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ABSTRACT: A detailed investigation of the magnetization processes in epitaxial MnSi
thin film reveals the existence of elliptically distorted skyrmion strings that
lie in the plane of the film. We provide proof that the uniaxial anisotropy
stabilizes this state over extended regions of the magnetic phase diagram.
Theoretical analysis of an observed cascade of first-order phase transitions is
based on rig- orous numerical calculations of competing chiral modulations,
which shows the existence of helicoids, elliptic skyrmions, and cone phases.
10/2012;
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ABSTRACT: We report on magnetic susceptibility and specific heat measurements of the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at TC = 278.2(3) K. Pronounced anomalies in the field-dependent χac(H) data as well as in the corresponding imaginary part χ''ac(H) reveal a precursor region around TC in the magnetic phase diagram. The occurrence of a maximum at T0 = 279.6 K in the zero-field specific heat data indicates a second-order transition into a magnetically ordered state. A shoulder evolves above this maximum as a magnetic field is applied. The field dependence of both features coincides with crossover lines from the field-polarized to the paramagnetic state deduced from χac(T) at constant magnetic fields. The experimental findings are analyzed within the standard Dzyaloshinskii theory for cubic helimagnets. The remarkable multiplicity of modulated precursor states and the complexity of the magnetic phase diagram near the magnetic ordering are explained by the change of the character of solitonic inter-core interactions and the onset of specific confined chiral modulations in this area.
Journal of Physics Condensed Matter 07/2012; 24(29):294204. · 2.55 Impact Factor
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ABSTRACT: We present an experimental and theoretical investigation of the influence of a uniaxial magnetocrystalline anisotropy on the magnetic textures that are formed in a chiral magnetic system. We show that the epitaxially induced tensile stress in MnSi thin films grown on Si(111) creates an easy-plane uniaxial anisotropy. The magnetoelastic shear stress coefficient is derived from SQUID magnetometry measurements in combination with transmission electron microscopy and x-ray diffraction data. Density functional calculations of the magnetoelastic coefficient support the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. Theoretical calculations based on a Dzyaloshinskii model that includes an easy-plane anisotropy predict a variety of modulations to the magnetic order that are not observed in bulk MnSi crystals. Evidence for these states is found in the magnetic hysteresis and polarized neutron reflectometry measurements.
Phys. Rev. B. 03/2012; 85(9).
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Physical Review Letters 10/2011; 107(17):179701. · 7.37 Impact Factor
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ABSTRACT: 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 T(C)=278.2(3) K. Precursor phenomena display a complex succession of temperature-driven crossovers and phase transitions in the vicinity of T(C). The A-phase region, present below T(C) and fields H<0.5 kOe, is split in several pockets. The complexity of the magnetic phase diagram is theoretically explained by the confinement of solitonic kinklike or Skyrmionic units that develop an attractive and oscillatory intersoliton coupling owing to the longitudinal inhomogeneity of the magnetization.
Physical Review Letters 09/2011; 107(12):127203. · 7.37 Impact Factor
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ABSTRACT: We employ a general phenomenological domain theory for ferromagnetic shape memory (FSM) materials to model equilibrium domain states in martensites with tetragonal twin variants having axes ratios c/a > 1 and easy-plane magnetic anisotropy. The model is evaluated for realistic material parameters of non-modulated (NM) Ni2MnGa alloys. Phase diagrams under competing external stresses and magnetic fields are presented that show regions of fully magnetized single-variant states and various multiphase regions. The theoretical elastic stresses that block single-variant states are found to be much smaller in NM than in the 5M structure of Ni2MnGa alloys.
Journal of Physics Conference Series 07/2011; 303(1):012083.
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ABSTRACT: 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.
02/2011;
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ABSTRACT: 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 $T_C= 278.2(3)$ K. Precursor phenomena display a complex succession of
temperature-driven crossovers and phase transitions in the vicinity of $T_C$.
The A-phase region, present below $T_C$ and fields $H<0.5$ 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 $+\pi$
Skyrmion lattice, the adjacent A$_2$ pocket, however, appears to be related to
helicoids propagating in directions perpendicular to the applied field.
01/2011;
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01/2011;
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ABSTRACT: Curling magnetic vortices with chiral rotation sense and polarity of the core magnetization can exist in magnetic circular nanostructures. Broken mirror symmetry at surfaces/interfaces of magnetic nanostructures induces chiral Dzyaloshinskii-Moriya interactions which may strongly affect the magnetic properties. Using a micromagnetic approach, we investigate the influence of these chiral interactions on the vortex states in magnetic nanodisks. We calculate numerically the shape, size, and stability of the vortices in equilibrium as functions of magnetic field and the material and geometrical parameters. As a result, under the influence of the chiral magnetic interactions vortices of opposite chirality should have different sizes. We provide detailed numerical analysis of this effect, which can be applied to measure the strength of the induced Dzyaloshinskii-Moriya coupling in magnetic thin film elements.
Journal of Physics Conference Series 02/2010; 200(4):042012.
