Magnetic switching in granular FePt layers promoted by near-field laser enhancement

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle x-ray scattering at an x-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, one order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material, with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer

Steplike versus continuous domain propagation in Co Pd multilayer films

We investigate the microscopic reversal behavior in perpendicular anisotropy magnetic thin films using an integrated mask sample design that allows probing a nanoscale sample region with soft x rays. Local hysteresis loops and spectroholography images are obtained from the transmitted signal exploiting x ray magnetic circular dichroism. Our data provide direct evidence of microscopic spin flip avalanches, such as responsible for Barkhausen noise. In comparison with macroscopic magnetometry measurements we find evidence for the sputter pressure dependent introduction of local defects that prevent a continuous domain wall motion but are not strong enough to introduce the appearance of microscopic return point memor

Untersuchung unabhängiger N H und N C Bindungsverformungen auf ultrakurzen Zeitskalen mit resonanter inelastischer Röntgenstreuung

Die Femtosekundendynamik nach resonanten Photoanregungen mit optischen und Röntgenpulsen ermöglicht eine selektive Verformung von chemischen N H und NC Bindungen in 2 Thiopyridon in wässriger Lösung. Die Untersuchung der orbitalspezifischen elektronischen Struktur und ihrer Dynamik auf ultrakurzen Zeitskalen mit resonanter inelastischer Röntgenstreuung an der N1s Resonanz am Synchrotron und dem Freie Elektronen Laser LCLS in Kombination mit quantenchemischen Multikonfigurationsberechnungen erbringen den direkten Nachweis dieser kontrollierten photoinduzierten Molekglverformungen und ihrer ultrakurzen Zeitskal

Measurement of collective excitations in VO₂ by resonant inelastic x-ray scattering

Vanadium dioxide is of broad interest as a spin-1/2 electron system that realizes a metal-insulator transition near room temperature, due to a combination of strongly correlated and itinerant electron physics. Here, resonant inelastic X-ray scattering is used to measure the excitation spectrum of charge, spin, and lattice degrees of freedom at the vanadium L-edge under different polarization and temperature conditions. These spectra reveal the evolution of energetics across the metal-insulator transition, including the low temperature appearance of a strong candidate for the singlet-triplet excitation of a vanadium dimer.Comment: 5 pages, 4 figure

Anti Stokes resonant x ray Raman scattering for atom specific and excited state selective dynamics

Ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x rays have been poor at picking up excited state moieties from the unexcited ones. With time resolved anti Stokes resonant x ray Raman scattering AS RXRS performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbital and magnetic selectivity of x rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe CO 5 in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS RXRS with transform limited x ray pulses is given which will be covered experimentally by upcoming transform limited x ray source