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Register a new userCrystalline structure and XMCD studies of Co40Fe40B20 grown on Bi2Te3, BiTeI and Bi2Se3
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Epitaxial films of Co40Fe40B20 (further - CoFeB) were grown on Bi2Te3(001) and Bi2Se3(001) substrates by laser molecular beam epitaxy (LMBE) technique at 200-400C. Bcc-type crystalline structure of CoFeB with (111) plane parallel to (001) plane of Bi2Te3 was observed, in contrast to polycrystalline CoFeB film formed on Bi2Se3(001) at RT using high-temperature seeding layer. Therefore, structurally ordered ferromagnetic thin films were obtained on the topological insulator surface for the first time. Using high energy electron diffraction (RHEED) 3D reciprocal space mapping, epitaxial relations of main crystallographic axes for the CoFeB/ Bi2Te3 heterostructure were revealed. MOKE and AFM measurements showed the isotropic azimuthal in-plane behavior of magnetization vector in CoFeB/ Bi2Te3, in contrast to 2nd order magnetic anisotropy seen in CoFeB/Bi2Se3. XPS measurements showed more stable behavior of CoFeB grown on Bi2Te3 to the oxidation, in compare to CoFeB grown on Bi2Se3. XAS and XMCD measurements of both concerned nanostructures allowed calculation of spin and orbital magnetic moments for Co and Fe. Additionally, crystalline structure and XMCD response of the CoFeB/BiTeI and Co55Fe45/BiTeI systems were studied, epitaxial relations of main crystallographic axes were found, and spin and orbital magnetic moments were calculated.
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We report comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1). The films are single crystals of high crystalline quality, which strongly depends on that of their substrates, with in-plane epitaxial relationships of Bi2Te3 [2 1 -3 0] || Si [1 -1 0] and Bi2Te3 [0 1 -1 0] || Si [1 1 -2] along which the lattices of 1x3 Bi2Te3 and 2x2 Si supercells are well matched. As the samples age, we observe loss of crystalline Bi2Te3 film thickness accompanied with roughening of the crystalline interfaces, formation of new crystalline phases as well as compositional and structural modification of the Si substrate, consistent with the diffusion of Te into the Si substrate.
Co40Fe40B20 layers were grown on the Pb0.71Sn0.29Te topological insulator substrates by laser molecular beam epitaxy (LMBE) method, and the growth conditions were studied. The possibility of growing epitaxial layers of a ferromagnet on the surface of a topological insulator was demonstrated for the first time. The Co40Fe40B20 layers obtained have a bcc crystal structure with a crystalline (111) plane parallel to the (111) PbSnTe plane. The use of three-dimensional mapping in the reciprocal space of reflection high electron diffraction (RHEED) patterns made it possible to determine the epitaxial relationship of main crystallographic axes between the film and the substrate of topological insulator. Quenching of some reflections in diffraction pattern allows confirmation of the substrate stoichiometry.
Electronic structures of the tetradymites, Bi$_2$Te$_3$, Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, containing various dopants and vacancies, are studied using the first principles calculations methods. We focus on the possibility of formation of the resonant levels (RL), confirming the formation of the RL by Sn in Bi$_2$Te$_3$, and predicting similar behavior of Sn in Bi$_2$Te$_2$Se and Bi$_2$Se$_3$. Vacancies, which are likely present on the chalcogen atoms in the real samples of Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, are also studied and their charged donor and resonant behavior is discussed. Doping of the vacancy-containing materials with regular acceptors, like Ca or Mg, is shown to compensate the donor effect of vacancies, and $n-p$ crossover, while increasing the dopant concentration, is observed. We verify, that RL on Sn is not disturbed by the chalcogen vacancies in Bi$_2$Te$_2$Se and Bi$_2$Se$_3$, and for the Sn-doped materials with Se or Te vacancies, double-doping, instead of heavy doping with Sn, is suggested as an effective way of reaching the resonant level. This should help to avoid the smearing of the RL, which was a possible reason for an earlier unsuccessful experimental observation of the influence of the RL on thermoelectric properties of Sn doped Bi$_2$Te$_2$Se. Finally we show, that Al and Ga are possible new resonant impurities in the tetradymites, hoping that it will stimulate further experimental studies.
Using high-resolution angle-resolved photoemission spectroscopy, the electronic structure near the Fermi level and the topological property of the Bi(111) films grown on the Bi$_2$Te$_3$(111) substrate were studied. Very different from the bulk Bi, we found another surface band near the $bar{M}$ point besides the two well-known surface bands on the Bi(111) surface. With this new surface band, the bulk valence band and the bulk conduction band of Bi can be connected by the surface states. Our band mapping revealed odd number of Fermi crossings of the surface bands, which provided a direct experimental signature that Bi(111) thin films of a certain thickness on the Bi$_2$Te$_3$(111) substrate can be topologically nontrivial in three dimension.
Magnetic interaction with the gapless surface states in topological insulator (TI) has been predicted to give rise to a few exotic quantum phenomena. However, the effective magnetic doping of TI is still challenging in experiment. Using first-principles calculations, the magnetic doping properties (V, Cr, Mn and Fe) in three strong TIs (Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$) are investigated. We find that for all three TIs the cation-site substitutional doping is most energetically favorable with anion-rich environment as the optimal growth condition. Further our results show that under the nominal doping concentration of 4%, Cr and Fe doped Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Cr doped Sb$_{2}$Te$_{3}$ remain as insulator, while all TIs doped with V, Mn and Fe doped Sb$_{2}$Te$_{3}$ become metal. We also show that the magnetic interaction of Cr doped Bi$_{2}$Se$_{3}$ tends to be ferromagnetic, while Fe doped Bi$_{2}$Se$_{3}$ is likely to be antiferromagnetic. Finally, we estimate the magnetic coupling and the Curie temperature for the promising ferromagnetic insulator (Cr doped Bi$_{2}$Se$_{3}$) by Monte Carlo simulation. These findings may provide important guidance for the magnetism incorporation in TIs experimentally.
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