227 research outputs found
STM study of the preparation of clean Ta(110) and the subsequent growth of two-dimensional Fe islands
This report deals with the preparation of a clean Ta(110) surface,
investigated by means of scanning tunneling microscopy/spectroscopy as well as
by low-energy electron diffraction and Auger electron spectroscopy. The surface
initially exhibits a surface reconstruction induced by oxygen contamination.
This reconstruction can be removed by annealing at high temperatures under
ultrahigh vacuum conditions. The reconstruction-free surface reveals a surface
resonance at a bias voltage of about -500 mV. The stages of the transformation
are presented and discussed. In a next step, Fe islands were grown on top of
Ta(110) and investigated subsequently. An intermixing regime was identified for
annealing temperatures of (550 - 590) K
Probing the Nanoskyrmion Lattice on Fe/Ir(111) with Magnetic Exchange Force Microscopy
We demonstrate that the magnetic nanoskyrmion lattice on the Fe monolayer on
Ir(111) and the positions of the Fe atoms can be resolved simultaneously using
magnetic exchange force microscopy. Thus, the relation between magnetic and
atomic structure can be determined straightforwardly by evaluating the Fourier
transformation of the real space image data. We further show that the magnetic
contrast can be mapped on a Heisenberg-like magnetic interaction between tip
and sample spins. Since our imaging technique is based on measuring forces, our
observation paves the way to study skyrmions or other complex spin textures on
insulating sample systems with atomic resolution
Effective damping enhancement in noncollinear spin structures
Damping mechanisms in magnetic systems determine the lifetime, diffusion and
transport properties of magnons, domain walls, magnetic vortices, and
skyrmions. Based on the phenomenological Landau-Lifshitz-Gilbert equation, here
the effective damping parameter in noncollinear magnetic systems is determined
describing the linewidth in resonance experiments or the decay parameter in
time-resolved measurements. It is shown how the effective damping can be
calculated from the elliptic polarization of magnons, arising due to the
noncollinear spin arrangement. It is concluded that the effective damping is
larger than the Gilbert damping, and it may significantly differ between
excitation modes. Numerical results for the effective damping are presented for
the localized magnons in isolated skyrmions, with parameters based on the
Pd/Fe/Ir(111) model-type system.Comment: Paper: 6 pages, 3 figures; Supplemental Material: 5 pages, 2 figure
Tuning non-collinear magnetic states by hydrogenation
Two different superstructures form when atomic H is incorporated in the Fe
monolayer on Ir(111). Depending on the amount of H provided, either a highly
ordered p(2x2) hexagonal superstructure or an irregular roughly square
structure is created. We present here spin-polarized scanning tunneling
microscopy (SP-STM) measurements which reveal that in both cases the magnetic
nanoskyrmion lattice state of the pristine Fe monolayer is modified. Our
measurements of the magnetic states in these hydrogenated films are in
agreement with superpositions of cycloidal spin spirals which follow the
pattern and the symmetry dictated by the H superstructures. We thus demonstrate
here the possibility to vary the symmetry of a non-collinear magnetic state in
an ultrathin film without changing its substrate.Comment: 7 pages, 6 figure
Domain imaging across the magneto-structural phase transition in FeTe
The investigation of the magnetic phase transitions in the parent compounds
of Fe-based superconductors is regarded essential for an understanding of the
pairing mechanism in the related superconducting compounds. Even though the
chemical and electronic properties of these materials are often strongly
inhomogeneous on a nanometer length scale, studies of the magnetic phase
transitions using spatially resolved experimental techniques are still scarce.
