79 research outputs found
Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets
We report a computational theoretical investigation of electron spin-flip
scattering induced by the electron-phonon interaction in the transition-metal
ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon
spin-flip scattering is computed from first-principles, employing a generalized
spin-flip Eliashberg function as well as ab initio computed phonon dispersions.
Aiming at investigating the amount of electron-phonon mediated demagnetization
in femtosecond laser-excited ferromagnets, the formalism is extended to treat
laser-created thermalized as well as nonequilibrium, nonthermal hot electron
distributions. Using the developed formalism we compute the phonon-induced spin
lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated
demagnetization rate is evaluated for laser-created thermalized and
nonequilibrium electron distributions. Nonthermal distributions are found to
lead to a stronger demagnetization rate than hot, thermalized distributions,
yet their demagnetizing effect is not enough to explain the experimentally
observed demagnetization occurring in the subpicosecond regime.Comment: 14 pages, 8 figures, to appear in PR
Ab initio investigation of Elliott-Yafet electron-phonon mechanism in laser-induced ultrafast demagnetization
The spin-flip (SF) Eliashberg function is calculated from first-principles
for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet
electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization.
This is used to compute the SF probability and demagnetization rate for
laser-created thermalized as well as non-equilibrium electron distributions.
Increased SF probabilities are found for thermalized electrons, but the induced
demagnetization rate is extremely small. A larger demagnetization rate is
obtained for {non-equilibrium} electron distributions, but its contribution is
too small to account for femtosecond demagnetization.Comment: 5 pages, 3 figures, to appear in PR
Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys
A hierarchical multiscale approach to model the magnetization dynamics of
ferromagnetic ran- dom alloys is presented. First-principles calculations of
the Heisenberg exchange integrals are linked to atomistic spin models based
upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate
temperature-dependent parameters (e.g., effective exchange interactions,
damping param- eters). These parameters are subsequently used in the
Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate
numerically and analytically the ultrafast demagnetization times. The developed
multiscale method is applied here to FeNi (permalloy) as well as to copper-
doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice
demagnetizes faster than the Fe sublattice for the here-studied FeNi-based
alloys
Spin-Mixing Conductances of Ni-Based Films Attached to Cu(100) Leads
The complex spin-mixing conductance of epitaxial Cu/Ni/Cu(100) systems is predicted to oscillate as a function of Ni thickness. The oscillation period is explained in terms of spin-resolved Fermi surface properties of bulk nickel. Stability of the oscillations with respect to interface Cu-Ni interdiffusion and to alloying in the Ni film is investigated as well
Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
In pump-probe experiments an intensive laser pulse creates non-equilibrium
excited electron distributions in the first few hundred femtoseconds after the
pulse. The influence of non-equilibrium electron distributions caused by a pump
laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is
investigated theoretically here for the first time, considering electron
distributions immediately after the pulse as well as thermalized ones, that are
not in equilibrium with the lattice or spin systems. The XMCD signal is shown
not to be simply proportional to the spin momentum in these situations. The
computed spectra are compared to recent pump-probe XMCD experiments on Ni. We
find that the majority of experimentally observed features considered to be a
proof of ultrafast spin momentum transfer to the lattice can alternatively be
attributed to non-equilibrium electron distributions. Furthermore, we find the
XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid,
even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure
Landauer theory of ballistic torkances in non-collinear spin valves
We present a theory of voltage-induced spin-transfer torques in ballistic
non-collinear spin valves. The torkance on one ferromagnetic layer is expressed
in terms of scattering coefficients of the whole spin valve, in analogy to the
Landauer conductance formula. The theory is applied to Co/Cu/Ni(001)-based
systems where long-range oscillations of the Ni-torkance as a function of Ni
thickness are predicted. The oscillations represent a novel quantum size effect
due to the non-collinear magnetic structure. The oscillatory behavior of the
torkance contrasts a thickness-independent trend of the conductance.Comment: Version 3: 6 pages, 3 figures. Corrected version that passed the
peer-review proces
Ab initio formulation of the four-point conductance of interacting electronic systems
We derive an expression for the four-point conductance of a general quantum junction in terms of the density response function. Our formulation allows us to show that the four-point conductance of an interacting electronic system possessing either a geometrical constriction and/or an opaque barrier becomes identical to the macroscopically measurable two-point conductance. Within time-dependent density-functional theory the formulation leads to a direct identification of the functional form of the exchange-correlation kernel that is important for the conductance. We demonstrate the practical implementation of our formula for a metal-vacuum-metal interface
Exploring the impact of the inverse Faraday effect on all-optical helicity-dependent magnetization switching
All-optical helicity-dependent magnetization switching (AO-HDS) is the
quickest deterministic technique for data storage by solely using ultrashort
laser pulses. Granular high data density magnetic storage media developed for
heat-assisted magnetic recording (HAMR) provide an ideal playground to
investigate the interplay of effects leading to magnetization switching. In the
latest perception, we identify two effects, the magnetic circular dichroism
(MCD) and the inverse Faraday effect (IFE), as the forces driving the switching
process. During photon absorption, which leads to a rapid temperature rise and
thus to magnetization quenching, the MCD ensures two distinct electron
temperatures due to helicity-dependent absorption. This effect already holds a
nonvanishing probability for magnetization switching. At the same time, the IFE
induces a magnetic moment within the material, enhancing the switching
probability. We present AO-HDS experiments using ultrashort laser pulses
() in the near-infrared range from
to . The experiments demonstrate a strong
dependence of the switching efficiency on the absorbed energy density,
elevating the electron temperature in the vicinity of the Curie point, allowing
for the IFE to take full effect, inducing a magnetic moment for deterministic
switching in the quenched magnetization state. While we do not observe an
enhanced switching due to an increased MCD, a higher induced magnetization
usually improves the switching rate if the electron temperature reaches the
transition temperature vicinity. Therefore, we conclude that the magnetic
moment generated by the IFE is crucial for the switching efficiency and the
distinct deterministic character of the switching process. Laser pulses with a
higher absorption induce a higher magnetic moment and switch magnetization at
lower fluences
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