159 research outputs found
Skyrmion Lattice in a Chiral Magnet
Skyrmions represent topologically stable field configurations with
particle-like properties. We used neutron scattering to observe the spontaneous
formation of a two-dimensional lattice of skyrmion lines, a type of magnetic
vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice
stabilizes at the border between paramagnetism and long-range helimagnetic
order perpendicular to a small applied magnetic field regardless of the
direction of the magnetic field relative to the atomic lattice. Our study
experimentally establishes magnetic materials lacking inversion symmetry as an
arena for new forms of crystalline order composed of topologically stable spin
states
Long-range crystalline nature of the skyrmion lattice in MnSi
We report small angle neutron scattering of the skyrmion lattice in MnSi
using an experimental set-up that minimizes the effects of demagnetizing fields
and double scattering. Under these conditions the skyrmion lattice displays
resolution-limited Gaussian rocking scans that correspond to a magnetic
correlation length in excess of several hundred {\mu}m. This is consistent with
exceptionally well-defined long-range order. We further establish the existence
of higher-order scattering, discriminating parasitic double-scattering with
Renninger scans. The field and temperature dependence of the higher-order
scattering arises from an interference effect. It is characteristic for the
long-range crystalline nature of the skyrmion lattice as shown by simple mean
field calculations.Comment: 4 page
Spin Transfer Torques in MnSi at Ultra-low Current Densities
Spin manipulation using electric currents is one of the most promising
directions in the field of spintronics. We used neutron scattering to observe
the influence of an electric current on the magnetic structure in a bulk
material. In the skyrmion lattice of MnSi, where the spins form a lattice of
magnetic vortices similar to the vortex lattice in type II superconductors, we
observe the rotation of the diffraction pattern in response to currents which
are over five orders of magnitude smaller than those typically applied in
experimental studies on current-driven magnetization dynamics in
nanostructures. We attribute our observations to an extremely efficient
coupling of inhomogeneous spin currents to topologically stable knots in spin
structures
Skyrmion Lattice in a Doped Semiconductor
We report a comprehensive small angle neutron scattering study (SANS) of the
magnetic phase diagram of the doped semiconductor Fe_{1-x}Co_{x}Si for x=0.2
and 0.25. For magnetic field parallel to the neutron beam we observe a six-fold
intensity pattern under field-cooling, which identifies the A-phase of
Fe_{1-x}Co_{x}Si as a skyrmion lattice. The regime of the skyrmion lattice is
highly hysteretic and extents over a wide temperature range, consistent with
the site disorder of the Fe and Co atoms. Our study identifies Fe_{1-x}Co_{x}Si
is a second material after MnSi in which a skyrmion lattice forms and
establishes that skyrmion lattices may also occur in strongly doped
semiconductors
Theory of current-driven motion of Skyrmions and spirals in helical magnets
We study theoretically the dynamics of the spin textures, i.e., Skyrmion
crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets
under external current. By numerically solving the Landau-Lifshitz-Gilbert
equation, it is found that (i) the critical current density of the motion is
much lower for SkX compared with SS in agreement with the recent experiment,
(ii) there is no intrinsic pinning effect for SkX and the deformation of the
internal structure of Skyrmion reduces the pinning effect dramatically, (iii)
the Bragg intensity of SkX shows strong time-dependence as can be observed by
neutron scattering experiment.Comment: 4 pages, 3 figure
Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Magnetic skyrmions are chiral spin structures with a whirling configuration.
Their topological properties, nanometer size and the fact that they can be
moved by small current densities have opened a new paradigm for the
manipulation of magnetisation at the nanoscale. To date, chiral skyrmion
structures have been experimentally demonstrated only in bulk materials and in
epitaxial ultrathin films and under external magnetic field or at low
temperature. Here, we report on the observation of stable skyrmions in
sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero
applied magnetic field. We use high lateral resolution X-ray magnetic circular
dichroism microscopy to image their chiral N\'eel internal structure which we
explain as due to the large strength of the Dzyaloshinskii-Moriya interaction
as revealed by spin wave spectroscopy measurements. Our results are
substantiated by micromagnetic simulations and numerical models, which allow
the identification of the physical mechanisms governing the size and stability
of the skyrmions.Comment: Submitted version. Extended version to appear in Nature
Nanotechnolog
Schönfeld (Schönfeldt; Schooveld; Sconfeld), Johann Heinrich (Hans Hendrick; Giovan Henrigo)
Vasari als Paradigma : Rezeption, Kritik, Perspektiven = The paradigm of Vasari : reception, criticism, perspectives (Collana del Kunsthistorisches Institut in Florenz, Max-Planck-Institut ; 20)
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