SwePub - search: WFRF:(Knut Ronny)

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1.	Knut, Ronny, et al. (author) Photoinduced reduction of surface states in the visible light sensitized photocatalyst Fe doped ZnO Other publication (other academic/artistic)
2.	Bailey, W. E., et al. (author) Detection of microwave phase variation in nanometre-scale magnetic heterostructures 2013 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 4, s. 2025- Journal article (peer-reviewed)abstract The internal phase profile of electromagnetic radiation determines many functional properties of metal, oxide or semiconductor heterostructures. In magnetic heterostructures, emerging spin electronic phenomena depend strongly upon the phase profile of the magnetic field (H) over tilde at gigahertz frequencies. Here we demonstrate nanometre-scale, layer-resolved detection of electromagnetic phase through the radio frequency magnetic field (H) over tilde (rf) in magnetic heterostructures. Time-resolved X-ray magnetic circular dichroism reveals the local phase of the radio frequency magnetic field acting on individual magnetizations (M) over tilde (i) through the susceptibility as (M) over tilde = (chi) over tilde(H) over tilde (rf). An unexpectedly large phase variation, similar to 40 degrees, is detected across spin-valve trilayers driven at 3 GHz. The results have implications for the identification of novel effects in spintronics and suggest general possibilities for electromagnetic-phase profile measurement in heterostructures.
3.	Berholts, Marta, et al. (author) Quantum watch and its intrinsic proof of accuracy 2022 In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 4:4 Journal article (peer-reviewed)abstract We have investigated the rich dynamics of complex wave packets composed of multiple high-lying Rydbergstates in He. A quantitative agreement is found between theory and time-resolved photoelectron spectroscopyexperiments. We show that the intricate time dependence of such wave packets can be used for investigatingquantum defects and performing artifact-free timekeeping. The latter relies on the unique fingerprint that iscreated by the time-dependent photoionization of these complex wave packets. These fingerprints determinehow much time has passed since the wave packet was formed and provide an assurance that the measured time iscorrect. Unlike any other clock, this quantum watch does not utilize a counter and is fully quantum mechanicalin its nature. The quantum watch has the potential to become an invaluable tool in pump-probe spectroscopy dueto its simplicity, assurance of accuracy, and ability to provide an absolute timestamp, i.e., there is no need to findtime zero.
4.	Chen, Cong, et al. (author) Tomographic reconstruction of circularly polarized high-harmonic fields : 3D attosecond metrology. 2016 In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 2:2 Journal article (peer-reviewed)abstract Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerge as a complex series of rotating linearly polarized bursts, varying rapidly in amplitude, frequency, and polarization. We extend attosecond metrology techniques to circularly polarized light by simultaneously irradiating a copper surface with circularly polarized high-harmonic and linearly polarized infrared laser fields. The resulting temporal modulation of the photoelectron spectra carries essential phase information about the EUV field. Utilizing the polarization selectivity of the solid surface and by rotating the circularly polarized EUV field in space, we fully retrieve the amplitude and phase of the circularly polarized harmonics, allowing us to reconstruct one of the most complex coherent light fields produced to date.
5.	Choudhury, D., et al. (author) Near-Room-Temperature Colossal Magnetodielectricity and Multiglass Properties in Partially Disordered La2NiMnO6 2012 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:12, s. 127201- Journal article (peer-reviewed)abstract We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.
6.	Coleman, V. A., et al. (author) Defect Formation In Graphene Nanosheets By Acid Treatment : An X-Ray Absorption Spectroscopy And Density Functional Theory Study 2008 In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:6, s. 062001-4 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.
7.	Elhanoty, Mohamed F., et al. (author) Different fingerprints for the OISTR mechanism in the magnetic alloys experiments 2022 In: ADVANCES IN ULTRAFAST CONDENSED PHASE PHYSICS III. - : SPIE - The International Society for Optics and Photonics. - 9781510651418 - 9781510651401 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.
