Time-resolved x-ray imaging shows that the magnetization dynamics of a micron-sized pattern containing a ferromagnetic vortex is determined by its handedness, or chirality. The out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness in the planar magnetic structure, leading to a precessional motion of the core parallel to a subnanosecond field pulse. The core velocity was an order of magnitude higher than expected from the static susceptibility. These results demonstrate that handedness, already well known to be important in biological systems, plays an important role in the dynamics of microscopic magnets.
Current-induced spin-orbit torques (SOTs) represent one of the most effective ways to manipulate the magnetization in spintronic devices. The orthogonal torquemagnetization geometry, the strong damping, and the large domain wall velocities inherent to materials with strong spin-orbit coupling make SOTs especially appealing for fast switching applications in nonvolatile memory and logic units. So far, however, the timescale and evolution of the magnetization during the switching process have remained undetected. Here, we report the direct observation of SOTdriven magnetization dynamics in Pt/Co/AlO x dots during current pulse injection.Time-resolved x-ray images with 25 nm spatial and 100 ps temporal resolution reveal that switching is achieved within the duration of a sub-ns current pulse by the fast nucleation of an inverted domain at the edge of the dot and propagation of a tilted domain wall across the dot. The nucleation point is deterministic and alternates between the four dot quadrants depending on the sign of the magnetization, current, and external field. Our measurements reveal how the magnetic symmetry is broken by the concerted action of both damping-like and field-like SOT and show that reproducible switching events can be obtained for over 10 12 reversal cycles. arXiv:1704.06402v1 [cond-mat.mtrl-sci] 21 Apr 2017Controlling the magnetic state of ultrathin heterostructures using electric currents is key to developing nonvolatile memory devices with minimal static and dynamic power consumption 1 . A promising approach for magnetic switching is based on injecting an in-plane current into a ferromagnet/heavy metal bilayer, where the spin-orbit torques (SOTs) 2,3 resulting from the spin Hall effect and interface charge-spin conversion 4-8 provide an efficient mechanism to reverse the magnetization 1,9,10,12-15 and manipulate domain walls (DWs) [16][17][18][19] .SOT switching schemes can be easily integrated into three-terminal magnetic tunnel junctions having either in-plane 10 or out-of-plane 20 magnetization. Although the threeterminal geometry is more demanding in terms of size, it offers desirable features compared to the two-terminal spin-transfer torque (STT) approach presently used in magnetic random access memories (MRAM) 21 . One such feature is the separation of the read and write current paths in the tunnel junction, which avoids electrical stress of the oxide barrier during writing and allows for independent optimization of the tunneling magnetoresistance during reading. The other crucial feature is the switching speed, which is expected to be extremely fast because the spin accumulation inducing the SOTs is orthogonal to the quiescent magnetization, leading to a negligible incubation delay. Such a delay is a major issue for STT devices, since thermal fluctuations result in a switching time distribution that is several ns wide 22,23 . Furthermore, the SOT-induced magnetization dynamics is governed by strong damping in the monodomain regime 24,25 and fast domain wall motion in the m...
The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum 1, 2 . Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation 3-5 . The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these femtosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a majority of the angular momentum lost from the spin system on the laser-induced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system. 2Broadly speaking, proposed mechanisms for ultrafast demagnetisation fall into two categories: spin-flip scattering mechanisms and spin transport mechanisms. The first category explains the demagnetisation process as a sudden increase in scattering processes that ultimately result in a decrease of spin order. These scattering processes can include electron-electron, electron-phonon, electron-magnon and even direct spin-light interactions. On average, such scattering must necessarily involve a transfer of angular momentum from the electronic spins to some other subsystem(s). Candidates include the lattice, the electromagnetic field, and the orbital angular momentum of the electrons. Numerical estimates and experiments using circularly polarised light strongly suggest that the amount of angular momentum given to the electromagnetic field interaction is negligible 6 , and experiments using femtosecond x-ray magnetic dichroism (XMCD) indicate that the angular momentum of both electronic spins and orbitals decrease in magnitude nearly simultaneously 7-9 . The only remaining possibility for a spin-flip induced change in angular momentum therefore appears to be a transfer to the lattice via spin-orbit coupling, but this remains to be experimentally verified.The second category of proposed mechanisms relies on the idea that laser excitation causes a ...
We performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution v(k) of tungsten. The 4D spectral function ρ(E; k) in the entire bulk Brillouin zone and 6 eV binding-energy (E) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining full-field k-microscopy with time-of-flight parallel energy recording) and the high brilliance of the soft X-rays used. A direct comparison of bulk and surface spectral functions (taken at low photon energies) reveals a time-reversal-invariant surface state in a local bandgap in the (110)-projected bulk band structure. The surface state connects hole and electron pockets that would otherwise be separated by an indirect local bandgap. We confirmed its Dirac-like spin texture by spin-filtered momentum imaging. The measured 4D data array enables extraction of the 3D dispersion of all bands, all energy isosurfaces, electron velocities, hole or electron conductivity, effective mass and inner potential by simple algorithms without approximations. The high-Z bcc metals with large spin-orbit-induced bandgaps are discussed as candidates for topologically non-trivial surface states.
We report on imaging of three-dimensional precessional orbits of the magnetization vector in a magnetic field by means of a time-resolved vectorial Kerr experiment that measures all three components of the magnetization vector with picosecond resolution. Images of the precessional mode taken with submicrometer spatial resolution reveal that the dynamical excitation in this time regime roughly mirrors the symmetry of the underlying equilibrium spin configuration and that its propagation has a non-wavelike character. These results should form the basis for realistic models of the magnetization dynamics in a largely unexplored but technologically increasingly relevant time scale.
Time-resolved images of the magnetization switching process in a spin transfer structure, obtained by ultrafast x-ray microscopy, reveal the limitations of the macrospin model. Instead of a coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. Our measurements reveal the fundamental roles played independently by the torques due to charge and spin currents in breaking the magnetic symmetry on picosecond time scales.
Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 µm in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.
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