Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO 3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO 3 : the relevant excited charge carriers must remain localized to be consistent with the data. Multiferroics have a great potential for application due to their possible coupling of ferroelectricity and magnetism [1][2][3]. BiFeO 3 (BFO) is one of the few room temperature multiferroics today [4][5][6][7][8], and of these, the only one that is a stable phase. Its relatively small band gap of approximately 2.7 eV [9] renders BFO an ideal candidate for applications in spintronics and memory devices [5] with a perspective for ultrafast optical switching similar to purely ferroelectric [10] or magnetic materials [11]. The photovoltaic effect in this complex material and the underlying ultrafast carrier dynamics after above-band-gap femtosecond (fs) optical excitation have been studied thoroughly [12][13][14]. The photoinduced currents in BFO lead to THz emission [15,16] and to a photostrictive response [17]. Alloptical experiments showed that the rapid photoinduced mechanical stress excites coherent phonons [18,19]. The dynamics of photoinduced strains were directly and quantitatively measured in a recent synchrotron-based ultrafast x-ray diffraction (UXRD) study with a temporal resolution of 100 ps [20]. Combined optical measurements revealed a linear dependence of the transient strain and the number of excited carriers over several nanoseconds (ns). This led to the conclusion that depolarization field screening (DFS) including macroscopic transport of the carriers to the surface and interface could be the dominant stress generating process, although the effect of excited antibonding orbitals was not ruled out [20].In this Letter, we report complementing UXRD experiments at a laser-driven plasma x-ray source (PXS) in order to monitor the coherent and incoherent lattice dynamics in a BFO thin film sample with subpicosecond (ps) temporal resolution after above-band-gap excitation. We observe a sound-velocity limited evolution of the structural response within 10 ps indicating a quasi-instantaneous stress. The substantial Bragg peak broadening is a direct evidence of an inhomogeneous spatial stress profile. It appears quasiinstantaneously and decays on nanosecond time scales as reconfirmed by new synchrotron-based UXRD data recorded at the Advanced Photon Source (APS). We obtain quantitative agreement of the transient peak shift and broadening measured with both setups and can firmly conclude that the photogenerated stress driving the film expansion has a strongly inhomogeneous sp...

The udkm1Dsim toolbox is a collection of matlab (MathWorks Inc.) classes and routines to simulate the structural dynamics and the according X-ray diffraction response in one-dimensional crystalline sample structures upon an arbitrary time-dependent external stimulus, e.g. an ultrashort laser pulse. The toolbox provides the capabilities to define arbitrary layered structures on the atomic level including a rich database of corresponding element-specific physical properties. The excitation of ultrafast dynamics is represented by an N -temperature model which is commonly applied for ultrafast optical excitations. Structural dynamics due to thermal stress are calculated by a linear-chain model of masses and springs. The resulting X-ray diffraction response is computed by dynamical X-ray theory. The udkm1Dsim toolbox is highly modular and allows for introducing user-defined results at any step in the simulation procedure.

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By comparing femtosecond laser pulse induced ferro- and antiferromagnetic dynamics in one and the same material-metallic dysprosium-we show both to behave fundamentally different. Antiferromagnetic order is considerably faster and much more efficiently reduced by optical excitation than its ferromagnetic counterpart. We assign the fast and extremely efficient process in the antiferromagnet to an interatomic transfer of angular momentum within the spin system. Our findings imply that this angular momentum transfer channel is effective in other magnetic metals with nonparallel spin alignment. They also point out a possible route towards energy-efficient spin manipulation for magnetic devices.