| 1. |
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| 2. |
- Emmel, M., et al.
(författare)
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Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism
- 2014
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 368, s. 364-373
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Tidskriftsartikel (refereegranskat)abstract
- We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co2FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the L-3,L-2 edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co2FeSi0.6Al0.4. Thus, we assume that during deposition Al atoms are being implanted into the subsurface of Co2FeSi. Such an interdiffusion is not observed for the corresponding Cr-capped films, which makes Cr the material of choice for capping Co2FeSi films. We report stronger XMLD intensities at the L-3,L-2 Co and Fe egdes for films with a higher saturation magnetization. Additionally, we compare the spectra with ab initio predictions and obtain a reasonably good agreement. Furthermore, we were able to detect an XMCD signal at the Si Ledge, indicating the presence of a magnetic moment at the Si atoms.
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| 3. |
- Legut, D., et al.
(författare)
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Influence of the crystal structure of thin Co films on X-ray magnetic linear dichroism-Comparison of ab initio theory and reflectometry experiments
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:17, s. 17E132-
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Tidskriftsartikel (refereegranskat)abstract
- We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure. (C) 2014 AIP Publishing LLC.
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| 4. |
- Legut, D., et al.
(författare)
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Interference Effects in T-MOKE Spectra of Fe Thin Films at the 3p Edges - Theory and Experiment
- 2015
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Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 127:2, s. 466-468
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Tidskriftsartikel (refereegranskat)abstract
- We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.
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| 5. |
- Tesch, M. F., et al.
(författare)
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Magnetocrystalline anisotropy in x-ray magnetic linear dichroism at the 3 p edges of crystalline Fe thin films
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:14, s. 140404-
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Tidskriftsartikel (refereegranskat)abstract
- X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3 p absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the 3 p semicore states on an equal footing.
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| 6. |
- Wdowik, U. D., et al.
(författare)
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CoO under pressure from first principles
- 2008
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 69:7, s. 1698-1703
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory and the generalized gradient approximation with correction for Hubbard energy was used to study the behavior of cobaltous oxide (CoO) under pressure. COO undergoes an insulator-metal transition which is accompanied by a magnetic collapse. The antiferromagnetic phase of CoO transforms to nonmagnetic phase with the 6-7% reduction in the fractional volume. The magnetic collapse and the energy band gap closure are driven by the lost of correlation which results from the delocalization of 3d electrons. Delocalization process is due to the band broadening with compression. The Hubbard energy influences the transitions pressure. The lower Hubbard terms result in the lower values of transition pressure. The evolution of magnetic moment, energy band gap, and the bandwidth versus increasing pressure is analyzed. The results of calculations are compared to the existing theoretical and experimental data.
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| 7. |
- Arapan, S., et al.
(författare)
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Computational screening of Fe-Ta hard magnetic phases
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:1
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we perform a systematic calculation of the Fe-Ta phase diagram to discover hard magnetic phases. By using structure prediction methods based on evolutionary algorithms, we identify two energetically stable magnetic structures: a tetragonal Fe3Ta (space group 122) and a cubic Fe5Ta (space group 216) binary phase. The tetragonal structure is estimated to have both high saturation magnetization (mu M-0(s) = 1.14 T) and magnetocrystalline anisotropy (K-1 = 2.17 MJ/m(3)) suitable for permanent magnet applications. The high-throughput screening of magnetocrystalline anisotropy also reveals two low-energy metastable hard magnetic phases: Fe5Ta2 (space group 156) and Fe6Ta (space group 194), that may exhibit intrinsic magnetic properties comparable to SmCo5 and Nd2Fe14B, respectively.
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| 8. |
- Arapan, S., et al.
(författare)
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From virtual to reality : A practical route to design new materials
- 2024
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- Modern computational techniques that use a combination of electronic structure calculations, adaptive genetic algorithms, and machine learning data analysis have been recently predicting many new unknown structures that may exhibit desired physical properties. Yet, most of these theoretically discovered structures belong to the realm of virtual phase space, and the great challenge to experimentally observe them still remains. Based on the example of the C36 Laves phase in a Co-Fe-Ta system, we demonstrate a practical route to design and produce a material with desired properties.
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| 9. |
- Carva, Karel, et al.
(författare)
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Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:18, s. 184425-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Carva, K, et al.
(författare)
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Influence of laser-excited electron distributions on the X-ray magnetic circular dichroism spectra : Implications for femtosecond demagnetization in Ni
- 2009
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 86:5, s. 57002-
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Tidskriftsartikel (refereegranskat)abstract
- 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 nonequilibrium 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. Copyright (C) EPLA, 2009
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| 11. |
- 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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| 12. |
- Frietsch, B., et al.
