338 research outputs found
Runge-Kutta methods for third order weak approximation of SDEs with multidimensional additive noise
A new class of third order Runge-Kutta methods for stochastic differential
equations with additive noise is introduced. In contrast to Platen's method,
which to the knowledge of the author has been up to now the only known third
order Runge-Kutta scheme for weak approximation, the new class of methods
affords less random variable evaluations and is also applicable to SDEs with
multidimensional noise. Order conditions up to order three are calculated and
coefficients of a four stage third order method are given. This method has
deterministic order four and minimized error constants, and needs in addition
less function evaluations than the method of Platen. Applied to some examples,
the new method is compared numerically with Platen's method and some well known
second order methods and yields very promising results.Comment: Two further examples added, small correction
Target-skyrmions and skyrmion clusters in nanowires of chiral magnets
In bulk non-centrosymmetric magnets the chiral Dzyaloshinskii-Moriya exchange
stabilizes tubular skyrmions with a reversed magnetization in their centers.
While the double-twist is favorable in the center of a skyrmion, it gives rise
to an excess of the energy density at the outskirt. Therefore, magnetic
anisotropies are required to make skyrmions more favorable than the conical
spiral state in bulk materials. Using Monte Carlo simulations, we show that in
magnetic nanowires unusual skyrmions with a doubly twisted core and a number of
concentric helicoidal undulations (target-skyrmions) are thermodynamically
stable even in absence of single-ion anisotropies. Such skyrmions are free of
magnetic charges and, since the angle describing the direction of magnetization
at the surface depends on the radius of the nanowire and an applied magnetic
field, they carry a non-integer skyrmion charge s > 1. This state competes with
clusters of spatially separated s=1 skyrmions. For very small radii, the
target-skyrmion transforms into a skyrmion with s < 1, that resembles the
vortex-like state stabilized by surface-induced anisotropies
Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te
A rapid and anisotropic modification of the Fermi-surface shape can be
associated with abrupt changes in crystalline lattice geometry or in the
magnetic state of a material. In this study we show that such an electronic
topological transition is at the basis of the formation of an unusual
pressure-induced tetragonal ferromagnetic phase in FeTe. Around 2 GPa,
the orthorhombic and incommensurate antiferromagnetic ground-state of
FeTe is transformed upon increasing pressure into a tetragonal
ferromagnetic state via a conventional first-order transition. On the other
hand, an isostructural transition takes place from the paramagnetic
high-temperature state into the ferromagnetic phase as a rare case of a `type
0' transformation with anisotropic properties. Electronic-structure
calculations in combination with electrical resistivity, magnetization, and
x-ray diffraction experiments show that the electronic system of FeTe
is instable with respect to profound topological transitions that can drive
fundamental changes of the lattice anisotropy and the associated magnetic
order.Comment: 7 pages, 4 figur
Chiral skyrmions in thin magnetic films: new objects for magnetic storage technologies?
Axisymmetric magnetic lines of nanometer sizes (chiral vortices or skyrmions)
have been predicted to exist in a large group of noncentrosymmetric crystals
more than two decades ago. Recently these magnetic textures have been directly
observed in nanolayers of cubic helimagnets and monolayers of magnetic metals.
