Electronic and magnetic properties of SnO_{2}/CrO_{2} thin superlattices
 Pablo D Borges^{1}Email author,
 Luísa MR Scolfaro^{2},
 Horácio W Leite Alves^{3},
 Eronides F da SilvaJr^{4} and
 Lucy VC Assali^{1}
DOI: 10.1186/1556276X6146
© Borges et al; licensee Springer. 2011
Received: 25 August 2010
Accepted: 15 February 2011
Published: 15 February 2011
Abstract
In this article, using firstprinciples electronic structure calculations within the spin density functional theory, alternated magnetic and nonmagnetic layers of rutileCrO_{2} and rutileSnO_{2} respectively, in a (CrO_{2})_{ n }(SnO_{2})_{ n }superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A halfmetallic behavior is observed for the (CrO_{2})_{ n }(SnO_{2})_{ n }SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μ_{B} per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutileCrO_{2} is unstable at ambient temperature, and known to be stabilized when on top of SnO_{2}, the authors suggest that (CrO_{2})_{ n }(SnO_{2})_{ n }SLs may be applied to spintronic technologies since they provide efficient spinpolarized carriers.
Introduction
A variety of heterostructures have been studied for spintronics applications, and they have proved to have a great potential for highperformance spinbased electronics [1]. A key requirement in developing most devices based on spins is that the host material must be ferromagnetic (FM) above 300 K. In addition, it is necessary to have efficient spinpolarized carriers. One approach to achieve the spin injection is to create builtup superlattices (SLs) of alternating magnetic and nonmagnetic materials. One attempt has already been made by Zaoui et al. [2], through ab initio electronic structure calculations for the one monolayer (ZnO)_{1}(CuO)_{1} SL, with the aim of obtaining a halfmetallic behavior material, since they are 100% spin polarized at the Fermi level and therefore appear ideal for a welldefined carrier spin injection.
In this study, the magnetic and electronic properties of (CrO_{2})_{ n }(SnO_{2})_{ n }SLs with n = 1, 2, ..., 10 being the number of monolayers are investigated. These systems are good candidates to obtain a halfmetallic behavior material since bulk rutileCrO_{2} has shown experimentally this behavior [3] and recently magnetic tunnel junctions based on CrO_{2}/SnO_{2} epitaxial layers have been obtained [4].
Theoretical method
Results and discussion
Experimental and calculated values for the lattice parameters of the SnO_{2}, CrO_{2}, and of the (CrO_{2})_{1}(SnO_{2})_{1} and (CrO_{2})_{10}(SnO_{2})_{10} SLs in the rutile structure
a(Å)  c/a  u  

SnO_{2}  4.737^{a}  0.673^{a}  0.307^{a} 
4.839^{b}  0.670^{b}  0.306^{b}  
CrO_{2}  4.421^{c}  0.6596^{c}  0.301^{c} 
4.455^{d}  0.6569^{d}  0.304^{d}  
(CrO_{2})_{1}(SnO_{2})_{1}  4.625^{d}  0.658^{d}  ^{} 
(CrO_{2})_{10}(SnO_{2})_{10}  4.640^{d}  6.546^{d}  ^{} 
The advantage in using the SnO_{2}/CrO_{2} SLs, despite the fact that CrO_{2} is unstable at room temperature, is that its stability becomes possible when grown on SnO_{2}[12]. Our results showed that the interface effects due to the lattice mismatch do not change the chromium dioxide magnetism characteristics. If the distances between two planes perpendicular to the rutile caxis containing the Cr_{2} and Sn_{1} are compared (see Figure 1a), at the interface region of the SL, before and after full relaxations, then changes of only approximately 4% are observed for all the studied SLs.
Conclusions
In conclusion, the results of firstprinciples electronic structure calculations, within the spin density functional theory, carried out for (CrO_{2})_{ n }(SnO_{2})_{ n }SLs formed by alternating magnetic and nonmagnetic layers of rutileCrO_{2} and rutileSnO_{2}, where the number of monolayers n was varied from 1 to 10, have been reported in this article. A halfmetallic behavior is observed for all the studied (CrO_{2})_{ n }(SnO_{2})_{ n }SLs. The ground state is FM, with a magnetic moment of 2 μ_{B} per chromium atom, which is independent of the number of monolayers. As the FM rutileCrO_{2} is unstable at ambient temperature, and known to be stabilized when on top of SnO_{2}, it is suggested that (CrO_{2})_{ n }(SnO_{2})_{ n }SLs may be applied to spintronic technologies since they provide efficient spinpolarized carriers.
Abbreviations
 AFM:

