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 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 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. 2 , 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 3 and recently magnetic tunnel junctions based on CrO₂/SnO₂ epitaxial layers have been obtained 4 .

For the (CrO₂)₁(SnO₂)₁ SL, the calculation was started with the experimental lattice parameters of the tin dioxide, a = 4.737 Å, c/a = 0.673, and u = 0.307 8 9 10 . The system was relaxed until the residual forces on the ions were less than 10 meV/Å. Good agreement between the calculated and the available experimental values for the lattice parameters is obtained, as seen in Table 1. Figure 1b shows that the ground state is ferromagnetic (FM), being the most stable state compared with the non-magnetic (NM) and anti-ferromagnetic (AFM) ones. For the ground state, the total magnetic moment gives a value of 2 μ_B per chromium atom. Figure 2a,b presents the total density of states (TDOS) and the projected density of states (PDOS), respectively for the Cr 3d orbital, showing that the system has a half metallic behavior, with the Cr 3d orbital appearing in the gap region, characterizing a metallic-like behavior for the majority spin and a semiconductor-like behavior for the minority spin. The band structures of the SL for spin up and spin down are depicted in Figure 2c. A band gap of approximately 1.71 eV is obtained for the minority spin at the Г-point. There is a smaller gap for spin flip excitations from the Fermi level, which is approximately 0.86 eV. For the (SnO₂) _n (CrO₂) _n SLs with n >1, considered here up to n = 10, it was observed that the ground state remains as FM. The interplay of the SnO₂ and CrO₂ layer thicknesses does not change the half-metallic behavior, as can be verified through the DOS shown in Figure 3a,b for n = 10. The magnetic moment per Cr atom, in all the studied cases, is the same and equal to 2 μ_B. Moreover, the SL magnetization does not depend on the number of monolayers. This has been verified by performing calculations with one monolayer of CrO₂ grown between 3, 7, and 11 monolayers of SnO₂. It was observed that the SL magnetization remained equal to 2 μ_B. Our results show a 100% spin polarization at the Fermi level, ideal for a well-defined carrier spin injection.

An investigation, related to strain effects along the z-direction for the rutile phase of CrO₂, was made by simulating bulk rutile-CrO₂, on top of tin dioxide, assuming for CrO₂ the lattice parameter a of SnO₂, i.e., a situation in which the chromium dioxide is tensile. By varying the ratio c/a _SnO2 and minimizing the total energy of the system, the authors obtained the curves shown in Figure 4a for the FM, AFM, and NM states, showing that the transition from a FM to an AFM state occurs when c/a _SnO2 is about 0.544. At this value, a magnetic moment reduction is observed, as depicted in Figure 4b. These results suggest a magnetization change when the SL is under strain or, in other words, when CrO₂ is compressed. A similar behavior was found by Srivastava et al. for bulk rutile-CrO₂ under pressure 11 .

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 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 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 μ_B 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.

AFM: anti-ferromagnetic; FM: ferromagnetic; GGA-PBE: generalized gradient approximation and the Perdew, Burke, and Ernzerhof; NM: non-magnetic; PDOS: projected density of states; SL: superlattice; TDOS: total density of states; VASP-PAW: Vienna Ab-initio Simulation Package and the Projected Augmented Wave.

The authors declare that they have no competing interests.

PB performed the ab initio calculations, participated in the analysis, and drafted the manuscript. LS and PB conceived of the study. HA, ES, LA, and LS participated in the analysis and in the production of a final version of the manuscript. All authors read and approved the final manuscript.