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Tuning Magnetism in Transition-Metal-Doped 3C Silicon Carbide Polytype Jing Zhou,† Haiming Li,† Linjuan Zhang,† Jie Cheng,‡ Haifeng Zhao,† Wangsheng Chu,† Jinlong Yang,*,§ Yi Luo,*,‡,§ and Ziyu Wu*,†,‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China, National Synchrotron Radiation Lab, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China, and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: October 27, 2010
Structural and magnetic properties of 3d transition-metal-doped silicon carbide in cubic (3C) polytype have been systematically studied from first principles to reconcile conflicting experimental findings. The most energetically favorable structures fall in two distinct sets depending on the character of the 3d transition metal and the Si atomic chemical potential. The structure of substitutional TMSi is the most stable one for early transition metals like Ti, V, Cr, and Mn, while the clustering of TMSi-TMI dimers formed by the neighboring substitutional TMSi and interstitial TMI is energetically favored for late transition metals such as Co, Ni, and Cu. For Fe, the most stable structure is the substitutional configuration under C-rich conditions, while under Si-rich conditions the clustering of the FeSi-FeI dimer is energetically favored. It is found in the doped silicon carbide that the Co dimer is nonmagnetic, while both Ni and Cu atoms interact ferromagnetically and make the whole doped system half metallic. Fe atoms show a ferrimagnetic order with a local magnetic moment of 2.0 and -0.34 µB at substitutional and interstitial sites, respectively. Such intrinsically tunable magnetic properties of 3d transition-metal-doped silicon carbide could find many exciting potential applications in spintronics. Diluted magnetic semiconductors (DMS) have attracted much attention in recent years triggered by the possible development of novel functional spintronics materials. In DMS-based materials, the interplay between charge and spin degrees of freedom is well beyond conventional semiconductor materials,1-4 which provides the opportunity of extending the functionality of traditional materials to design new types of spintronics devices such as nonvolatile transistors5 and light-induced spin-crossover materials6 and for applications in quantum computers.7 Many theoretical and experimental works have shown that ferromagnetism above room temperature can be observed in certain wide band gap semiconductors, including a family of III-nitrides and ZnO, doped with 3d transition-metal (TM) elements.8-11 Silicon carbide (SiC) is a wide gap semiconductor with great potential in high-temperature, high-power, high-frequency, and radiation-hardened devices. Several experimental studies point out that SiC could be a good candidate for diluted magnetic semiconductor since Ni-, Mn-, and Fe-doped SiC show interesting magnetic properties.12-16 These three TM-doped SiC systems show a ferromagnetic order with different Curie temperature TC varying from very low to room temperature. However, the origin of the magnetic interaction has been attributed to the intrinsic DMS property,12-14 defects,15 or a secondary phase formation.14,16 Most strikingly, a typical glassy ferromagnetic response could be observed in an Al-doped 4H SiC even up to 300 K.17 First-principles calculations have been carried out for * To whom correspondence should be addressed. E-mail: wuzy@ ustc.edu.cn;
[email protected];
[email protected]. † Chinese Academy of Sciences. ‡ National Synchrotron Radiation Lab, University of Science and Technology of China. § Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China.