Here, we present a real space spin-resolved scanning tunneling microscopy
investigation of the surface of FeTe single crystals with different
excess Fe content, , which are continuously driven through the magnetic
phase transition. For FeTe, the transition into the low-temperature
monoclinic commensurate antiferromagnetic phase is accompanied by the sudden
emergence of ordering into four rotational domains with different orientations
of the monoclinic lattice and of the antiferromagnetic order, showing how
structural and magnetic order are intertwined. In the low-temperature phase of
FeTe one type of the domain boundaries disappears, and the transition
into the paramagnetic phase gets rather broad, which is assigned to the
formation of a mixture of orthorhombic and monoclinic phases
Coupling of Coexisting Non-Collinear Spin States in the Fe Monolayer on Re(0001)
Spin-polarized scanning tunneling microscopy is used to investigate the
magnetic state of the Fe monolayer on Re(0001). Two coexisting atomic-scale
non-collinear spin textures are observed with a sharp transition between them
on the order of one atomic lattice spacing. A strict position correlation
between the two spin states is observed both in experiments and in Monte Carlo
simulations, demonstrating their strong coupling behavior.Comment: 5 pages, 3 figure
Magnetic domain walls in strain-patterned ultrathin films
We present a comparison of the characteristics of the magnetic domain walls
in an atomic monolayer of Co on Pt(111) and a Ni/Fe atomic bilayer on Ir(111),
based on spin-polarized scanning tunneling microscopy measurements. In both
cases, the films exhibit a roughly triangular dislocation line pattern created
by epitaxial strain relief, as well as out-of-plane ferromagnetic order.
Domains with opposite magnetization are separated by domain walls with a unique
rotational sense, demonstrating the important role of the Dzyaloshinskii-Moriya
interaction induced by the Co/Pt and the Fe/Ir interfaces. The domain walls in
Co/Pt(111) are straight and usually found in geometrical constrictions of the
film, where they can minimize their length. In contrast, the domain walls in
Ni/Fe/Ir(111) follow complicated paths, which can be correlated to the
structural triangular pattern. The comparison between the two systems shows
that the structural patterns, despite their similarity, have a different impact
on the domain walls. In the Co/Pt(111) case, the magnetic state is not
influenced by the dislocation line network, in contrast to the Ni/Fe/Ir(111)
system in which the formation of the walls is favored at specific positions of
the structural pattern
Field-Dependent Size and Shape of Single Magnetic Skyrmions
The atomic-scale spin structure of individual isolated skyrmions in an
ultrathin film is investigated in real space by spin-polarized scanning
tunneling microscopy. Their axial symmetry as well as their unique rotational
sense is revealed by using both out-of-plane and in-plane sensitive tips. The
size and shape of skyrmions change as a function of magnetic field. An
analytical expression for the description of skyrmions is proposed and applied
to connect the experimental data to the original theoretical model describing
chiral skyrmions. Thereby, the relevant material parameters responsible for
skyrmion formation can be obtained.Comment: Accepted for publication in Phys. Rev. Lett. (2015
Stochastic dynamics and pattern formation of geometrically confined skyrmions
Ensembles of magnetic skyrmions in confined geometries are shown to exhibit
thermally driven motion on two different time scales. The intrinsic fluctuating
dynamics (ps) is governed by short-range symmetric and antisymmetric
exchange interactions, whereas the long-time limit (ns) is
determined by the coaction of skyrmion-skyrmion-repulsion and the system's
geometry. Micromagnetic simulations for realistic island shapes and sizes are
performed and analyzed, indicating the special importance of skyrmion dynamics
at finite temperatures. We demonstrate how the competition between skyrmion
mobility and observation time directly affects the addressability of skyrmionic
bits, which is a key challenge on the path of developing skyrmion-based
room-temperature applications. The presented quasiparticle Monte Carlo approach
offers a computationally efficient description of the diffusive motion of
skyrmion ensembles in confined geometries, like racetrack memory setups.Comment: 12 pages, 11 figure
Spectroscopic signature of the Stark-shifted Tamm-type surface state of La(0001)
We have studied the Tamm-type surface state of La(0001) by tunneling
spectroscopy within a wide range of tunneling currents from 0.1 nA to 8000 nA,
thereby tuning the electric-field strength in a tip-vacuum-sample tunnel
junction. We observe a significant shift of the unoccupied Tamm-type surface
state toward the Fermi energy with increasing electric-field strength,
accompanied by a broadening of the width of the resonance peak indicating a
decrease of the surface-state lifetime. Our experimental results are contrary
to previous reports for Stark-shifted Shockley-type surface states of noble
metal (111) surfaces.Comment: 12 pages, 3 figure
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