8.	Elhanoty, Mohamed F., et al. (author) Element-selective ultrafast magnetization dynamics of hybrid Stoner-Heisenberg magnets 2022 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:10 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.
9.	Eriksson, Olle, et al. (author) Ordering in diluted magnetic semiconductors : A magnetic percolation phenomenon (invited) 2007 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:9, s. 09H114- 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.
10.	Fan, Tingting, et al. (author) Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism 2015 In: 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 Journal article (peer-reviewed)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.
11.	Forssén, Jens, 1968, et al. (author) Toolbox from the EC FP7 HOSANNA project for the reduction of road and rail traffic noise in the outdoor environment 2014 In: Transport Research Arena 2014, Paris, France. ; , s. 10- Conference paper (peer-reviewed)abstract This paper offers a brief overview of innovative methods for road and rail traffic noise reduction between sourceand receiver. These include using new barrier designs, planting of trees, treatments of ground and road surfacesand greening of building façades and roofs using natural materials, like vegetation, soil and other substrates incombination with recycled materials and artificial elements. The abatements are assessed in terms of numericallypredicted sound level reductions, perceptual effects and cost–benefit analysis. Useful reductions of noise fromurban roads and tramways are predicted for 1-m-high urban noise barriers and these are increased by addinginter-lane barriers. A 3 m wide 0.3 m high lattice ground treatment, a carefully planted 15-m-wide tree belt andreplacing 50 m of paved areas by grassland are predicted to give similar reductions. Tree belts are shown to bevery cost-effective and combining tall barriers with a row of trees reduces the negative impact of wind. Greenroofs may significantly reduce the noise at the quiet side of buildings.
12.	Forssén, Jens, 1968, et al. (author) Åtgärder i utomhusmiljön för att minska buller från väg- och tågtrafik – resultat från EU-projektet HOSANNA 2013 In: LjudBladet, Svenska akustiska sällskapet. - 1403-2317. ; 3, s. 16-28 Journal article (other academic/artistic)abstract Denna artikel ger en kort översikt av bulleråtgärder, resulterande från EU-projektet HOSANNA, för minskning av buller från väg- och spårtrafik under ljudets utbredning. Dessa verktyg inkluderar användning av ny skärmdesign, plantering av träd, förbättringar av mark- och vägytor samt vegetation på byggnadsfasader och tak, med hjälp av naturmaterial, såsom vegetation, jord och andra substrat i kombination med återvunnet material och artificiella element. De akustiska insättningsdämpningarna bedöms genom numeriska beräkningar, perceptuella effekter och kostnadsnyttoanalys. En betydande minskning av buller från väg- och spårtrafik förutsägs för ett 1-m-högt urbant bullerskydd. Detta kan förbättras, upp till ca 10 dB insättningsdämpning, genom att placera ytterligare skydd mellan körfälten. Ett 3 m brett och 0,3 m högt rutnät, ett omsorgsfullt planterat 15 m brett trädbälte samt att ersätta 50 meter asfalterad mark med gräsmark förutspås ge bullerminskningar om 5–8 dB, medan användning av fasadvegetation och gröna tak kan minska nivån på innergårdar med upp till ca 4 respektive 7 dB. Trädbälten visar sig vara mycket kostnadseffektiva och kombinationer av höga skärmar med en rad av träd minskar de negativa effekterna av vind på bullerskärmars insättningsdämpning. Gröna tak kan minska bullret på den tysta sidan av byggnader.