substitutional TM-doped SiC DMSs. Gubanov et al.18 and Miao et al.19 found that in a 3C SiC a series of 3d TM impurities favor substitution at the Si site and not at the C site. Miao et al.19 further analyzed the magnetic interaction for early 3d TMs doped 3C SiC. It was shown that Mn and Cr are ferromagnetically stable in their high-spin state,20 while Fe is paramagnetic.18,19,21 However, Los et al. pointed out that TMs in a SiC host may exist in both magnetic and nonmagnetic states which sensitively depend on the environment around the TM atom in the host matrix.22-24 On the basis of these studies, one could anticipate that a TM-doped SiC might indeed be an excellent DMS system with tunable magnetic properties. However, a good understanding of the underlying mechanism for different magnetic properties of a TM-doped SiC is still lacking. A systematic study on the TM-doped SiC is highly desirable and will be extremely useful to explore future spintronics applications. In this work, we carried out first-principles calculations to determine the energetically most stable microstructure of all possible 3d TMs in the 3C SiC. It is found that in SiC late 3d TMs, such as Co, Ni, and Cu, favor a dimer structure, a quite unusual behavior that leads to peculiar magnetic properties. The pseudo-potential method as implemented in the Vienna Ab Initio Simulation Package (VASP)25,26 has been employed for all calculations. The exchange-correlation interaction was described by the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof.27 The electronic wave functions were expanded using a plane-wave basis set with a cutoff energy of 450 eV. The Brillouin-zone integration for geometry optimization was performed by using a 4 × 4 × 4 Monkhost-Pack k-points grid.28 Test calculations showed that using more k points did not lead to any noticeable changes in the converged energy. Atomic positions of the structure were relaxed until all force components became 0.7 eV under Si-rich conditions and >0.5 eV under C-rich conditions) that of the substitutional structure and of the interstitial structure. In the dimer structure, a total magnetic moment of 2.0 µB is obtained. The interstitial Cu is slightly spin polarized with a local magnetic moment of 0.1 µB, and the substitutional Cu atom contributes also to a small local magnetic moment of about 0.35 µB. On the other hand, bonded C atoms are largely spin polarized, with a relatively large magnetic moment of 0.18 µB. Figure 4c and 4d shows the total and Cu 3d partial DOS of Cu-doped SiC in the dimer structure. It can be clearly seen that the Cu-doped SiC in the dimer structure is also a half-metallic system. However, contrary to the Ni-doped SiC, its Fermi energy crosses the minority spin band and its semiconductor behavior occurs for the majority spin band. Previous calculations showed that the Cu-doped SiC with Cu substitution at the Si site was AFM,19 while our calculations clearly point out the existence of the dimer structure with a ferromagnetic Cu-Cu interaction. From an energetic point of view, a single ferromagnetic Cu-doped SiC phase with
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Zhou et al. to half-metallic systems for which a variety of spintronics applications may occur. Acknowledgment. This work was partly supported by the National Outstanding Youth Fund (Project No. 10125523 to Z.W.) and by the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-N42) References and Notes
Figure 5. (a) Total and (b) Fe 3d partial DOS for the FeSi-FeI dimer of the Fe-doped SiC. All energies refer to the position of the Fermi energy. Positive (negative) values correspond to the majority (minority) spin contribution.
a half-metallic property can be easily obtained if the growth temperature is adequate. Our calculations also show that the substitutional structure of the Fe-doped SiC is nonmagnetic, in good agreement with previous results.23 However, under Si-rich conditions as for other late 3d TMs, the dimer structure is the most stable one in the Fe-doped SiC. The total and partial DOS for the Fe-doped SiC in the dimer structure is shown in Figure 5. For the dimer structure, a total magnetic moment of 2.0 µB is obtained. More specifically, the interstitial Fe is spin polarized with a local magnetic moment of 0.34 µB, and the substitutional Fe atom contributes with a local magnetic moment of 2.0 µB. More strikingly, the spin moments of the two Fe atoms are antiparallel, in other words, that Fe-doped SiC with a dimer structure is ferrimagnetic, a configuration really different from both Cuand Ni-doped SiC systems. To conclude, our first-principles calculations point out many interesting magnetic properties of TM-doped SiC systems. Actually, we found that in SiC the second half 3d TMs favor a dimer structure. The local spins and their mutual arrangements in the different TM-doped SiC systems lead to different magnetic properties, ranging from the nonmagnetism of Co, half metallic for Ni and Cu, and ferrimagnetism for Fe. We claim that careful control of the growth conditions of TM-doped SiC systems may lead to different magnetic systems, in particular
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