| 1. |
- Coleman, V. A., et al.
(author)
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Defect Formation In Graphene Nanosheets By Acid Treatment : An X-Ray Absorption Spectroscopy And Density Functional Theory Study
- 2008
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In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:6, s. 062001-4
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Journal article (peer-reviewed)abstract
- In-plane defects have been introduced into graphene nanosheets by treatment with hydrochloric acid. Acid treatment induces bond cleavage in the C–C network via electrophilic attack. These resultant vacancy sites will then undergo further reactions with the surrounding ambient to produce C–O and C–H bonds. A σ* resonance at 287 eV in the carbon K-edge x-ray absorption spectra is observed with acid treatment and is assigned to C–O states. Theoretical modelling of a di-vacancy in a graphene bilayer reproduces all essential features of this resonance and in addition predicts a metallic conductivity of states around this vacancy. The possibility of engineering the properties of graphene via the routes explored here is an important step towards establishing strategies for building devices based on this material.
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| 2. |
- Elhanoty, Mohamed F., et al.
(author)
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Different fingerprints for the OISTR mechanism in the magnetic alloys experiments
- 2022
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In: ADVANCES IN ULTRAFAST CONDENSED PHASE PHYSICS III. - : SPIE - The International Society for Optics and Photonics. - 9781510651418 - 9781510651401
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Conference paper (peer-reviewed)abstract
- The interplay between various degrees of freedom in laser induced ultrafast magnetization dynamics (LIUMD) of magnetic alloys is intricate due to the competition between different mechanisms and processes. In this work, we resolve the element specific magnetization dynamics of FePd alloy and further elucidate the dependency of the OISTR mechanism on the laser pulse parameters using ultrashort, short and relatively longer pulse duration with weak and strong fluence. Remarkably, our results illustrate potential discrepancies in experiments measuring the optical inter site spin transfer (OISTR) effect in magnetic alloys.
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| 3. |
- Elhanoty, Mohamed F., et al.
(author)
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Element-selective ultrafast magnetization dynamics of hybrid Stoner-Heisenberg magnets
- 2022
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:10
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Journal article (peer-reviewed)abstract
- Stoner and Heisenberg excitations in magnetic materials are inherently different. The first involves an effective reduction of the exchange splitting, whereas the second comprises excitation of spin waves. In this work, we test the impact of these two excitations in the hybrid Stoner-Heisenberg system of FePd. We present a microscopic picture of ultrafast demagnetization dynamics in this alloy, which represents both components of strong local exchange splitting in Fe and induced polarization in Pd. We identify the spin-orbit coupling (SOC) and the optical intersite spin transfer (OISTR) as the two dominant factors for demagnetization at ultrashort timescales. Remarkably, the drastic difference in the origin of the magnetic moment of the Fe and Pd species is not deciding the initial magnetization dynamics in this alloy. By tuning the external laser pulse, the extrinsic OISTR can be manipulated for site-selective demagnetization on femtosecond timescales providing the fastest way for optical and selective control of the magnetization dynamics in alloys. Saliently, our results signify why various experiments demonstrating OISTR might obtain conflicting results.
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| 4. |
- Eriksson, Olle, et al.
(author)
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Ordering in diluted magnetic semiconductors : A magnetic percolation phenomenon (invited)
- 2007
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:9, s. 09H114-
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Journal article (peer-reviewed)abstract
- We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped III-V and II-VI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electron-electron interaction.
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| 5. |
- Grånäs, Oscar, 1979-, et al.
(author)
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Ultrafast modification of the electronic structure of a correlated insulator
- 2022
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In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 4:3
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Journal article (peer-reviewed)abstract
- A nontrivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use of electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the time scales and energies involved in using quantum effects for possible applications. We use element-specific transient x-ray absorption spectroscopy and high-harmonic generation to measure the response to ultrashort off-resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V/angstrom lead to no detectable changes in the correlated Ni 3d orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first-principles theory. Our results highlight the importance of retardation effects in electronic screening and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operation.
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| 6. |
- Iusan, Diana, et al.
(author)
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Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al : Experiments and ab initio density-functional calculations
- 2008
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:8, s. 085319-
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Journal article (peer-reviewed)abstract
- We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thin-film samples fabricated by solution-based methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valence-band edge with pinning of the Fermi level primarily due to native defects yielding a heavily n-doped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the local-density approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.
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| 7. |
- Jana, Somnath, et al.
(author)
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Analysis of the linear relationship between asymmetry and magnetic moment at the M edge of 3d transition metals
- 2020
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In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 2:1
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Journal article (peer-reviewed)abstract
- The magneto-optical response of Fe and Ni during ultrafast demagnetization is studied experimentally and theoretically. We have performed pump-probe experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry using photon energies that cover the M absorption edges of Fe and Ni between 40 and 72 eV. The magnetic asymmetry was obtained by forming the difference of reflected intensities obtained for two opposite orientations of the sample magnetization. Density functional theory (DFT) was used to calculate the magneto-optical response of different magnetic configurations, representing different types of excitations: long wavelength magnons, short wavelength magnons, and Stoner excitations. In the case of Fe, we find that the calculated asymmetry is strongly dependent on the specific type of magnetic excitation. Our modeling also reveals that during remagnetization Fe is, to a reasonable approximation, described by magnons, even though small nonlinear contributions could indicate some degree of Stoner excitations as well. In contrast, we find that the calculated asymmetry in Ni is rather insensitive to the type of magnetic excitations. However, there is a weak nonlinearity in the relation between asymmetry and the off-diagonal component of the dielectric tensor, which does not originate from the modifications of the electronic structure. Our experimental and theoretical results thus emphasize the need to consider a coupling between asymmetry and magnetization that may be more complex than a simple linear relationship. This insight is crucial for the microscopic interpretation of ultrafast magnetization experiments.
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| 8. |
- Jana, Somnath, et al.
(author)
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Atom-specific magnon-driven ultrafast spin dynamics in Fe1-xNix alloys
- 2023
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18
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Journal article (peer-reviewed)abstract
- By employing element-specific spectroscopy in the ultrafast time scale in Fe1-xNix alloys, we find a composition-dependent effect in the demagnetization that we relate to electron-magnon scattering and changes in the spin-wave stiffness. In all six measured alloys of different composition, the demagnetization of Ni compared to Fe exhibits a delay, an effect which we find is inherent in alloys but not in elemental Fe and Ni. Using a model based on electron-magnon scattering, we extract a spin-wave stiffness from all alloys that show excellent agreement with values obtained from other techniques.
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| 9. |
- Malik, Rameez Saeed, 1987-, et al.
(author)
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Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl
- 2021
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
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Journal article (peer-reviewed)abstract
- We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (tau(M)) in films of Co2FeAl is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by tau(R), is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter (alpha). Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which might have general applicability.
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| 10. |
- Paul, Souvik, et al.
(author)
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Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory
- 2018
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:12
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Journal article (peer-reviewed)abstract
- Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of BiFeO3, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the LDA+U approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe 3d states, whereas the LDA+U method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe 3d orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe 3d states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.
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