13.	Granroth, Sari, et al. (author) Investigation of interface properties of Ni/Cu multilayers by high kinetic energy photoelectron spectroscopy 2009 In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121 .- 1550-235X. ; 80:9 Journal article (peer-reviewed)abstract High kinetic-energy photoelectron spectroscopy (HIKE) or hard x-ray photoelectron spectroscopy has been used to investigate the alloying of Ni/Cu (100) multilayers. Relative intensities of the corelevels and their chemical shifts derived from binding energy changes are shown to give precise information on physicochemical properties and quality of the buried layers. Interface roughening, including kinetic properties such as the rate of alloying, and temperature effects on the processes can be analyzed quantitatively. Using HIKE, we have been able to precisely follow the deterioration of the multilayer structure at the atomic scale and observe the diffusion of the capping layer into the multilayer structure which in turn is found to lead to a segregation in the ternary system. This is of great importance for future research on multilayered systems of this kind. Our experimental data are supplemented by first-principles theoretical calculations of the core-level shifts for a ternary alloy to allow for modeling of the influence of capping materials on the chemical shifts.
14.	Granroth, Sari, et al. (author) Understanding interface properties from high kinetic energy photoelectron spectroscopy and first principles theory 2011 In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 183:1-3, s. 80-93 Research review (peer-reviewed)abstract Advances in instrumentation regarding 3rd generation synchrotron light sources and electron spectrometers has enabled the field of high kinetic energy photoelectron spectroscopy (HIKE) (also often denoted hard X-ray photoelectron spectroscopy (HX-PES or HAXPES)). Over the last years, the amount of investigations that relies on the HIKE method has increased dramatically and can arguably be said to have given a rebirth of the interest in photoelectron spectroscopy in many areas. It is in particular the much increased mean free path at higher kinetic energies in combination with the elemental selectivity of the core level spectroscopies in general that has lead to this fact, as it makes it possible to investigate the electronic structure of materials with a substantially reduced surface sensitivity. In this review we demonstrate how HIKE can be used to investigate the interface properties in multilayer systems. Relative intensities of the core level photoelectron peaks and their chemical shifts derived from binding energy changes are found to give precise information on physico-chemical properties and quality of the buried layers. Interface roughening, including kinetic properties such as the rate of alloying, and temperature effects on the processes can be analyzed quantitatively. We will also provide an outline of the theoretical framework that is used to support the interpretation of data. We provide examples from our own investigations of multilayer systems which comprises both systems of more model character and a multilayer system very close to real applications in devices that are considered to be viable alternative to the present read head technology. The experimental data presented in this review is exclusively recorded at the BESSY-II synchrotron at the Helmholtz-Zentrum Berlin fur-Materialien und Energie. This HIKE facility is placed at the bending magnet beamline KMC-1, which makes it different from several other facilities which relies on undulators as the source. We will therefore also briefly describe some of the salient design features of this facility.
15.	Grånäs, Oscar, 1979-, et al. (author) Ultrafast modification of the electronic structure of a correlated insulator 2022 In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 4:3 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.
16.	Gupta, Rahul, et al. (author) Element-resolved evidence of superdiffusive spin current arising from ultrafast demagnetization process 2023 In: Phys. Rev. B. - : American Physical Society. ; 108:6 Journal article (peer-reviewed)abstract Using element-specific measurements of the ultrafast demagnetization of Ru/Fe65Co35 hetero-structures, we show that Ru can exhibit a significant magnetic contrast (3% asymmetry) resulting from ultrafast spin currents emanating from the demagnetization process of the FeCo layer. We use this magnetic contrast to investigate how superdiffusive spin currents are affected by the doping of heavy elements in the FeCo layer. We find that the spin currents are strongly suppressed, and that the recovery process in Ru slows down by Re doping. This is in accordance with a change in interface reflectivity of spin currents as found by the superdiffusive spin transport model.
17.	Gupta, Rahul, et al. (author) Ultrafast Spin Dynamics of Ru in Ru/Fe65Co35/Ru heterostructures: Effect of Re doping Other publication (other academic/artistic)
18.	Hickstein, Daniel D., et al. (author) Non-collinear generation of angularly isolated circularly polarized high harmonics 2015 In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 9:11, s. 743-750 Journal article (peer-reviewed)abstract We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization.
19.	Iusan, Diana, et al. (author) Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al : Experiments and ab initio density-functional calculations 2008 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:8, s. 085319- 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.