(författare)
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The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals
- 2020
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Ingår i: Science Advances. - : American Association for the Advancement of Science. - 2375-2548. ; 6:39
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.
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| 13. |
- Jansing, C., et al.
(författare)
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X-ray natural birefringence in reflection from graphene
- 2016
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Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 94:4
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Tidskriftsartikel (refereegranskat)abstract
- The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon 1s absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to 30 degrees, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to 17 degrees. Both graphene systems show resonantly enhanced effects at the pi* and sigma* energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of 140 degrees has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the pi* resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of pi* and sigma* bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced sigma* excitation energy and a strong tilt of 50% of the p(z) orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the p(z) orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting lambda/4 or lambda/2 plates in the design of future high-intensity light sources.
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| 14. |
- Kallmayer, M., et al.
(författare)
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Element-specific magnetic properties of Co(2)(Mn(1-x)Fe(x))Si films probed by x-ray magnetic circular/linear dichroism
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:5, s. 054448-
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Tidskriftsartikel (refereegranskat)abstract
- Element-specific magnetic properties of epitaxial Co(2)Fe(x)Mn(1-x)Si/MgO(100) Heusler films prepared by laser ablation are measured by circular dichroism in x-ray absorption spectroscopy (XMCD). Surface and bulk magnetization of the 100-nm-thick films are equal as tested by comparing the total electron yield and transmission measurements and follow the generalized Slater-Pauling rule. For Co(2)FeSi, the large magnetization of 6 mu(B) per formula unit indicates half-metallic behavior. Calculations using the local spin density approximation (LDA) result in a half-metallic band structure for this compound when an additional electron-electron-correlation correction (LDA + U) is considered. Simulated XMCD spectra using the LDA and LDA + U approaches are compared to the experimental spectra. While the electron-electron Coulomb correlation has only a small effect on the observable XMCD spectra, the calculations predict that it will cause a significant change in the x-ray magnetic linear dichroism (XMLD).
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| 15. |
- Legut, D., et al.
(författare)
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Magnetic exchange interactions and estimation of T-N in CsNiF3 from first principles
- 2008
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Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 113:1, s. 503-506
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Tidskriftsartikel (refereegranskat)abstract
- The CsNiF3 has been for long time studied as a prototype of quasi-1D planar ferromagnetic system. At very low temperature the studied system is insulator and therefore the magnetic exchange interactions should decay very rapidly. We treated the magnetic exchange coupling within the Heisenberg model for the nearest neighbor interaction between the antiferromagnetically coupled Ni-chains. The influence of up to the second-nearest neighbors on the ferromagnetic exchange coupling along the Ni-chains was determined. The exchange interactions were calculated for the experimental volume by the density functional theory, within the all-electron approach using the local density approximation for the exchange and correlation. The Neel temperature was calculated by means of the mean-field theory and by the random-phase approximation method.
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| 16. |
- Mertins, H-Ch., et al.
(författare)
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Magneto-optical reflection spectroscopy on graphene/Co in the soft x-ray range
- 2017
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 903
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Tidskriftsartikel (refereegranskat)abstract
- The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.
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| 17. |
- Tarasenko, R., et al.
(författare)
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Extraordinary two-dimensionality in the S = 1/2 spatially anisotropic triangular quantum magnet Cu(1,3−diaminopropane)Cl2 with modulated structure
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:21
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Tidskriftsartikel (refereegranskat)abstract
- The growth of sufficiently large single crystals of Cu(tn)Cl2 (tn=1,3−diaminopropane) enabled specific heat and susceptibility studies in various field orientations. The nearly invisible broad hump in zero-field specific heat at 0.6 K coincides with the change in the characteristic parameters of zero-field muon relaxation spectra. The lack of oscillations in the time spectra and their exponential character preserved down to 40 mK suggest the coexistence of static and fluctuating local fields associated with the prevalence of low-dimensional correlations. The extreme two-dimensionality is also manifested by the nonmonotonous character of the magnetic phase diagram. First-principle calculations of exchange couplings introduced a concept of a quasi-two-dimensional magnetic lattice with many couplings within the magnetic layers. The strongest ones lead to the model of a rectangular lattice with the intrachain coupling J/kB = 4.3 K and the interchain coupling J'/J = 0.46, which provides excellent agreement with the zero-field specific heat. However, the susceptibility data suggest the importance of other weaker interactions in accord with first-principle studies. Significant broadening of field-induced specific heat anomalies may be ascribed to potential intrinsic partial magnetic disorder associated with the gradual modulation of tn positions in the crystal structure.
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| 18. |
- 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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