Correlating EUV TMOKE and ARPES measurements to understand the temporal and spatial length scales underlying ultrafast demagnetization in ferromagnets
arXiv (Cornell University), Dec 19, 2017
Time-resolved magneto-optical Kerr effect spectroscopies provide powerful probes of the spin dyna... more Time-resolved magneto-optical Kerr effect spectroscopies provide powerful probes of the spin dynamics in magnetic materials. Moreover, using extreme ultraviolet high harmonics that span the entire M-shell absorption edges, spin dynamics corresponding to individual elements in alloys or layers in multilayers can be simultaneously extracted. However, to date, the disparate spin dynamics that occur at different depths in a laser-heated material were not disentangled. Here we show that this spatial averaging masks the true behavior of the material, where material at the surface can go through the ferromagnetic-paramagnetic phase transition, while deeper layers will not. Here we quantitatively compare time-resolved transverse magneto-optical Kerr effect measurements and time- and angle-resolved photoemission spectroscopy measurements on nickel. We show that the critical behavior of the material completely explains the results obtained from these two experiments, taking into account the three universal timescales observed, as well as the probing depth for each method. Our findings show that because of the existence of a critical fluence above which the phase transition will occur, to understand the dynamics in magnetic materials both the incident fluence and probe depth must be taken into account.
Journal of Magnetism and Magnetic Materials, Apr 1, 2021
The current study aimed to investigate the effects of a Namaste care program on the quality of li... more The current study aimed to investigate the effects of a Namaste care program on the quality of life of 25 women with late-stage Alzheimer's disease. The program was implemented two hours daily and four days per week for six months in a nursing facility, Tehran, Iran. Women's quality of life was measured using the Persian version of the Quality of Life in Late-Stage Dementia scale before and after the program implementation. After a six-month intervention with the Namaste care program, the total score of quality of life significantly decreased (17.79 § 1.10 at the end of trial compared with 24.67 § 1.62 at baseline, P = 0.01), indicating improved quality of life. This effect was obtained after controlling for demographic variables and comorbidities. Hence, it seems that the Namaste care program might be an effective supportive method to improve the quality of life of women with late-stage Alzheimer's disease in Iranian culture. However, further large-sample studies are needed to investigate the generalizability of the findings.
Inverse faraday effect as mechanism for ultrafast all-optical magnetic switching
The mechanisms underlying helicity dependent AOS are currently under debate. So far, models have ... more The mechanisms underlying helicity dependent AOS are currently under debate. So far, models have been based on the existence of the inverse Faraday effect, a magnetization component which is induced by the circularly polarised light. In this abstract, we investigate helicity dependent AOS in FePt theoretically. For that, in a first step, we employ ab initio methods to calculate the laser-power and photon frequency dependence of the inverse Faraday field and, in a second step, we calculate the magnetization dynamics triggered by the laser pulse via the thermal heating effect and the inverse Faraday field based on the Landau-Lifshitz-Bloch equation of motion for high-temperature magnetization dynamics.
We theoretically investigate the optically-induced demagnetization of ferromagnetic FePt using th... more We theoretically investigate the optically-induced demagnetization of ferromagnetic FePt using the time-dependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light-matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field E in the perturbative limit, i.e., ∆Mz ∝ E 2 . Moreover, the magnetization dynamics happens dominantly at even multiples nω0, (n = 0, 2, • • • ) of the pump-laser frequency ω0, whereas odd multiples of ω0 do not contribute. We further investigate the demagnetization in conjunction to the optically-induced change of electron occupations and electron correlations. Correlations within the Kohn-Sham local-density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the ab initio computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time-dependent occupation numbers poorly describe the ultrafast demagnetization.
Capturing the coupled dynamics of the charge, spin and lattice degrees of freedom through the ferromagnetic phase transition using time-resolved spectroscopies
Bulletin of the American Physical Society, Mar 7, 2018
Ab initio calculations of the “giant” magnetoresistance in uranium intermetallics (abstract)
Journal of Applied Physics, Apr 15, 1997
Based on the coherent electronic structure of the ideal crystal, the “giant” magnetoresistance (G... more Based on the coherent electronic structure of the ideal crystal, the “giant” magnetoresistance (GMR) of UNiGa and U2Pd2X (X=In, Sn) at the field-induced transition from the antiferromagnetic ground state to the ferromagnetic state is calculated using linear transport theory. Assuming that the scattering is well represented by a single averaged relaxation time, good agreement with experimental data is obtained for the GMR of UNiGa. The effect can be understood from topological changes of particular Fermi surface sheets at the magnetic transition, bringing about conductivity changes of a factor of 2. No experimental data are available for the high-field resistivity of U2Pd2X. Here, in contrast to UNiGa, the Brillouin zone is invariant under the metamagnetic transition. Nonetheless, conductivity changes up to a factor of 2 are found, again caused by changed connectivity of the Fermi surface. Moreover, we predict the possibility of an inverse GMR effect for this system without introducing spin-dependent scattering. The latter is shown to have minor influence due to strong spin–orbit coupling.