20.	Jana, Somnath, et al. (author) A setup for element specific magnetization dynamics using the transverse magneto-optic Kerr effect in the energy range of 30-72 eV 2017 In: Review of Scientific Instruments. - : AMER INST PHYSICS. - 0034-6748 .- 1089-7623. ; 88:3 Journal article (peer-reviewed)abstract In this paper, we present a spectrometer that is designed for element-specific and time-resolved transverse magneto-optic Kerr effect experiments at the high-harmonic generation pump-probe facility High Energy Laser Induced Overtone Source (HELIOS) laboratory. HELIOS delivers photons with energies between 30 eV and 72 eV with an overall time resolution of less than 40 fs. The spectrometer is based on a Rowland-circle geometry and allows for simultaneous measurements of all magnetic transition-metal elements. The setup also features easy sample transfer and alignment, and it combines high photon throughput, optimized data acquisition, and a fast switching of the magnetic field at the sample. The spectrometer performance is demonstrated by measuring the ultrafast demagnetization of permalloy. Our data are, for all practical purposes, identical to what have been reported in the earlier high-order harmonic generation work of a similar sample by Mathias et al. [Proc. Natl. Acad. Sci. U.S.A. 109, 4792-4797 (2012)], however, obtained within 15% of the acquisition time compared to their study. Furthermore, our data show a shift of the demagnetization curve of Ni relative to Fe, which has previously been interpreted as a delay of the Ni demagnetization to that of Fe [S. Mathias et al., Proc. Natl. Acad. Sci. U. S. A. 109, 4792-4797 (2012)].
21.	Jana, Somnath, et al. (author) Analysis of the linear relationship between asymmetry and magnetic moment at the M edge of 3d transition metals 2020 In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 2:1 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.
22.	Jana, Somnath, et al. (author) Atom-specific magnon driven ultrafast spin dynamics in Fe1-xNix alloys Other publication (other academic/artistic)abstract By employing element specific ultrafast spectroscopy in Fe1-xNix alloys alloys, we find a composition dependent effect in the demagnetization that we relate to changes in electron-magnon scattering. 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. The result establishes the atom-specific sd-exchange induced magnon generation as an underlying mechanism during ultrafast demagnetization in Fe1-xNix alloys
23.	Jana, Somnath, et al. (author) Atom-specific magnon-driven ultrafast spin dynamics in Fe1-xNix alloys 2023 In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18 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.
24.	Jana, Somnath, et al. (author) Atomic-scale chemical fluctuation in LaSrVMoO6, a proposed half-metallic antiferromagnet 2010 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:18 Journal article (peer-reviewed)abstract Half-metallic antiferromagnets (HMAFMs) have been proposed theoretically long ago but have not been realized experimentally yet. Recently, a double perovskite compound, LaSrVMoO6, has been claimed to be an almost real HMAFM system. Here, we report detailed experimental and theoretical studies on this compound. Our results reveal that the compound is neither a half-metal nor an ordered antiferromagnet. Most importantly, an unusual chemical fluctuation is observed locally, which finally accounts for all the electronic and magnetic properties of this compound.
25.	Jana, S., et al. (author) Experimental confirmation of the delayed Ni demagnetization in FeNi alloy 2022 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 120 Journal article (peer-reviewed)abstract Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials, such as magnetic alloys. Recently, however, it turned out that the commonly used technique of the transverse magneto-optical Kerr effect (T-MOKE) in the extreme ultraviolet range may have issues with unwanted crosstalk between different elemental signals and energy-dependent non-linear response. This problem can be sizeable, which puts recent observations of ultrafast spin transfer from Fe to Ni sites in FeNi alloys into question. In this study, we investigate the Fe-to-Ni spin transfer in a crosstalk-free time-resolved x-ray magnetic circular dichroism (XMCD) experiment with a reliable time reference. With XMCD near the absorption maxima, we find a very similar Fe and Ni dynamics as with T-MOKE from identical samples. Considering the potential non-linearities of the T-MOKE response, such a good agreement in our findings is remarkable. Our data provide the ongoing discussion about ultrafast spin-transfer mechanisms in FeNi systems with a broader experimental basis.
