TY - JOUR A1 - Zalden, Peter A1 - Quirin, Florian A1 - Schumacher, Mathias A1 - Siegel, Jan A1 - Wei, Shuai A1 - Koc, Azize A1 - Nicoul, Matthieu A1 - Trigo, Mariano A1 - Andreasson, Pererik A1 - Enquist, Henrik A1 - Shu, Michael J. A1 - Pardini, Tommaso A1 - Chollet, Matthieu A1 - Zhu, Diling A1 - Lemke, Henrik A1 - Ronneberger, Ider A1 - Larsson, Jörgen A1 - Lindenberg, Aaron M. A1 - Fischer, Henry E. A1 - Hau-Riege, Stefan A1 - Reis, David A. A1 - Mazzarello, Riccardo A1 - Wuttig, Matthias A1 - Sokolowski-Tinten, Klaus T1 - Femtosecond x-ray diffraction reveals a liquid-liquid phase transition in phase-change materials JF - Science N2 - In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid-liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics. Y1 - 2019 U6 - https://doi.org/10.1126/science.aaw1773 SN - 0036-8075 SN - 1095-9203 VL - 364 IS - 6445 SP - 1062 EP - 1067 PB - American Assoc. for the Advancement of Science CY - Washington, DC ER - TY - JOUR A1 - Iurchuk, V. A1 - Schick, D. A1 - Bran, J. A1 - Colson, D. A1 - Forget, A. A1 - Halley, D. A1 - Koc, Azize A1 - Reinhardt, Mathias A1 - Kwamen, C. A1 - Morley, N. A. A1 - Bargheer, Matias A1 - Viret, M. A1 - Gumeniuk, R. A1 - Schmerber, G. A1 - Doudin, B. A1 - Kundys, B. T1 - Optical Writing of Magnetic Properties by Remanent Photostriction JF - Physical review letters N2 - We present an optically induced remanent photostriction in BiFeO3, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO3/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO3. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications. Y1 - 2016 U6 - https://doi.org/10.1103/PhysRevLett.117.107403 SN - 0031-9007 SN - 1079-7114 VL - 117 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Reinhardt, Matthias A1 - Koc, Azize A1 - Leitenberger, Wolfram A1 - Gaal, Peter A1 - Bargheer, Matias T1 - Optimized spatial overlap in optical pump-X-ray probe experiments with high repetition rate using laser-induced surface distortions JF - Journal of synchrotron radiation N2 - Ultrafast X-ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X-ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented. KW - ultrafast X-ray diffraction KW - nanostructures KW - surface deformation KW - heat diffusion KW - optical pump Y1 - 2016 U6 - https://doi.org/10.1107/S1600577515024443 SN - 1600-5775 VL - 23 SP - 474 EP - 479 PB - International Union of Crystallography CY - Chester ER - TY - JOUR A1 - Koc, Azize A1 - Reinhardt, M. A1 - von Reppert, Alexander A1 - Roessle, Matthias A1 - Leitenberger, Wolfram A1 - Dumesnil, K. A1 - Gaal, Peter A1 - Zamponi, Flavio A1 - Bargheer, Matias T1 - Ultrafast x-ray diffraction thermometry measures the influence of spin excitations on the heat transport through nanolayers JF - Physical review : B, Condensed matter and materials physics N2 - We investigate the heat transport through a rare earth multilayer system composed of yttrium (Y), dysprosium (Dy), and niobium (Nb) by ultrafast x-ray diffraction. This is an example of a complex heat flow problem on the nanoscale, where several different quasiparticles carry the heat and conserve a nonequilibrium for more than 10 ns. The Bragg peak positions of each layer represent layer-specific thermometers that measure the energy flow through the sample after excitation of the Y top layer with fs-laser pulses. In an experiment-based analytic solution to the nonequilibrium heat transport problem, we derive the individual contributions of the spins and the coupled electron-lattice system to the heat conduction. The full characterization of the spatiotemporal energy flow at different starting temperatures reveals that the spin excitations of antiferromagnetic Dy speed up the heat transport into the Dy layer at low temperatures, whereas the heat transport through this layer and further into the Y and Nb layers underneath is slowed down. The experimental findings are compared to the solution of the heat equation using macroscopic temperature-dependent material parameters without separation of spin and phonon contributions to the heat. We explain why the simulated energy density matches our experiment-based derivation of the heat transport, although the simulated thermoelastic strain in this simulation is not even in qualitative agreement. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevB.96.014306 SN - 2469-9950 SN - 2469-9969 VL - 96 PB - American Physical Society CY - College Park ER -