Spintronics is a research field that aims to understand and control spins on the nanoscale and should enable next-generation data storage and manipulation. One technological and scientific key challenge is to stabilize small spin textures and to move them efficiently with high velocities. For a long time, research focused on ferromagnetic materials, but ferromagnets show fundamental limits for speed and size. Here, we circumvent these limits using compensated ferrimagnets. Using ferrimagnetic Pt/GdCo/TaO films with a sizeable Dzyaloshinskii-Moriya interaction, we realize a current-driven domain wall motion with a speed of 1.3 km s near the angular momentum compensation temperature (T) and room-temperature-stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature (T). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. Our work shows that high-speed, high-density spintronics devices based on current-driven spin textures can be realized using materials in which T and T are close together.

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During ultrafast demagnetization of a magnetically ordered solid, angular momentum has to be transferred between the spins, electrons, and phonons in the system on femto- and picosecond timescales. Although the intrinsic spin-transfer mechanisms are intensely debated, additional extrinsic mechanisms arising due to nanoscale heterogeneity have only recently entered the discussion. Here we use femtosecond X-ray pulses from a free-electron laser to study thin film samples with magnetic domain patterns. We observe an infrared-pump-induced change of the spin structure within the domain walls on the sub-picosecond timescale. This domain-topography-dependent contribution connects the intrinsic demagnetization process in each domain with spin-transport processes across the domain walls, demonstrating the importance of spin-dependent electron transport between differently magnetized regions as an ultrafast demagnetization channel. This pathway exists independent from structural inhomogeneities such as chemical interfaces, and gives rise to an ultrafast spatially varying response to optical pump pulses.

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Knowledge of atomic diffusion is a fundamental issue in synthesis and stability of materials. Direct studies of the elementary diffusion event, that is, how the individual atoms 'jump', are scarce, as the available techniques are limited to selected systems. Here we show how by monitoring the spatial and temporal variations of the scattered coherent X-ray intensity the diffusion of single atoms can be studied. This is demonstrated for the intermetallic alloy Cu(90)Au(10). By measuring along several directions in reciprocal space, we can elucidate the dynamical behaviour of single atoms as a function of their neighbourhood. This method, usually referred to as X-ray photon correlation spectroscopy (XPCS), does not rely on specific atomic species or isotopes and can thus be applied to almost any system. Thus, given the advent of the next-generation X-ray sources, XPCS has the potential to become the main method for quantitatively understanding diffusion on the atomic scale.

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Ultrashort, coherent x-ray pulses of a free-electron laser are used to holographically image the magnetization dynamics within a magnetic domain pattern after creation of a localized excitation via an optical standing wave. We observe a spatially confined reduction of the magnetization within a couple of hundred femtoseconds followed by its slower recovery. Additionally, the experimental results show evidence of a spatial evolution of magnetization, which we attribute to ultrafast transport of nonequilibrium spin-polarized electrons for early times and to a fluence-dependent remagnetization rate for later times. DOI: 10.1103/PhysRevLett.112.217203 PACS numbers: 75.78.Jp, 42.40.Kw, 75.25.−j, 78.70.Ck Progress in the field of light-induced, ultrafast manipulation of magnetic order has recently led to all-optical, ultrafast magnetic switching [1][2][3] and to an increased control of its dynamics by designing tailored nanostructured samples [4][5][6][7] as well as by exploiting nanoscale magnetic inhomogeneities [8,9]. The influence of interfaces between different materials and magnetic domain boundaries has cast doubt on our theoretical understanding of the underlying fundamental mechanism responsible for femtosecond magnetization dynamics. The model explaining the ultrafast loss of magnetic order after optical excitation by (e.g., electron-phonon or impurity-mediated) spin-flip scattering events [10] has in part been challenged by an approach based on nonlocal superdiffusive spin transport [11]. In spite of their very different microscopic origins, both have been successful in explaining a wide range of experimental data, suggesting that both mechanisms play an important role and that their respective magnitudes depend on the specific experimental conditions [7]. More specifically, in the case of superdiffusive spin transport, energy-and spin-dependent electron lifetimes and velocities induce spin-polarized currents, leading to significant ultrafast spatial rearrangement of magnetic order.To gain control of magnetization dynamics and all-optical switching in the lateral dimension, one relies on nanometer localization of the optical excitation, as well as detailed knowledge on how (spin-polarized) electron currents lead to a spatial transfer of magnetization. Technologically this plays an important role not only for all-optical approaches, but also for heat-assisted magnetic recording, which has the potential to increase the magnetic recording density by lowering the coercitivity of high-anisotropy materials [12]. Necessary to this end, it is required to deliver the optical energy to a sub-100-nm spot size, i.e., far beyond the diffraction limit of optical light. The most successful approaches include localization of the evanescent light from near-field optical probes [13], using metallic plates to excite surface plasmons [14,15] or a combination thereof [16].Here, we implement time-resolved Fourier transform holography (FTH) [17] and exploit x-ray magnetic circular dichroism (XMCD) to directly image the magnet...