26.	Kfir, Ofer, et al. (author) Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics 2015 In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 9:2, s. 99-105 Journal article (peer-reviewed)abstract Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution.
27.	Kfir, Ofer, et al. (author) Helicity-selective phase-matching and quasi-phase matching of circularly polarized high-order harmonics : towards chiral attosecond pulses 2016 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 49:12 Journal article (peer-reviewed)abstract Phase matching of circularly polarized high-order harmonics driven by counter-rotating bi-chromatic lasers was recently predicted theoretically and demonstrated experimentally. In that work, phase matching was analyzed by assuming that the total energy, spin angular momentum and linear momentum of the photons participating in the process are conserved. Here we propose a new perspective on phase matching of circularly polarized high harmonics. We derive an extended phase matching condition by requiring a new propagation matching condition between the classical vectorial bi-chromatic laser pump and harmonics fields. This allows us to include the influence of the laser pulse envelopes on phase matching. We find that the helicity dependent phase matching facilitates generation of high harmonics beams with a high degree of chirality. Indeed, we present an experimentally measured chiral spectrum that can support a train of attosecond pulses with a high degree of circular polarization. Moreover, while the degree of circularity of the most intense pulse approaches unity, all other pulses exhibit reduced circularity. This feature suggests the possibility of using a train of attosecond pulses as an isolated attosecond probe for chiral-sensitive experiments.
28.	Klewe, C., et al. (author) Observation of coherently coupled cation spin dynamics in an insulating ferrimagnetic oxide 2023 In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 122:13 Journal article (peer-reviewed)abstract Many technologically useful magnetic oxides are ferrimagnetic insulators, which consist of chemically distinct cations. Here, we examine the spin dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite (Ni0.65Zn0.35Al0.8Fe1.2O4) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe2+/3+ and Ni2+ cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within ≈2°. Moreover, the effective spin relaxation is identical to within <10% for all magnetic cations in the ferrite. Thus, we validate the oft-assumed “ferromagnetic-like” dynamics in the resonantly driven ferrimagnetic oxide: the magnetic moments from different cations precess as a coherent, collective magnetization, despite the high contents of nonmagnetic Zn2+ and Al3+ diluting the exchange interactions.
29.	Knut, Ronny, et al. (author) High energy photoelectron spectroscopy in basic and applied science : Bulk and interface electronic structure 2013 In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 190, s. 278-288 Journal article (peer-reviewed)abstract With the access of new high-performance electron spectrometers capable of analyzing electron energies up to the order of 10 keV, the interest for photoelectron spectroscopy has grown and many new applications of the technique in areas where electron spectroscopies were considered to have limited use have been demonstrated over the last few decades. The technique, often denoted hard X-ray photoelectron spectroscopy (HX-PES or HAXPES), to distinguish the experiment from X-ray photoelectron spectroscopy performed at lower energies, has resulted in an increasing interest in photoelectron spectroscopy in many areas. The much increased mean free path at higher kinetic energies, in combination with the elemental selectivity of the core level spectroscopies in general has led to this fact. It is thus now possible to investigate the electronic structure of materials with a substantially enhanced bulk sensitivity. In this review we provide examples from our own research using HAXPES which to date has been performed mainly at the HIKE facility at the KMC-1 beamline at HZB, Berlin. The review exemplifies the new opportunities using HAXPES to address both bulk and interface electronic properties in systems relevant for applications in magnetic storage, energy related research, but also in purely curiosity driven problems.
