10 research outputs found
Spin-flip hot spots in ultrathin films of monovalent metals: Enhancement and anisotropy of the Elliott-Yafet parameter
In contrast to the long-known fact that spin-flip hot spots, i.e., special
\vc{k}-points on the Fermi surface showing a high spin-mixing parameter, do not
occur in the bulk of monovalent (noble and alkali) metals, we found them on the
surface Brillouin-zone boundary of ultrathin films of these metals.
Density-functional calculations within the Korringa-Kohn-Rostoker Green
function method for ultrathin (001) oriented Cu, Ag, and Au films of 10-layer
thickness show that the region around the hot spots can have a substantial
contribution, e.g.\ 52\% in Au(001), to the integrated spin-mixing parameter,
that could lead to a significant enhancement of the spin-relaxation rate or
spin-Hall angle in thin films. Owing to the appearance of spin-flip hot-spots,
a large anisotropy of the Elliott-Yafet parameter [50\% for Au(001)] is also
found in these systems. The findings are important for spintronics applications
in which noble-metals are frequently used and in which the dimensionality of
the sample is reduced.Comment: 6 pages, 2 figure
Lifetime reduction of surface states at Cu, Ag and Au(111) caused by impurity scattering
We present density-functional results on the lifetime of the (111) surface
state of the noble metals. We consider scattering on the Fermi surface caused
by impurity atoms belonging to the 3d and 4sp series. The results are analyzed
with respect to film thickness and with respect to separation of scattering
into bulk or into surface states. While for impurities in the surface layer the
overall trends are similar to the long-known bulk-state scattering, for
adatom-induced scattering we find a surprising behavior with respect to the
adatom atomic number. A plateau emerges in the scattering rate of the 3d
adatoms, instead of a peak characteristic of the d resonance. Additionally, the
scattering rate of 4sp adatoms changes in a zig-zag pattern, contrary to a
smooth parabolic increase following Linde's rule that is observed in bulk. We
interpret these results in terms of the weaker charge-screening and of
interference effects induced by the lowering of symmetry at the surface
Spin-polarization of platinum (111) induced by the proximity to cobalt nanostripes
We measured a spin polarization above a Pt (111) surface in the vicinity of a
Co nanostripe by spin-polarized scanning tunneling spectroscopy. The spin
polarization is exponentially decaying away from the Pt/Co interface and is
detectable at distances larger than 1 nm. By performing self-consistent
ab-initio calculations of the electronic-structure for a related model system
we reveal the interplay between the induced magnetic moments within the Pt
surface and the spin-resolved electronic density of states above the surface.Comment: 19 pages, 6 figure
Effect of spin-orbit scattering on transport properties of low-dimensional dilute alloys
The scoppe of this thesis is to gain insight, by means of -calculations, into the physics of monumentum and spin relaxation phenomena induced by electron scattering at impurities and defects in the noble metals copper, silver and gold. The main results are subdivided in three parts. In the first part, momentum- and spin-relaxation times due to scattering at adn impurities in copper and gold fcc bulk are invetigated. The inversion symmetry of the crystls leads to a two-fold degeneracy of all states on the Fermi surfae, adn therefore spin relaxaation is dominated by the elliott-Yafet mechanism as well as the spin-orbit coupling of the impurity. For impurities in gold , we calculate much shorter spin-relaxation times than in copper because of the stronger spin-orbit coupling of the fold host. Furthermore, we have found important qualitative differences between the relaxation times obtained for the and the impurities. As scattering at impurities is resonant, the electrons spend much more time at the impurity sites than in the case of the impurities; therefore, they are much longer exhibited to the spin-orbit coupling of the impurity. This results in considerably shorter spin-relaxation times, even if the momentum scattering rates are in the same order of magnitude. Finally, the investigation of interference of scattering processes at impurity dimers reveals that relevant differences to the independent-impurity approximation appear only for strong scatterer, palced at nearest neighbouringsites. in the second part we investigate the reduction of spin-conservng surface-state lifetimes induced by adatom- and impurity-scattering on the (111) surfaces of copper, silver and gold films with different thickness. We have found strong qualitative differences in the lifetimes when comparing the results for adatoms to those of impurities in the first and second layer. The trends for the latter ones are similar to those calculated in bulk in the first part of the thesis. In the third part, we investigate spin-orbit induced effects on thin (001) and (111) copper and gold films with focus on spin-relaxation mechanisms. We consider both symmetric and asymmetric systems, where the asymmetry of the latter ones is created by covering one side of the film with one layer of Zn. For the symmetric films, spin-mixing parameters and momentum- and spin-relaxation times due to scattering at self-adatoms are calculated. Whereas the largest spin-mixing in (111) films has been obtained for the surface states, on the Fermi surfaces of the (001) films spin hot spots occur, which are caused by anticrossings of bands and lead to locally very high spin mixing
Anisotropy of Spin Relaxation in Metals
The concept of anisotropy of spin relaxation in non-magnetic metals with
respect to the spin direction of the injected electrons relative to the crystal
orientation is introduced. The effect is related to an anisotropy of the
Elliott-Yafet parameter, arising from a modulation of the decomposition of the
spin-orbit Hamiltonian into spin-conserving and spin-flip terms as the spin
quantization axis is varied. This anisotropy, reaching gigantic values for
uniaxial transition-metals (e.g. 830% for hcp Hf) as density-functional
calculations show, is related to extended "spin-flip hot areas" on the Fermi
surface created by the proximity of extended sheets of the surface, or
"spin-flip hot loops" at the Brillouin zone boundary, and has no theoretical
upper limit. Possible ways of measuring the effect as well as consequences in
application are briefly outlined.Comment: 5 pages, 1 figure. arXiv admin note: text overlap with
arXiv:1206.058
Spin relaxation and the Elliott-Yafet parameter in W(001) ultrathin films: Surface states, anisotropy, and oscillation effects
Using first-principles methods based on density-functional theory we
investigate the spin relaxation in W(001) ultrathin films. Within the framework
of the Elliott-Yafet theory we calculate the spin mixing of the Bloch states
and we explicitly consider spin-flip scattering off self-adatoms. We find an
oscillatory behavior of the spin-mixing parameter and relaxation rate as a
function of the film thickness, which we trace back tosurface-state properties.
We also analyze the Rashba effect experienced by the surface states and discuss
its influence on the spin relaxation. Finally we calculate the anisotropy of
the spin-relaxation rate with respect to the polarization direction of the
excited spin population relative to the crystallographic axes of thefilm. We
find that the spin-relaxation rate can increase by as much as 47% when the spin
polarization is directed out of plane, compared to the case when it is in
plane. Our calculations are based on the multiple-scattering formalism of the
Korringa-Kohn-Rostoker Green-function method.Comment: 10 page