Ultrafast spin-dependent electron dynamics in fcc Co (original) (raw)
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Spin-polarization effects for electrons passing through thin iron and cobalt films
Solid state communications, 1993
Spin-dependent effects of the inelastic mean free path (IMFP) are evident for low-energy electrons passing through magnetized ferromagnetic films caused by a different attenuation within the layer. Values of IMFP for both spin components were determined for ultrathin iron and cobalt layers on W(1 1 0) by means of spin-resolving photoelectron spectroscopy.
Theory of spin excitations in Fe(110) multilayers
Physical Review B, 2003
We present theoretical studies of short-wavelength spin excitations in ferromagnetic Fe͑110͒ monolayers either adsorbed on a W͑110͒ substrate or free standing. We use an itinerant model of electrons as the basis for our analysis, with nine bands ͑the five 3d bands and the 4sp complex͒ included. The bands are described within an empirical tight-binding scheme, and the ferromagnetic ground state is generated from on-site intraatomic Coulomb interactions, described in mean-field theory. The random phase approximation ͑RPA͒ is employed to describe the spin excitations through analysis of the wave vector and frequency dependence of the dynamic transverse susceptibility. Several issues are explored. We compare the spin-wave stiffness and other features of the spin-wave spectrum for the free standing film and that adsorbed on the substrate to find substantial quantitative differences with origin in spin-spin interactions mediated by the substrate. We also compare the spin-wave spectrum calculated through use of the RPA, an approximate theory, but a scheme that does not invoke the adiabatic approximation, with results generated within the framework of the adiabatic approach. While the spin-wave exchange stiffnesses produced by the two methods are in agreement, there are substantial differences between excitation spectra at short wavelengths. We argue that effective interspin exchange couplings generated within the framework of the adiabatic approximation fail to provide a description of the spin-wave spectrum in the itinerant ferromagnets, beyond the low-frequency, long-wavelength regime where the spin-wave exchange stiffness suffices to describe the spectrum. We also discuss apparent hybridization gaps in the spin-wave spectrum. We show that in some cases they can be artifact of a poorly converged numerical analysis and, in one instance, on use of an inappropriate form for the intra-atomic Coulomb interaction.
Ultrafast spin transport as key to femtosecond demagnetization
Nature Materials, 2013
Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer. U ltrafast demagnetization triggered by shining a femtosecond laser pulse onto a ferromagnetic transition-metal sample has been extensively studied since its discovery 1 . In spite of numerous investigations, the mechanism underlying light-induced demagnetization could not yet be clearly identified. A variety of different microscopic models 2-6 has been put forward over the past years to explain how an ultrashort laser pulse could modify the magnetic system within a few hundred femtoseconds after laser excitation 7 . Most are based on ultrafast spin-flip scattering of some kind, such as Elliott-Yafet electron-phonon spin-flip scattering 2 , electron-magnon spin-flip scattering 3 , and Coulomb exchange spin-flip scattering 4 . Other possibilities are direct laserinduced 4 or relativistic electromagnetic-radiation-induced 5 spinflips. Regardless of the diverse nature of these mechanisms, all scenarios adopt the same first step in the excitation process, namely direct absorption of a femtosecond laser pulse in the ferromagnetic film. Conversely, in this work we explore a different process that leads to very efficient ultrafast demagnetization, namely transport of laser-excited non-spin-polarized electrons into the ferromagnet.