30.	Knut, Ronny, et al. (author) High energy photoelectron spectroscopy in basic and applied science : Bulk and interface electronic structure 2013 In: Journal of Electron Spectroscopy and Related Phenomena. ; 190, s. 278-288 Journal article (peer-reviewed)
31.	Knut, Ronny, et al. (author) Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers Other publication (other academic/artistic)
32.	Knut, Ronny, et al. (author) Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers 2013 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:13, s. 134407- Journal article (peer-reviewed)abstract To address the amount of disorder and interface diffusion induced by annealing, all-Heusler multilayer structures, consisting of ferromagnetic Co2MnGe and nonmagnetic Rh2CuSn layers of varying thicknesses, have been investigated by means of hard x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism. We find evidence for a 4 angstrom thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the unexpectedly small magnetoresistance found for current-perpendicular-to-plane giant magnetoresistance devices based on this all-Heusler system. We find that diffusion begins already at comparably low temperatures between 200 and 250 degrees C, where Mn appears to be most prone to diffusion.
33.	Knut, Ronny, et al. (author) Magnetic and electronic characterization of highly Co-doped ZnO : An annealing study at the solubility limit 2010 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:9, s. 094438- Journal article (peer-reviewed)abstract We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co3+, indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.
34.	Knut, Ronny, et al. (author) Monte Carlo simulation as a sensitive characterization tool for exchange interactions and magnetic ion distributions Journal article (other academic/artistic)
35.	Knut, Ronny (author) New Materials for Spintronics : Electronic structure and magnetism 2012 Doctoral thesis (other academic/artistic)abstract Materials exhibiting new functionalities due to interdependent electric (e.g. conductivity) and magnetic properties are potentially interesting for spintronics applications. We have investigated electronic and magnetic properties by means of x-ray spectroscopies and SQUID magnetometry in several magnetic materials, often in the form of thin films, which have shown promising properties for applications.One of the main subjects has been studies of inter-diffusion between layers in multilayer structures, which is an important factor for spin-dependent transport and magnetic properties. These studies have been performed by high kinetic (HIKE) photoemission spectroscopy where high photon energies increase the bulk sensitivity in comparison to soft x-ray photoemission spectroscopy. Cu/Ni multilayers were studied mainly as a model system and revealed a diffusion process that was dependent on layer thicknesses and capping materials. CoFeB/MgO/CoFeB, which is used as a magnetic field sensor in hard drives, has recently been shown to exhibit a perpendicular magnetic anisotropy (PMA) switchable by electric fields. We have studied both the interface quality and magnetic properties of thin CoFeB layers exhibiting PMA. Layered structures of full Heusler alloys Co2MnGe/Rh2CuSn have been proposed as a promising candidate for current-perpendicular-to-plane giant magneto-resistance sensors. Using HIKE,we have shown that diffusion of atoms, mainly Mn, occurs at temperatures lower than what is used in device fabrication, which likely contributes to the limited magneto-resistance values obtained.Lately, a large body of research has been performed on semiconductors doped with transition metal elements with the hope to find a ferromagnetic semiconductor at room temperature, a foundation for new devices combining spin and charge in their functionality. We have investigated Co and Fe doping in ZnO for different concentrations of the dopants and different annealing temperatures. The Co and Fe atoms are shown to forms clusters for which antiferromagnetic interactions are dominating.
36.	Knut, Ronny, et al. (author) Perpendicular and In-Plane Hole Asymmetry in a Strained NiFe2O4 film 2021 In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33 Journal article (peer-reviewed)abstract Strained materials can exhibit drastically modified physical properties in comparison to their fully relaxed analogues. We report on the x-ray absorption and magnetic circular dichroism of a strained \NFO inverse spinel film grown on a symmetry matched single crystal MgGa2O4 substrate. The Ni XAS spectra exhibit a sizable difference in the white line intensity for measurements with the x-ray electric field parallel to the film plane (normal incidence) vs. when the electric field is at an angle (off-normal). A considerable difference is also observed in the Fe L2,3 XMCD spectrum. Modeling of the XAS and XMCD spectra indicate that the modified energy ordering of the cation 3d states in the strained film leads to a preferential filling of 3d states with out-of-plane character. In addition, the results point to the utility of x-ray spectroscopy in identifying orbital populations even with elliptically polarized x-rays.
