11 research outputs found
Stability of magnetic vortex in soft magnetic nano-sized circular cylinder
Stability of magnetic vortex with respect to displacement of its center in a
nano-scale circular cylinder made of soft ferromagnetic material is studied
theoretically. The mode of vortex displacement producing no magnetic charges on
the cylinder side is proposed and the corresponding absolute single-domain
radius of the cylinder is calculated as a function of its thickness and the
exchange length of the material. In cylinders with the radii less than the
single-domain radius the vortex state is unstable and is absolutely prohibited
(except if pinned by material imperfections), so that the distribution of the
magnetization vector in such cylinders in no applied magnetic field is uniform
(or quasi-uniform). The phase diagram of nano-scale cylinders including the
stability line and the metastability region obtained here is presented.Comment: 3 pages, 2 figures, RevTex 4, presented at JEMS'01, accepted to JMM
Critical velocity for the vortex core reversal in perpendicular bias magnetic field
For a circular magnetic nanodot in a vortex ground state we study how the
critical velocity of the vortex core reversal depends on the magnitude
of a bias magnetic field applied perpendicularly to the dot plane. We find
that, similarly to the case = 0, the critical velocity does not depend on
the size of the dot. The critical velocity is dramatically reduced when the
negative (i.e. opposite to the vortex core direction) bias field approaches the
value, at which a \emph{static} core reversal takes place. A simple analytical
model shows good agreement with our numerical result.Comment: 4 pages, 2 figure
Origin of the increased velocities of domain wall motions in soft magnetic thin-film nanostripes beyond the velocity-breakdown regime
It is known that oscillatory domain-wall (DW) motions in soft magnetic
thin-film nanostripes above the Walker critical field lead to a remarkable
reduction in the average DW velocities. In a much-higher-field region beyond
the velocity-breakdown regime, however, the DW velocities have been found to
increase in response to a further increase of the applied field. We report on
the physical origin and detailed mechanism of this unexpected behavior. We
associate the mechanism with the serial dynamic processes of the nucleation of
vortex-antivortex (V-AV) pairs inside the stripe or at its edges, the
non-linear gyrotropic motions of Vs and AVs, and their annihilation process.
The present results imply that a two-dimensional soliton model is required for
adequate interpretation of DW motions in the linear- and oscillatory-DW-motion
regimes as well as in the beyond-velocity-breakdown regime.Comment: 16 pages, 3 figure
Evolution and stability of a magnetic vortex in small cylindrical ferromagnetic particle under applied field
The energy of a displaced magnetic vortex in a cylindrical particle made of
isotropic ferromagnetic material (magnetic dot) is calculated taking into
account the magnetic dipolar and the exchange interactions. Under the
simplifying assumption of small dot thickness the closed-form expressions for
the dot energy is written in a non-perturbative way as a function of the
coordinate of the vortex center. Then, the process of losing the stability of
the vortex under the influence of the externally applied magnetic field is
considered. The field destabilizing the vortex as well as the field when the
vortex energy is equal to the energy of a uniformly magnetized state are
calculated and presented as a function of dot geometry. The results (containing
no adjustable parameters) are compared to the recent experiment and are in good
agreement.Comment: 4 pages, 2 figures, RevTe
Criterion for transformation of transverse domain wall to vortex or antivortex wall in soft magnetic thin-film nanostripes
We report on the criterion for the dynamic transformation of the internal
structure of moving domain walls (DWs) in soft magnetic thin-film nanostripes
above the Walker threshold field, Hw. In order for the process of
transformation from transverse wall (TW) to vortex wall (VW) or antivortex wall
(AVW) occurs, the edge-soliton core of the TW-type DW should grow sufficiently
to the full width at half maximum of the out-of-plane magnetizations of the
core area of the stabilized vortex (or antivortex) by moving inward along the
transverse (width) direction. Upon completion of the nucleation of the vortex
(antivortex) core, the VW (AVW) is stabilized, and then its core accompanies
the gyrotropic motion in a potential well (hill) of a given nanostripe. Field
strengths exceeding the Hw, which is the onset field of DW velocity breakdown,
are not sufficient but necessary conditions for dynamic DW transformation
Strong radiation of spin waves by core reversal of a magnetic vortex and their wave behaviors in magnetic nanowire waveguides
We conducted micromagnetic numerical studies on the strong radiation of spin waves (SWs) produced by the magnetic-field-induced reversal of a magnetic vortex core, as well as their wave behaviors in magnetic nanowires. It was found that the radial SWs can be emitted intensively from a vortex core in a circular dot by virtue of localized large torques employed at the core, and then can be injected into a long nanowire via their contact. These SWs exhibit wave characteristics such as propagation, reflection, transmission, interference, and dispersion. These results offer a preview of the generation, delivery, and manipulation of SWs in magnetic elements, which are applicable to information-signal processing in potential SW devices.open997
Universal criterion and phase diagram for switching a magnetic vortex core in soft magnetic nanodots
The universal criterion for ultrafast vortex-core switching between the up-
and down-core bistates in soft magnetic nanodots was investigated by
micromagnetic simulations along with analytical calculations. Vortex-core
switching occurs whenever the velocity of vortex-core motion reaches the
critical velocity that is expressed as (e.g. m/s for Permalloy), where Aex is
the exchange stiffness, and is the gyromagnetic ratio. On the basis of the
above results, phase diagrams for the vortex-core switching event and switching
times with respect to both the amplitude and frequency of applied circularly
rotating magnetic field were calculated, which offer practical guidance for
implementing nanodots in vortex states into future solid-state
information-storage devices.Comment: 25 pages, 5 figures. To whom all correspondence should be addressed:
[email protected]
Soliton pair dynamics in patterned ferromagnetic ellipses
Confinement alters the energy landscape of nanoscale magnets, leading to the
appearance of unusual magnetic states, such as vortices, for example. Many
basic questions concerning dynamical and interaction effects remain unanswered,
and nanomagnets are convenient model systems for studying these fundamental
physical phenomena. A single vortex in restricted geometry, also known as a
non-localized soliton, possesses a characteristic translational excitation mode
that corresponds to spiral-like motion of the vortex core around its
equilibrium position. Here, we investigate, by a microwave reflection
technique, the dynamics of magnetic soliton pairs confined in lithographically
defined, ferromagnetic Permalloy ellipses. Through a comparison with
micromagnetic simulations, the observed strong resonances in the subgigahertz
frequency range can be assigned to the translational modes of vortex pairs with
parallel or antiparallel core polarizations. Vortex polarizations play a
negligible role in the static interaction between two vortices, but their
effect dominates the dynamics.Comment: supplemental movies on
http://www.nature.com/nphys/journal/v1/n3/suppinfo/nphys173_S1.htm