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ABSTRACT: Skyrmionic states in noncentrosymmetric magnets with Lifshitz invariants are investigated within the phenomenological Dzyaloshinskii model. The interaction between the chiral Skyrmions, being repulsive in a broad temperature range, changes into attraction at high temperatures. This leads to a remarkable confinement effect: near the ordering temperature Skyrmions exist only as bound states, and Skyrmion lattices are formed by an unusual instability-type nucleation transition. Numerical investigations on two-dimensional models demonstrate the confinement and the occurrence of different Skyrmion lattice precursor states near the ordering transition that can become thermodynamically stable by anisotropy or longitudinal softness in cubic helimagnets. Comment: 4 pages, 4 figures
01/2010;
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01/2010;
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ABSTRACT: 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, 2009
07/2009;
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ABSTRACT: Vortex states in magnetic nanodisks are essentially affected by surface/interface induced Dzyaloshinskii-Moriya interactions. Within a micromagnetic approach we calculate the equilibrium sizes and shape of the vortices as functions of magnetic field, the material and geometrical parameters of nanodisks. It was found that the Dzyaloshinskii-Moriya coupling can considerably increase sizes of vortices with "right" chirality and suppress vortices with opposite chirality. This allows to form a bistable system of homochiral vortices as a basic element for storage applications. Comment: 8 pages, 8 figures
06/2009;
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ABSTRACT: In cubic noncentrosymmetric ferromagnets uniaxial distortions suppress the helical states and stabilize Skyrmion lattices in a broad range of thermodynamical parameters. Using a phenomenological theory for modulated and localized states in chiral magnets, the equilibrium parameters of the Skyrmion and helical states are derived as functions of the applied magnetic field and induced uniaxial anisotropy. These results show that due to a combined effect of induced uniaxial anisotropy and an applied magnetic field Skyrmion lattices can be formed as thermodynamically stable states in large intervals of magnetic field and temperatures in cubic helimagnets, e.g., in intermetallic compounds MnSi, FeGe, (Fe,Co)Si. We argue that this mechanism is responsible for the formation of Skyrmion states recently observed in thin layers of Fe_{0.5}Co_{0.5}Si [X.Z.Yu et al., Nature \textbf{465}(2010) 901]. Comment: 5 pages, 3 figures
04/2009;
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C Pfleiderer,
A Neubauer,
S Mühlbauer,
F Jonietz,
M Janoschek,
S Legl,
R Ritz,
W Münzer,
C Franz,
P G Niklowitz,
T Keller,
R Georgii,
P Böni,
B Binz,
A Rosch,
U K Rößler, A N Bogdanov
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ABSTRACT: Systems lacking inversion symmetry, such as selected three-dimensional compounds, multilayers and surfaces support Dzyaloshinsky-Moriya (DM) spin-orbit interactions. In recent years DM interactions have attracted great interest, because they may stabilize magnetic structures with a unique chirality and non-trivial topology. The inherent coupling between the various properties provided by DM interactions is potentially relevant for a variety of applications including, for instance, multiferroic and spintronic devices. The, perhaps, most extensively studied material in which DM interactions are important is the cubic B20 compound MnSi. We review the magnetic field and pressure dependence of the magnetic properties of MnSi. At ambient pressure this material displays helical order. Under hydrostatic pressure a non-Fermi liquid state emerges, where a partial magnetic order, reminiscent of liquid crystals, is observed in a small pocket. Recent experiments strongly suggest that the non-Fermi liquid state is not due to quantum criticality. Instead it may be the signature of spin textures and spin excitations with a non-trivial topology.
Journal of Physics Condensed Matter 04/2009; 21(16):164215. · 2.55 Impact Factor
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01/2009;
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Phys. Rev. B. 01/2009; 80:134410.
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ABSTRACT: In antiferromagnetically coupled multilayers with perpendicular anisotropy
unusual multidomain textures can be stabilized due to a close competition
between long-range demagnetization fields and short-range interlayer exchange
coupling.
In particular, the formation and evolution of specific topologically stable
planar defects within the antiferromagnetic ground state, i.e. wall-like
structures with a ferromagnetic configuration extended over a finite width,
explain configurational hysteresis phenomena recently observed in
[Co/Pt(Pd)]/Ru and [Co/Pt]/NiO multilayers.
Within a phenomenological theory, we have analytically derived the
equilibrium sizes of these "ferroband" defects as functions of the
antiferromagnetic exchange, a bias magnetic field, and geometrical parameters
of the multilayers. In the magnetic phase diagram, the existence region of the
ferrobands mediates between the regions of patterns with sharp
antiferromagnetic domain walls and regular arrays of ferromagnetic stripes.
The theoretical results are supported by magnetic force microscopy images of
the remanent states observed in [Co/Pt]/Ru.
11/2008;
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ABSTRACT: The interplay between intrinsic and surface/interface-induced magnetic anisotropies strongly influences magnetization processes in nanomagnetic systems. We develop a micromagnetic theory to describe the field-driven reorientation in nanomagnets with cubic and uniaxial anisotropies. Spin configurations in competing phases and parameters of accompanying multidomain states are calculated as functions of the applied field and the magnetic anisotropies. The constructed magnetic phase diagrams allow to classify different types of the magnetization reversal and to provide detailed analysis of the switching processes in magnetic nanostructures. The calculated magnetization profiles of isolated domain walls show that the equilibrium parameters of such walls are extremely sensitive to applied magnetic field and values of the competing anisotropies and can vary in a broad range. For nanolayers with perpendicular anisotropy the geometrical parameters of stripe domains have been calculated as functions of a bias field. The results are applied to analyze the magnetization processes as observed in various nanosystems with competing anisotropies, mainly, in diluted magnetic semiconductor films (Ga,Mn)As.
Journal of Applied Physics 11/2008; · 2.17 Impact Factor