37.	Knut, Ronny, et al. (author) Photoinduced reduction of surface states in Fe:ZnO 2015 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 142:20 Journal article (peer-reviewed)abstract We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe2+ are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.
38.	Knut, Ronny, et al. (author) Reactive ZnO/Ti/ZnO interfaces studied by hard x-ray photoelectron spectroscopy 2014 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:4, s. 043714-043714-7 Journal article (peer-reviewed)abstract The chemistry and intermixing at buried interfaces in sputter deposited ZnO/Ti/ZnO thin layers were studied by hard x-ray photoelectron spectroscopy. The long mean free path of the photoelectrons allowed for detailed studies of the oxidation state, band bending effects, and intrinsic doping of the buried interfaces. Oxidation of the Ti layer was observed when ZnO was deposited on top. When Ti is deposited onto ZnO, Zn Auger peaks acquire a metallic character indicating a strong reduction of ZnO at the interface. Annealing of the stack at 200 °C results in further reduction of ZnO and oxidation of Ti. Above 300 °C, oxygen transport from the bulk of the ZnO layer takes place, leading to re-oxidation of ZnO at the interface and further oxidation of Ti layer. Heating above 500 °C leads to an intermixing of the layers and the formation of a ZnxTiOy compound.
39.	Knut, Ronny, et al. (author) Spectroscopic investigations of electronic structure 2012 In: Advanced Functional Materials. - : Elsevier. - 9780444536815 ; , s. 45-70 Book chapter (peer-reviewed)
40.	Knut, Ronny, et al. (author) XMCD study of PMA in CFB/MgO Other publication (other academic/artistic)
41.	Lagerqvist, Ulrika, 1980-, et al. (author) ZnO-ZnxFeyO Superlattices - Influence of Metastable Phases on Structure and Properties in Fe:Zn:O Other publication (other academic/artistic)
42.	Liu, H., et al. (author) Optical and extreme UV studies of spin dynamics in metallic and insulating ferrimagnets 2021 In: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 130:24 Journal article (peer-reviewed)abstract We present all-optical studies of spin dynamics in two classes of ferrimagnets. Both sets of experiments use table-top laser-based pump–probe techniques to examine the ultrafast and longer timescale spin excitations. We use the visible/near infra-red time-resolved magneto-optical Kerr effect (tr-MOKE) to follow the spin dynamics of a series of metallic (FeCo)1−xGdx thin films with varying Gd content. Magnetic compensation in the films occurs at a Gd concentration of ≈26%, and the spin dynamics of the films exhibit a non-monotonic variation in effective magnetization. We also examine spin dynamics in an insulating NiFe2O4 spinel using ultrafast techniques up at extreme ultraviolet energies, which permit element-specific investigations. The element and time-resolved delay scans reveal a non-trivial interaction between spin excitations on the different magnetic sub-lattices of the magnetic insulator.
43.	Locht, Inka L. M., 1986-, et al. (author) Magnetic asymmetry around the 3p absorption edge in Fe and Ni Other publication (other academic/artistic)abstract This work is a joint theoretical and experimental study of the relation between the magneto- optical response of a material in the sub-picosecond timescale and its instantaneous magnetisation. We perform pump-probe experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry. We measure the magnetic asymmetry of elemental Fe and Ni before and after the laser pulse. The observed differences between the magnetic asymmetry curves for various photon energies suggest that the relation between asymmetry and sample magnetization is more complex than a simple proportionality. Further insight is obtained by means of theoretical simulations based on density-functional theory. Our calculations show that non-linear effects in the asymmetry are most prominent at energies corresponding to the absorption edge and that the proportionality is recovered outside of this region. In conclusion, our experimental and theoretical results emphasize the need of including the complex relation between asymmetry and magnetization in the interpretation of ultrafast magnetization experiments in terms of microscopic properties.
