and theMarie Curie European ITNs FUNMOLS and NANOCTMfor funding.1G. Rahman, S. Cho, and S. C. Hong, Phys. Status Solidi B 244, 44352007.2G. Rahman, S. Cho, and S. C. Hong, Phys. Status Solidi A 205, 18602008.3H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y.Iye, Appl. Phys. Lett. 69, 363 1996.4S. B. Ogale, R. J. Choudhary, J. P. Buban, S. E. Lofland, S. R. Shinde, S.N. Kale, V. N. Kulkarni, J. Higgins, C. Lanci, J. R. Simpson, N. D.Browning, S. Das Sarma, H. D. Drew, R. L. Greene, and T. Venkatesan,Phys. Rev. Lett. 91, 077205 2003.5J. M. D. Coey, A. P. Douvalis, C. B. Fitzgerald, and M. Venkatesan, Appl.Phys. Lett. 84, 1332 2004.6N. H. Hong, J. Sakai, W. Prellier, and A. Hassini, J. Phys.: Condens.Matter 17, 1697 2005.7N. H. Hong and J. Sakai, Physica B 358, 265 2005.8C.-W. Zhang and S.-W. Yan, J. Appl. Phys. 106, 063709 2009.9H. Wang, Y. Yan, Y. S. Mohammed, X. Du, K. Li, and H. Jin, J. Magn.Magn. Mater. 321, 337 2009.10X. L. Wang, Z. Zeng, and X. H. Zheng, J. Appl. Phys. 101, 09H1042007.11S.-J. Hu, S.-S. Yan, X.-X. Yao, Y.-X. Chen, G.-L. Liu, and L.-M. Mei,Phys. Rev. B 75, 094412 2007.12G. Rahman, V. M. García-Suárez, and S. C. Hong, Phys. Rev. B 78,184404 2008.13C. Kılıç and A. Zunger, Phys. Rev. Lett. 88, 095501 2002.14W.-Z. Xiao, L.-L. Wang, L. Xu, Q. Wan, and B. S. Zou, Solid StateCommun. 149, 1304 2009.15W. Zhou, L. Liu, and P. Wu, J. Magn. Magn. Mater. 321, 3356 2009.16K. S. Yang, Y. Dai, B. B. Huang, and M. Whangbo, Appl. Phys. Lett. 93,132507 2008.17H. Pan, Y. P. Feng, Q. Y. Wu, Z. G. Huang, and J. Y. Lin, Phys. Rev. B 77,125211 2008.18R. Long and N. J. English, Phys. Rev. B 80, 115212 2009.19L. Shen, R. Q. Wu, H. Pan, G. W. Peng, M. Yang, Z. D. 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Surface-induced magnetism in C-doped SnO $_{2}$

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Surface-induced magnetism in C-doped SnO $_{2}$