ALBERT

Description: For an isolated quantum particle, such as an electron, the orbital (L) and spin (S) magnetic moments can change provided that the total angular momentum of the particle is conserved. In condensed matter, an efficient transfer between L and S can occur owing to the spin-orbit interaction, which originates in the relativistic motion of electrons. Disentangling the absolute contributions of the orbital and spin angular momenta is challenging, however, as any transfer between the two occurs on femtosecond timescales. Here we investigate such phenomena by using ultrashort optical laser pulses to change the magnetization of a ferromagnetic film and then probe its dynamics with circularly polarized femtosecond X-ray pulses. Our measurements enable us to disentangle the spin and orbital components of the magnetic moment, revealing different dynamics for L and S. We highlight the important role played by the spin-orbit interaction in the ultrafast laser-induced demagnetization of ferromagnetic films, and show also that the magneto-crystalline anisotropy energy is an important quantity to consider in such processes. Our study provides insights into the dynamics in magnetic systems as well as perspectives for the ultrafast control of information in magnetic recording media.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Boeglin, C -- Beaurepaire, E -- Halte, V -- Lopez-Flores, V -- Stamm, C -- Pontius, N -- Durr, H A -- Bigot, J-Y -- England -- Nature. 2010 May 27;465(7297):458-61. doi: 10.1038/nature09070.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institut de Physique et de Chimie des Materiaux de Strasbourg, UMR7504, CNRS et Universite de Strasbourg, 23, rue du Loess, 67034 Strasbourg, France. christine.boeglin@ipcms.u-strasbg.fr〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20505724" target="_blank"〉PubMed〈/a〉

Description: Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10-100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Radu, I -- Vahaplar, K -- Stamm, C -- Kachel, T -- Pontius, N -- Durr, H A -- Ostler, T A -- Barker, J -- Evans, R F L -- Chantrell, R W -- Tsukamoto, A -- Itoh, A -- Kirilyuk, A -- Rasing, Th -- Kimel, A V -- England -- Nature. 2011 Apr 14;472(7342):205-8. doi: 10.1038/nature09901. Epub 2011 Mar 30.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands. i.radu@science.ru.nl〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21451521" target="_blank"〉PubMed〈/a〉

Description: Resonant inelastic X-ray scattering and X-ray emission spectroscopy can be used to probe the energy and dispersion of the elementary low-energy excitations that govern functionality in matter: vibronic, charge, spin and orbital excitations. A key drawback of resonant inelastic X-ray scattering has been the need for high photon densities to compensate for fluorescence yields of less than a per cent for soft X-rays. Sample damage from the dominant non-radiative decays thus limits the materials to which such techniques can be applied and the spectral resolution that can be obtained. A means of improving the yield is therefore highly desirable. Here we demonstrate stimulated X-ray emission for crystalline silicon at photon densities that are easily achievable with free-electron lasers. The stimulated radiative decay of core excited species at the expense of non-radiative processes reduces sample damage and permits narrow-bandwidth detection in the directed beam of stimulated radiation. We deduce how stimulated X-ray emission can be enhanced by several orders of magnitude to provide, with high yield and reduced sample damage, a superior probe for low-energy excitations and their dispersion in matter. This is the first step to bringing nonlinear X-ray physics in the condensed phase from theory to application.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Beye, M -- Schreck, S -- Sorgenfrei, F -- Trabant, C -- Pontius, N -- Schussler-Langeheine, C -- Wurth, W -- Fohlisch, A -- England -- Nature. 2013 Sep 12;501(7466):191-4. doi: 10.1038/nature12449. Epub 2013 Aug 21.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute for Methods and Instrumentation of Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany. martin.beye@helmholtz-berlin.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23965622" target="_blank"〉PubMed〈/a〉