We develop a micromagnetic theory of chiral skyrmions in thin magnetic layers
for magnetic materials with intrinsic and induced chirality. Such particle-like
and stable micromagnetic objects can exist in broad ranges of applied magnetic
fields including zero field. Chiral skyrmions can be used as a new type of
highly mobile nanoscale data carriers
Low-temperature phase diagram of Fe1+yTe
We used low-temperature synchrotron x-ray diffraction to investigate the
structural phase transitions of Fe1+yTe in the vicinity of a tricitical point
in the phase diagram. Detailed analysis of the powder diffraction patterns and
temperature dependence of the peak-widths in Fe1+yTe showed that two-step
structural and magnetic phase transitions occur within the compositional range
0.11 0.13. The phase transitions are sluggish indicating a
strong competition between the orthorhombic and the monoclinic phases. We
combine high-resolution diffraction experiments with specific heat,
resistivity, and magnetization measurements and present a revised
temperature-composition phase diagram for Fe1+yTe.Comment: 10 pages, 14 figure
Exchange shift of stripe domains in antiferromagnetically coupled multilayers
Antiferromagnetically coupled multilayers with perpendicular anisotropy, as
[CoPt]/Ru, Co/Ir, Fe/Au, display ferromagnetic stripe phases as the ground
states. It is theoretically shown that the antiferromagnetic interlayer
exchange causes a relative shift of domains in adjacent layers. This ``exchange
shift'' is responsible for several recently observed effects: an anomalous
broadening of domain walls, the formation of so-called ``tiger-tail'' patterns,
and a ``mixed state'' of antiferromagnetic and ferromagnetic domains in
[CoPt]/Ru multilayers. The derived analitical relations between the values of
the shift and the strength of antiferromagnetic coupling provide an effective
method for a quantitative determination of the interlayer exchange
interactions.Comment: 4 pages, 3 figure
Pressure-induced phase transitions and high-pressure tetragonal phase of Fe1.08Te
We report the effects of hydrostatic pressure on the temperature-induced
phase transitions in Fe1.08Te in the pressure range 0-3 GPa using synchrotron
powder x-ray diffraction (XRD). The results reveal a plethora of phase
transitions. At ambient pressure, Fe1.08Te undergoes simultaneous first-order
structural symmetry-breaking and magnetic phase transitions, namely from the
paramagnetic tetragonal (P4/nmm) to the antiferromagnetic monoclinic (P2_1/m)
phase. We show that, at a pressure of 1.33 GPa, the low temperature structure
adopts an orthorhombic symmetry. More importantly, for pressures of 2.29 GPa
and higher, a symmetry-conserving tetragonal-tetragonal phase transition has
been identified from a change in the c/a ratio of the lattice parameters. The
succession of different pressure and temperature-induced structural and
magnetic phases indicates the presence of strong magneto-elastic coupling
effects in this material.Comment: 11 page
Precursor phenomena at the magnetic ordering of the cubic helimagnet FeGe
We report on detailed magnetic measurements on the cubic helimagnet FeGe in
external magnetic fields and temperatures near the onset of long-range magnetic
order at K. Precursor phenomena display a complex succession of
temperature-driven crossovers and phase transitions in the vicinity of .
The A-phase region, present below and fields kOe, is split in
several pockets. Relying on a modified phenomenological theory for chiral
magnets, the main part of the A-phase could indicate the existence of a
Skyrmion lattice, the adjacent A pocket, however, appears to be related to
helicoids propagating in directions perpendicular to the applied field.Comment: 5 pages, 4 figure
Skyrmionic textures in chiral magnets
In non-centrosymmetric magnets the chiral Dzyaloshinskii-Moriya exchange
stabilizes Skyrmion-strings as excitations which may condense into multiply
modulated phases. Such extended Skyrmionic textures are determined by the
stability of the localized "solitonic" Skyrmion cores and their geometrical
incompatibility which frustrates regular space-filling. We present numerically
exact solutions for Skyrmion lattices and formulate basic properties of the
Skyrmionic states.Comment: Conference information: The International Conference on Magnetism
(ICM), Karlsruhe, July 26 - 31, 200
Chiral Skyrmionic matter in non-centrosymmetric magnets
Axisymmetric magnetic strings with a fixed sense of rotation and nanometer
sizes (chiral magnetic vortices or Skyrmions) have been predicted to exist in a
large group of non-centrosymmetric crystals more than two decades ago. Recently
these extraordinary magnetic states have been directly observed in thin layers
of cubic helimagnet (Fe,Co)Si. In this report we apply our earlier theoretical
findings to review main properties of chiral Skyrmions, to elucidate their
physical nature, and to analyse these recent experimental results on
magnetic-field-driven evolution of Skyrmions and helicoids in chiral
helimagnets.Comment: 13 pages, 7 figures, invited talk - JEMS-2010 ( 23-28 August, Krakow,
Poland
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