Théorie LSDA + U Ordre multipolaire Magnetic phase transitions that involve multipolar degrees of... more Théorie LSDA + U Ordre multipolaire Magnetic phase transitions that involve multipolar degrees of freedom have been widely studied during the last couple of decades, challenging the common approximation which assumes that the physical properties of a magnetic material could be effectively described by purely dipolar degrees of freedom. Due to the complexity of the problem and to the large number of competing interactions involved, the simple (fcc) crystal structure of the actinide dioxides made them the ideal playground system for such theoretical and experimental studies. In the present paper, we summarize our recent attempts to provide an ab initio description of the ordered phases of UO 2 , NpO 2 , and AmO 2 by means of stateof-the-art LDA + U first-principles calculations. This systematic analysis of the electronic structures is here naturally connected to the local crystalline fields of the 5f states in the actinide dioxide series. Related to these we find that the mechanisms which lead to the experimentally observed insulating ground states work in distinctly different ways for each compound.
Inelastic electron scattering is a consequence of mostly Coulomb interaction between electrons in... more Inelastic electron scattering is a consequence of mostly Coulomb interaction between electrons in the sample and electron beam and, as such, it is a nonlocal event. In atomic resolution experiments, it thus opens the following question: How far is the origin of the inelastic scattering signal that is observed when the electron beam is passing nearby an atomic column or plane? We analyze computationally the delocalization of the magnetic signal in electron magnetic circular dichroism (EMCD) experiments in the so-called three-beam orientation, allowing one to image individual atomic planes. We compare the classical EMCD setup using the double-difference procedure (DD-EMCD) to a recently introduced atomic plane resolution EMCD (APR-EMCD) geometry, assuming the same probe size. We observe a strong localization of the EMCD signal to the closest atomic plane, confirming the potential of EMCD to study an evolution of magnetic properties near surfaces or interfaces with atomic plane resolution. The localization of the EMCD signal is remarkably higher than the localization of the nonmagnetic component of the inelastic scattering cross section. We also analyze double-channeling effects and find them particularly strong for the DD-EMCD method, while for APR-EMCD they appear to be minor. The DD-EMCD signal, on the other hand, appears to be more robust with respect to sample thickness than that of the APR-EMCD.
The electronic structure of the uranium dipnictides UX 2 ͑X = As, Sb, and Bi͒ is investigated by ... more The electronic structure of the uranium dipnictides UX 2 ͑X = As, Sb, and Bi͒ is investigated by means of ab initio calculations based on density functional theory. The calculated Fermi surfaces are presented and compared to available experimental models obtained from de Haas-van Alphen experiments. In agreement with experiments they are found to have a significant two-dimensional character. Also, the change of the electronic properties through the series is discussed.
5<i>f</i> -Shell correlation effects in dioxides of light actinides studied by O 1<i>s</i>x-ray absorption and emission spectroscopies and first-principles calculations
Journal of Physics: Condensed Matter, Jul 23, 2015
Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a singl... more Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
Pressure induced electride phase formation in calcium: A key to its strange high-pressure behavior
Nanometer-thin rare-earth-transition metal (RE-TM) alloys with precisely controlled compositions ... more Nanometer-thin rare-earth-transition metal (RE-TM) alloys with precisely controlled compositions and out-of-plane magnetic anisotropy are currently in the focus for ultrafast magnetophotonic applications. However, achieving lateral nanoscale dimensions, crucial for potential device downscaling, while maintaining designable optomagnetic functionality and out-of-plane magnetic anisotropy is extremely challenging. Here we integrate nanosized Tb$_{18}$Co$_{82}$ ferrimagnetic alloys, having strong out-of-plane magnetic anisotropy, within a gold plasmonic nanoantenna array to design micrometer-scale a magnetophotonic crystal that exhibit abrupt and narrow magneto-optical spectral features that are both magnetic field and light incidence direction controlled. The narrow Fano-type resonance arises through the interference of the individual nanoantenna's surface plasmons and a Rayleigh anomaly of the whole nanoantenna array, in both optical and magneto-optical spectra, which we demonstrate and explain using Maxwell-theory simulations. This robust magnetophotonic crystal opens the way for conceptually new high-resolution light incidence direction sensors, as well as for building blocks for plasmon-assisted all-optical magnetization switching in ferrimagnetic RE-TM alloys.
Direct Measurement of the Static and Transient Magneto-Optical Permittivity of Cobalt Across the Entire M-edge in a Reflection Geometry by Use of Polarization Scanning
Bulletin of the American Physical Society, Mar 6, 2018
New long-lived metastable state mediated by mode-selective electron-phonon coupling in 1 T -TaSe 2
Bulletin of the American Physical Society, Mar 4, 2019
Ab initio theory and multiscale modeling of ultrafast laser-induced magnetic processes
Bulletin of the American Physical Society, Mar 7, 2018
Manipulation of magnetisation with ultrashort laser pulses is promising for information storage d... more Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.
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Papers by Peter Oppeneer