Spin relaxation in magnetic nanostructures
2014
The thesis presents theoretical investigation of spin relaxation in magnetic nanostructures within the quantum-mechanical approach, including novel methods developed in this PhD project. The Gilbert damping constant α which enters the Landau-Lifshitz-Gilbert (LLG) equation describing the dynamics of magnetisation is found for bulk ferromagnetic transition metals and various ultrathin magnetic metallic layered systems employing the torque-correlation model by Kamberský. The expression for α is generalised to an arbitrary direction of magnetisation. Calculations are performed within a realistic tight-binding model including the spin-orbit interaction and their efficiency is remarkably improved by introducing finite temperature into the electronic occupation factors and subsequent summation over the Matsubara frequencies. Furthermore, two alternative formulas, not limited to the TB model, for α in terms of the Green function are derived. The results are reported for bulk ferromagnets, ferromagnetic films, ferromagnet/nonmagnet (Co/NM) bilayers (NM=Cu, Pd, Ag, Pt and Au) as well as NM/Co/NM, Co/NM1/NM2 trilayers, L1 0 Co/NM superlattices (ordered alloys) and [Co/NM] N multilayers. The obtained dependence of α on the electron scattering rate for bulk Fe, Co and Ni is in good agreement with the previous ab initio calculations. The dependence of α on Co and NM thicknesses and the effect of the nonmagnetic caps are investigated and found to be in accord with experiment. The calculated α in Co/NM bilayers and Co/NM1/NM2 trilayers is enhanced due to adding the nonmagnetic caps, particularly in the case of NM, NM2=Pd and Pt. This enhancement is explained by the large spin-orbit coupling of such NMs, combined with their large density of states at the Fermi level ϵ F. The enhancement in Co/NM1/Pt trilayers is shown to decay with the increasing thickness of the spacer NM1=Cu and Ag. Nonlocal origin of the damping enhancement is proved by visualising large contributions to α from the nonmagnetic part. Contributions to α from individual atomic layers and its k-space distribution are determined and analysed in several layered systems. It is revealed that in the Co/NM bilayers including NM metal with the d band crossing ϵ F the major contributions to the Gilbert damping come from a few atomic layers in the NM close to the Co/NM interfaces, whilst for those with NM d bands below ϵ F the main contributions originate from the Co part. Investigations in the k-space show the existence of hot spots in the Brillouin zone that give the largest contributions to α. Additionally, the nonadiabatic spin-transfer torque coefficient β entering an extended form of the LLG equation is calculated for bulk ferromagnets and ferromagnetic films. Its evaluation method is improved by using the Hellmann-Feynman theorem to calculate the electron velocity. In each case, comparison with results of other theoretical approaches, such as the ab initio calculations and the spin pumping theory, as well as experiment is performed. vi http://rcin.org.pl została ulepszona przez zastosowanie twierdzenia Hellmana-Feynmana do wyznaczania prędkości elektronowych. W każdym przypadku otrzymane rezulaty porównano z wynikami innych metod teoretycznych, takimi jak obliczenia ab initio i teoria pompowania spinowego, oraz wynikami eksperymentalnymi. vii http://rcin.org.pl
Progress in Surface Science, 2007
We report on a spin-, time-, angle-and energy-resolved two-photon photoemission experiment with unprecedented resolution and adequate sensitivity, which allows us to study spin-dependent electron dynamics. Image-potential-state electrons on iron and cobalt thin films serve as well-defined model systems. The observed exchange splitting of these states reflects the exchange-split boundaries of the bulkband gap. The temperature dependence of the spin polarization demonstrates that image-potential states are true sensors of the near surface magnetization. We have gained insight into quasielastic, i.e. resonant intra-and interband scattering processes and their inelastic counterparts. Lifetimes of minority and majority image-potential states differ primarily due to the spin-dependent density of states. In the minority channel of iron thin films quasielastic scattering processes become significant and are interpreted in terms of interband scattering between spin-up and spin-down image-potential-state bands. The latter process involves a spin flip on a sub-hundred femtosecond timescale and hints at quasielastic electron-magnon scattering.
Photo-Induced Spin Dynamics in Nanoelectronic Devices
The present research is devoted to the investigation of electron spin transmission through a nanoelectronic device. This device is modeled as nonmagnetic semiconductor quantum dot coupled to two diluted magnetic semiconductor leads. The spin transport characteristics through such a device are investigated under the effect of an ac-field of a wide range of frequencies. The present result shows a periodic oscillation of the conductance for both the cases of parallel and antiparallel spin alignment. These oscillations are due to Fano-resonance. Results for spin polarization and giant magneto-resistance show the coherency property. The present research might be useful for developing single spin-based quantum bits (qubits) required for quantum information processing and quantum spin-telecommunication. 85.75.Hh, 85.35
Physical Review Letters, 2006
The femtosecond magnetization dynamics of a thin cobalt film excited with ultrashort laser pulses has been studied using two complementary pump-probe techniques, namely spin-, energyand time-resolved photoemission and time-resolved magneto-optical Kerr effect. Combining the two methods it is possible to identify the microscopic electron spin-flip mechanisms responsible for the ultrafast macroscopic magnetization dynamics of the cobalt film. In particular, we show that electron-magnon excitation does not affect the overall magnetization even though it is an efficient spin-flip channel on the sub-200 fs timescale. Instead we find experimental evidence for the relevance of Elliott-Yafet type spin-flip processes for the ultrafast demagnetization taking place on a time scale of 300 fs.
Spin-dependent electron transport at the ferromagnet/semiconductor interface
Journal of Applied Physics, 1999
A search for spin-dependent electron transport at the ferromagnet/semiconductor interface has been made by measuring the bias dependence of a photon excited current through the interface. A circularly polarized laser beam was used to excite electrons with a spin polarization perpendicular to the film plane. In samples of the form 3 nm Au/5 nm Ni 80 Fe 20 /GaAs ͑110͒, a significant transport current was detected with a magnitude dependent on the relative orientation of the spin polarization and the magnetization vector. At perpendicular saturation, the bias dependence of the photocurrent is observed to change in the range 0.7-0.8 eV when the helicity is reversed.