| 1. |
- Carva, Karel, et al.
(författare)
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Theory of femtosecond laser-induced demagnetization
- 2015
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Ingår i: ULTRAFAST MAGNETISM I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 111-115
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Konferensbidrag (refereegranskat)abstract
- Using ab initio calculations we computed the ultrafast demagnetization that can be achieved by Elliott-Yafet electron-phonon spin-flip scatterings in laser-excited ferromagnets. Our calculations show that nonequilibrium laser-created distributions contribute mostly to the ultrafast demagnetization. Nonetheless, the total Elliott-Yafet contribution is too small to account for the fs-demagnetization.
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| 2. |
- Fan, Tingting, et al.
(författare)
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Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism
- 2015
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 112:46, s. 14206-14211
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 mu m, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N-4,N-5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.
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| 3. |
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| 4. |
- Mertins, Hans-Christoph, et al.
(författare)
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Detection of the Magnetocrystalline Anisotropy in X-Ray Magnetic Linear Dichroism Reflection Spectra Across the Fe 3p and 2p Edges
- 2014
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 50:11
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Tidskriftsartikel (refereegranskat)abstract
- The magnetocrystalline anisotropy of X-ray magnetic linear dichroism (XMLD) reflection spectra measured on single-crystalline bcc Fe films across the 3p and 2p edges are presented. The XMLD spectra were obtained from a series of reflection spectra by aligning the electric field vector of linearly polarized undulator radiation with respect to the crystal axes. Our results show the presence of a huge magnetocrystalline anisotropy in the XMLD reflection spectra. The XMLD signal is further investigated as a function of the Fe film thickness in Au/Fe/Ag/GaAs layered systems. Simulations of the reflection spectra reveal the influences of interference effects, which can enhance or diminish the XMLD signals. The measured spectra are in good agreement with ab initio calculated spectra.
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| 5. |
- Nolting, Frithjof, et al.
(författare)
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Anisotropy of the L-2,L-3 x-ray magnetic linear dichroism of Fe films on GaAs : Experiment and ab initio theory
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:18, s. 184415-
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Tidskriftsartikel (refereegranskat)abstract
- We report investigations of the anisotropy of the x-ray magnetic linear dichroism (XMLD) of thin single-crystalline Fe films on GaAs substrates with film thicknesses of 3 nm and 7 nm, respectively. The orientation of the in-plane Fe magnetization with respect to the crystal axes is stepwise rotated from the [1 1 0] to the [0 1 0] direction and the Fe L-2,L-3 XMLD spectrum is measured at every orientation step. A highly anisotropic XMLD signal at the Fe L-2 and L-3 edges is observed, which origin is analyzed in detail. We show that the measured anisotropic XMLD spectra are in reasonable agreement with ab initio calculated spectra. The XMLD anisotropy is shown to be a consequence of the cubic crystal-field split density of 3d states, which are selectively probed by transitions from the spin-orbit and exchange-split 2p core levels. We furthermore investigate computationally the influences of the spin-orbit interaction of the Fe 3d valence electrons as well as of stress in the Fe films. The Fe 3d spin-orbit interaction only leads to small modifications of the XMLD spectra, whereas tetragonal deformation of the films can cause a larger modification of the spectra.
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| 6. |
- Souvatzis, Petros, 1970-, et al.
(författare)
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Ab initio study of interacting lattice vibrations and stabilization of the beta phase in Ni-Ti shape-memory alloy
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:9, s. 092201-
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Tidskriftsartikel (refereegranskat)abstract
- Lattice dynamical methods used to predict phase transformations in crystals typically evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the beta structure when it appears as a high-temperature phase of the shape memory alloy Ni-Ti. Here it is shown by self-consistent ab initio lattice dynamical calculations that the critical temperature for the premartensitic R-to-beta phase transformation in Ni-Ti can be effectively calculated with good accuracy, and that the beta phase is a result primarily of the stabilizing interaction between different lattice vibrations.
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| 7. |
- Thielemann-Kühn, Nele, et al.
(författare)
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Optical control of 4f orbital state in rare-earth metals
- 2024
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 10:16
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Tidskriftsartikel (refereegranskat)abstract
- A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, altering the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals and more general for the excitation of localized electronic states in correlated materials.
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| 8. |
- Turgut, Emrah, et al.
(författare)
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Stoner versus Heisenberg : Ultrafast exchange reduction and magnon generation during laser-induced demagnetization
- 2016
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Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 94:22
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Tidskriftsartikel (refereegranskat)abstract
- Understanding how the electronic band structure of a ferromagnetic material is modified during laser-induced demagnetization on femtosecond time scales has been a long-standing question in condensed matter physics. Here, we use ultrafast high harmonics to measure time-, energy-, and angle-resolved M-edge magnetic asymmetry spectra for Co films after optical pumping to induce ultrafast demagnetization. This provides a complete data set that we can compare with advanced ab initio magneto-optical calculations. Our analysis identifies that the dominant mechanisms contributing to ultrafast demagnetization on time scales up to several picoseconds are a transient reduction in the exchange splitting and the excitation of ultrafast magnons. Surprisingly, we find that the magnon contribution to ultrafast demagnetization is already strong on subpicosecond time scales, while the reduction in exchange splitting persists to several picoseconds.
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| 9. |
- Valencia, S., et al.
(författare)
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Quadratic X-Ray Magneto-Optical Effect upon Reflection in a Near-Normal-Incidence Configuration at the M Edges of 3d-Transition Metals
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 104:18, s. 187401-
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Tidskriftsartikel (refereegranskat)abstract
- We have observed a quadratic x-ray magneto-optical effect in near-normal-incidence reflection at the M edges of iron. The effect appears as the magnetically induced rotation of similar to 0.1 degrees of the polarization plane of linearly polarized x-ray radiation upon reflection. A comparison of the measured rotation spectrum with results from x-ray magnetic linear dichroism data demonstrates that this is the first observation of the Schafer-Hubert effect in the x-ray regime. Ab initio density-functional theory calculations reveal that hybridization effects of the 3p core states necessarily need to be considered when interpreting experimental data. The discovered magneto-x-ray effect holds promise for future ultrafast and element-selective studies of ferromagnetic as well as antiferromagnetic materials.
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| 10. |
- Wdowik, Urszula D., et al.
(författare)
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Ab initio lattice dynamics of MnO
- 2009
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 21:27, s. 275402-
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Tidskriftsartikel (refereegranskat)abstract
- Vibrational dynamics of the MnO lattice has been studied using density functional theory combined with the direct method. Considerations have been limited to the harmonic approximation. Ab initio calculated Hellmann-Feynman forces were used to obtain density of states and the dispersion relations of phonons in the MnO crystal. Corrections for the local Coulomb interactions were applied. The Hubbard energies were varied from 1 to 7.9 eV. Increased Hubbard terms give a significant increase in the on-site force constants of cations, while the force constants on anions are affected indirectly. The density of phonon states and the dispersion curves are shifted to higher frequencies with the increasing Hubbard energies. The on-site Coulomb interactions influence mainly the optical phonon vibrations. The higher Hubbard terms lower the mean-squared vibrations of both cations and anions. The lattice contribution to the heat capacity experiences small changes upon variation of Coulomb repulsion. Results of the calculations are compared to the existing experimental data.
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