44.	Malik, Rameez Saeed, 1987-, et al. (author) Ultrafast dynamics in Fe65Co35 alloys: Effect of Re doping Other publication (pop. science, debate, etc.)abstract Soft magnetic FeCo alloys are of great interest due to their potential spintronics applications. The magnetic damping parameter plays a vital role in the performance of these spintronics devices. The Gilbert damping parameter increase in these alloys with doping of 5d elements. Here, we have investigated the effect of Re doping on the element-specific magnetization dynamics of Ru/Fe65Co35/Ru thin films using the time-resolved magneto-optical Kerr effect. When varying the concentration of Re from 0 to 12.6 %, no change of the demagnetization time constant is observed. However, a gradual change of the remagnetization time is observed with the increase of Re concentration. This remagnetization dynamics can be related to the Gilbert damping parameter of these films. An interesting time-resolved dynamics at the Ru-edge is observed. A significant increase (40%) of the asymmetry signal is observed for the undoped sample and drops down with the Re doping. This effect is possibly a super diffusive spin current going from the magnetic layer to the non magnetic capping layer.
45.	Malik, Rameez Saeed, 1987-, et al. (author) Ultrafast magnetization dynamics in half-metallic Co2FeAl Heusler alloy Other publication (pop. science, debate, etc.)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 resultsfrom electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we findthat the demagnetization time (τM) in films of Co2FeAl is almost independent of varying structuralorder, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower processof magnetization recovery, specified byτR, is found to occur on picosecond time scales, and isdemonstrated to correlate strongly with the Gilbert damping parameter (α). Our results showthat Co2FeAl is unique, in that it is the first material that clearly demonstrates the importance ofthe damping parameter in the remagnetization process. Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which mighthave general applicabilit
46.	Malik, Rameez Saeed, 1987-, et al. (author) Ultrafast magnetization dynamics in magnetically frustrated NiFe2O4 ferrimagnet Other publication (pop. science, debate, etc.)abstract NiFe2O4 shows complex dynamics between its elements after an ultrashort laser pulse excitation. The reflectivity shows oscillations, indicating either a reordering of the charge distribution several picoseconds after absorption of the laser pulse or a coherent phonon mode with a frequency of 0.5 THz. Also, the magnetic asymmetry exhibits oscillations that cannot be attributed to transverse magnetic excitations (magnons) but correspond to longitudinal excitations. The demagnetization shows different behavior depending on the excitation photon energy, for energies close to the band gap (1.55 eV) oscillations in the asymmetry are prominent, while for higher energies (3.1 eV) the demagnetization becomes dominant and the oscillations vanish. A fast demagnetization is observed when using 3.1 eV compared to 1.55 eV, while the electronic response remains similar, which indicates that the fast demagnetization dynamics is driven by holes created deeper in the valence band while the dynamics of electrons excited to d-states in the conduction band provides a smaller contribution to the demagnetization.The oscillations in the asymmetry can be a consequence of the competing exchange interactions that are antiferromagnetic between all three sublattices, but where a ferromagnetic alignment between octahedral Ni and octahedral Fe is the ground state.
47.	Malik, Rameez Saeed, 1987-, et al. (author) Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl 2021 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10 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.
48.	Mancuso, Christopher A., et al. (author) Controlling electron-ion rescattering in two-color circularly polarized femtosecond laser fields 2016 In: Phys. Rev. A. - : American Physical Society. ; 93 Journal article (peer-reviewed)
49.	Mancuso, Christopher A., et al. (author) Controlling Nonsequential Double Ionization in Two-Color Circularly Polarized Femtosecond Laser Fields 2016 In: Phys. Rev. Lett.. - : American Physical Society. ; 117 Journal article (peer-reviewed)
50.	Mancuso, Christopher A., et al. (author) Strong-field ionization with two-color circularly polarized laser fields 2015 In: Physical Review A. ; 91:3 Journal article (peer-reviewed)

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