Spin relaxation and spin coherence in nonmagnetic metallic ultrathin films and nanowires.
Final Report Abstract
We successfully implemented nonself-consistent as well as self-consistent solver for spin-orbit coupling in the FP-KKR approach. We also developed a new parallized code to calculate the Fermi surface using the tetrahedron method. The code enables us to calculate the Fermi surface of complicated systems with many band crossing such as 5d transition-metal thinfilms. In addition, we developed a KKR version, named as JÜSCATT, to calculate wavefunctions and scattering-matrix elements. In this code, Boltzmann equations for conductivities are also implemented to study spin-dependent transport properties, such as extrinsic spin Hall and spin Nernst effects. In this project, we particularly focused on the effects of the spin relaxation induced by the Elliott-Yafet mechanism as well as the extrinsic spin Hall transport due to the skew-scattering for 5d transition-metal thin films with self-adatom impurity in comparison with Au and Cu thin films. The Elliott-Yafet parameter and the spin relaxation are systematically examined as functions of the film thickness up to 10 layers as well as the crystallographic orientation of the film. The overall trends are in qualitative agreement with the Elliott approximation. We investigate for the first time the anisotropy of the spin relaxation. Owing to the reduced dimensionality in thin films, the anisotropy of the spin-mixing parameter and the spin-flip relaxation rate in thin films is different from that in bulk metals, but not in a universal manner. For cubic crystal structures, the anisotropy significantly increases in thin films compared to that in bulk systems, because of the crystal-symmetry reduction. On the contrary, in hcp materials where the symmetry in bulk is anyhow low, the anisotropy value in bulk is quite large and in all studied cases higher than the value in thin films, as a result of the Fermi surface formation. The large anisotropy in thin films is contributed mainly by the spin-flip hot spots. We have shown that the Fermi surface of monovalent metals in an ultrathin film geometry can show spinflip hot spots as the Fermi rings cross the surface Brillouin-zone boundary. This is in contrast to the bulk of such metals, where it is known that hot spots do not occur, as the Fermi surface is included within the Brillouin zone. Moreover, we have shown that the hot spots contribute to large anisotropy values of the spin-mixing parameter with respect to the relative orientation between the spin-quantization axis and the crystallographic directions. Since the presence of hot spots strongly influences the spin-relaxation time or the spin-Hall conductivity, our findings can have consequences in spintronics applications, in particular since ultrathin noble-metal films are used to transmit or probe spin currents. Furthermore, we find that the presence of Rashba surface states plays a crucial role in the spin relaxation. In particular, we observe that the Elliott–Yafet parameter and the spin-relaxation rate in bcc W(001) films exhibits an even-odd oscillation with respect to the number of layers of the films, which stems from an even-odd oscillation in the surface electronic structure of the films at and near the Fermi energy. The longitudinal charge conductivity and the transverse spin Hall current for 10-layer thin films with 1% self-adatom impurities are calculated by means of the self-consistent Boltzmann equation. The spin Hall angle found to strongly vary in 5d films with respect to the material but also with respect to film thickness and orientation.
Publications
- ”Spin relaxation in non-magnetic metallic systems: Effects of resonant scattering and anisotropy”, Workshop - Simulation and Manipulation of Quantum Systems for Information Processing (SMQS-IP-11), Jülich, Germany (2011)
P. Mavropoulos, S. Heers, N. H. Long, B. Zimmermann, Y. Mokrousov and S. Blügel
- ”Anisotropy of Spin Relaxation in Metals”, Phys. Rev. Lett. 109, 236603 (2012)
B. Zimmermann, P. Mavropoulos, S. Heers, N. H. Long, S. Blügel and Y. Mokrousov
(See online at https://doi.org/10.1103/PhysRevLett.109.236603) - ”Giant anisotropy of spin relaxation in metals and ultrathin metallic films”, Core to core Groningen workshop 2012: Spintronics and Moltronics, Groningen, The Netherlands (2012)
N. H. Long, P. Mavropoulos, S. Heers, B. Zimmermann, S. Blügel and Y. Mokrousov
- ”Spin relaxation in 5d transition-metal ultrathin films”, Osaka University, Japan (2012)
N. H. Long, P. Mavropoulos, S. Heers, B. Zimmermann, Y. Mokrousov and S. Blügel
- ”Spin relaxation in metals and metallic thin films”, 520. WE-Heraeus-Seminar on Spin-orbitdriven transverse transport phenomena, Bad Honnef, Germany (2012)
P. Mavropoulos, B. Zimmermann, N. H. Long, S. Heers, Y. Mokrousov and S. Blügel
- ”Anisotropy of spin relaxation and transverse transport in metals”, J. Phys.: Condens. Matter 25, 163201 (2013)
Y. Mokrousov, H. Zhang, F. Freimuth, B. Zimmermann, N. H. Long, J. Weischenberg, I. Souza, P. Mavropoulos and S. Blügel
(See online at https://doi.org/10.1088/0953-8984/25/16/163201) - ”Spin Hall and spin Nernst effect in 5d transition metals”, Joint Workshop of Interactive Materials Science Cadet Program and JSPS Core-to-Core Program Advanced Research Networks, Awaji, Japan (2013)
N. H. Long, P. Mavropoulos, D. S. G. Bauer, B. Zimmermann, S. Blügel and Y. Mokrousov
- ”Spin relaxation and the Elliott-Yafet parameter in W(001) ultrathin films: Surface states, anisotropy, and oscillation effects”, Phys. Rev. B 87, 224420 (2013)
N. H. Long, P. Mavropoulos, S. Heers, B. Zimmermann, D. S. G. Bauer, S. Blügel and Y. Mokrousov
(See online at https://doi.org/10.1103/PhysRevB.87.224420) - ”Spin-flip hot spots in ultrathin films of monovalent metals: Enhancement and anisotropy of the Elliott-Yafet parameter”, Phys. Rev. B 88, 144408 (2013)
N. H. Long, P. Mavropoulos, S. Heers, B. Zimmermann, Y. Mokrousov, and S. Blügel
(See online at https://doi.org/10.1103/PhysRevB.88.144408) - ”Spin relaxation and spin Hall transport in 5d transition-metal ultrathin films”, Phys. Rev. B 90, 064406 (2014)
N. H. Long, P. Mavropoulos, B. Zimmermann, D. S. G. Bauer, Y. Mokrousov and S. Blügel
(See online at https://doi.org/10.1103/PhysRevB.90.064406)