antiferromagnetic
 FM:

ferromagnetic
 GGAPBE:

generalized gradient approximation and the Perdew, Burke, and Ernzerhof
 NM:

nonmagnetic
 PDOS:

projected density of states
 SL:

superlattice
 TDOS:

total density of states
 VASPPAW:

Vienna Abinitio Simulation Package and the Projected Augmented Wave.
Declarations
Acknowledgements
The authors would like to thank the partial support from the Brazilian funding agencies FAPEMIG, FAPESP, CAPES, and CNPq, and from the Material, Science, Engineering and Commercialization Program at the Texas State University in San Marcos.
Authors’ Affiliations
References
 Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molnár S, Roukes ML, Chtchelkanova AY, Treger DM: Spintronics: A SpinBased Electronics Vision for the Future. Science 2001, 294: 1488. 10.1126/science.1065389View ArticleGoogle Scholar
 Zaoui A, Ferhat M, Ahuja R: Magnetic properties of (ZnO)1/(CuO)1 (001) superlattice. Appl Phys Lett 2009, 94: 102102. 10.1063/1.3095811View ArticleGoogle Scholar
 Anguelouch A, Gupta A, Xiao Gang, Abraham DW, Ji Y, Ingvarsson S, Chien CL: Nearcomplete spin polarization in atomicallysmooth chromiumdioxide epitaxial films prepared using a CVD liquid precursor. Phys Rev B 2001, 64: 180408R. 10.1103/PhysRevB.64.180408View ArticleGoogle Scholar
 Miao GX, LeClair P, Gupta A, Xiao G, Varela M, Pennycook S: Magnetic tunnel junctions based on CrO2/SnO2 epitaxial bilayers. Appl Phys Lett 2006, 89: 022511. 10.1063/1.2216109View ArticleGoogle Scholar
 Kresse G, Furthmuller J: Efficiency of abinitio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996, 6: 15. 10.1016/09270256(96)000080View ArticleGoogle Scholar
 Kresse G, Furthmuller J: Efficient iterative schemes for ab initio totalenergy calculations using a planewave basis set. Phys Rev B 1996, 54: 11169. 10.1103/PhysRevB.54.11169View ArticleGoogle Scholar
 Perdew JP, Burke K, Ernzerhof M: Generalized Gradient Approximation Made Simple. Phys Rev Lett 1996, 77: 3865. 10.1103/PhysRevLett.77.3865View ArticleGoogle Scholar
 Wycokoff R: Crystal Structures. Volume 1. 2nd edition. New York, London: John Wiley & Sons; 1963.Google Scholar
 Borges PD, Scolfaro LMR, Leite Alves HW, da Silva EF Jr: DFT study of the electronic, vibrational, and optical properties of SnO2. Theor Chem Acc 2010, 126: 39. 10.1007/s0021400906723View ArticleGoogle Scholar
 Maddox BR, Yoo CS, Kasinathan D, Pickett WE, Scalettar RT: Highpressure structure of halfmetallic CrO2. Phys Rev B 2006, 73: 144111. 10.1103/PhysRevB.73.144111View ArticleGoogle Scholar
 Srivastava V, Sanyal SP, Rajagopalan M: First Principles study of pressure induced magnetic transition in CrO2. Indian J Pure Appl Phys 2008, 46: 397.Google Scholar
 Zabel H, Bader SD, (Eds): Magnetic Heterostructures: Advances and Perspectives in Spinstructures and Spintransport STMP 227. Berlin: Springer; 2008.Google Scholar
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