Electronic and magnetic properties of SnO2/CrO2 thin superlattices
© Borges et al; licensee Springer. 2011
Received: 25 August 2010
Accepted: 15 February 2011
Published: 15 February 2011
In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2) n (SnO2) n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2) n (SnO2) n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.
A variety of heterostructures have been studied for spintronics applications, and they have proved to have a great potential for high-performance spin-based electronics . 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 spin-polarized carriers. One approach to achieve the spin injection is to create built-up superlattices (SLs) of alternating magnetic and non-magnetic materials. One attempt has already been made by Zaoui et al. , through ab initio electronic structure calculations for the one monolayer (ZnO)1(CuO)1 SL, with the aim of obtaining a half-metallic behavior material, since they are 100% spin polarized at the Fermi level and therefore appear ideal for a well-defined carrier spin injection.
In this study, the magnetic and electronic properties of (CrO2) n (SnO2) n SLs with n = 1, 2, ..., 10 being the number of monolayers are investigated. These systems are good candidates to obtain a half-metallic behavior material since bulk rutile-CrO2 has shown experimentally this behavior  and recently magnetic tunnel junctions based on CrO2/SnO2 epitaxial layers have been obtained .
Results and discussion
Experimental and calculated values for the lattice parameters of the SnO2, CrO2, and of the (CrO2)1(SnO2)1 and (CrO2)10(SnO2)10 SLs in the rutile structure
The advantage in using the SnO2/CrO2 SLs, despite the fact that CrO2 is unstable at room temperature, is that its stability becomes possible when grown on SnO2. 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 c-axis containing the Cr2 and Sn1 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.
In conclusion, the results of first-principles electronic structure calculations, within the spin density functional theory, carried out for (CrO2) n (SnO2) n SLs formed by alternating magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2, where the number of monolayers n was varied from 1 to 10, have been reported in this article. A half-metallic behavior is observed for all the studied (CrO2) n (SnO2) n SLs. The ground state is FM, with a magnetic moment of 2 μB per chromium atom, which is independent of the number of monolayers. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, it is suggested that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.
generalized gradient approximation and the Perdew, Burke, and Ernzerhof
projected density of states
total density of states
Vienna Ab-initio Simulation Package and the Projected Augmented Wave.
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.
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