107 research outputs found
Broadband probing magnetization dynamics of the coupled vortex state permalloy layers in nanopillars
Broadband magnetization response of coupled vortex state magnetic dots in
layered nanopillars was explored as a function of in-plane magnetic field and
interlayer separation. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25
nm) nanopillars of 600 nm diameter, a small in-plane field splits the
eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition
between in-phase and out-of-phase kinds of the low-lying coupled spin wave
modes. The critical field for this splitting is determined by antiparallel
chiralities of the vortices in the layers. Qualitatively similar (although more
gradual) changes occur also in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25
nm) tri-layer nanopillars. These findings are in qualitative agreement with
micromagnetic dynamic simulations
Magnetic Vortex Resonance in Patterned Ferromagnetic Dots
We report a high-resolution experimental detection of the resonant behavior
of magnetic vortices confined in small disk-shaped ferromagnetic dots. The
samples are magnetically soft Fe-Ni disks of diameter 1.1 and 2.2 um, and
thickness 20 and 40 nm patterned via electron beam lithography onto microwave
co-planar waveguides. The vortex excitation spectra were probed by a vector
network analyzer operating in reflection mode, which records the derivative of
the real and the imaginary impedance as a function of frequency. The spectra
show well-defined resonance peaks in magnetic fields smaller than the
characteristic vortex annihilation field. Resonances at 162 and 272 MHz were
detected for 2.2 and 1.1 um disks with thickness 40 nm, respectively. A
resonance peak at 83 MHz was detected for 20-nm thick, 2-um diameter disks. The
resonance frequencies exhibit weak field dependence, and scale as a function of
the dot geometrical aspect ratio. The measured frequencies are well described
by micromagnetic and analytical calculations that rely only on known properties
of the dots (such as the dot diameter, thickness, saturation magnetization, and
exchange stiffness constant) without any adjustable parameters. We find that
the observed resonance originates from the translational motion of the magnetic
vortex core.Comment: submitted to PRB, 17 pages, 5 Fig
Non-linear vortex dynamics and transient effects in ferromagnetic disks
We report a time resolved imaging and micromagnetic simulation study of the
relaxation dynamics of a magnetic vortex in the non-linear regime. We use
time-resolved photoemission electron microscopy and micromagnetic calculations
to examine the emergence of non-linear vortex dynamics in patterned Ni80Fe20
disks in the limit of long field pulses. We show for core shifts beyond ~20-25%
of the disk radius, the initial motion is characterized by distortions of the
vortex, a transient cross-tie wall state, and instabilities in the core
polarization that influence the core trajectories.Comment: 11 pages, 3 figures, submitted to Phys. Rev. Let
Ultrafast vortex-core reversal dynamics in ferromagnetic nanodots
To verify the exact underlying mechanism of ultrafast vortex-core reversal as well as the vortex state stability, we conducted numerical calculations of the dynamic evolution of magnetic vortices in Permalloy cylindrical nanodots under an oscillating in-plane magnetic field over a wide range of the field frequency and amplitude. The calculated results reveal different kinds of the nontrivial dynamic responses of vortices to the driving external field, including the vortex-core reversal. In particular, the results offer insight into the 10 ps scale underlying physics of the ultrafast vortex-core reversal driven by small-amplitude (similar to 10 Oe) oscillating in-plane fields. This work also provides fundamentals of how to effectively manipulate the vortex dynamics as well as the dynamical switching of the vortex-core orientation.open624
Dynamic origin of azimuthal modes splitting in vortex-state magnetic dots
A spin wave theory explaining experimentally observed frequency splitting of
dynamical excitations with azimuthal symmetry of a magnetic dot in a vortex
ground state is developed. It is shown that this splitting is a result of the
dipolar hybridization of three spin wave modes of a dot having azimuthal
indices |m|=1: two high-frequency azimuthal excitation modes of the in-plane
part of the vortex with indices m = +/-1 and a low-frequency m= +1 gyrotropic
mode describing the translational motion of the vortex core. The analytically
calculated magnitude of the frequency splitting is proportional to the ratio of
the dot thickness to its radius and quantitatively agrees with the results of
time resolved Kerr experiments.Comment: 10 pages, 5 figure
Bistability of vortex core dynamics in a single perpendicularly magnetized nano-disk
Microwave spectroscopy of individual vortex-state magnetic nano-disks in a
perpendicular bias magnetic field, , is performed using a magnetic resonance
force microscope (MRFM). It reveals the splitting induced by on the
gyrotropic frequency of the vortex core rotation related to the existence of
the two stable polarities of the core. This splitting enables spectroscopic
detection of the core polarity. The bistability extends up to a large negative
(antiparallel to the core) value of the bias magnetic field , at which the
core polarity is reversed. The difference between the frequencies of the two
stable rotational modes corresponding to each core polarity is proportional to
and to the ratio of the disk thickness to its radius. Simple analytic
theory in combination with micromagnetic simulations give quantitative
description of the observed bistable dynamics.Comment: 4 pages, 3 figures, 1 table, 16 references. Submitted to Physical
Review Letters on December 19th, 200
Localized domain-wall excitations in patterned magnetic dots probed by broadband ferromagnetic resonance
We investigate the magnetization dynamics in circular Permalloy dots with
spatially separated magnetic vortices interconnected by domain walls (double
vortex state). We identify a novel type of quasi one-dimensional (1D) localised
spin wave modes confined along domain walls, connecting each of two vortex
cores with two edge half-antivortices. Variation of the mode eigenfrequencies
with the dot size is in quantitative agreement with the developed model, which
considers a dipolar origin of the localized 1D spin waves or so-called
Winter\'s magnons [J.M. Winter, Phys.Rev. 124, 452 (1961)]. These spin waves
are analogous to the displacement waves of strings, and could be excited in a
wide class of patterned magnetic nanostructures possessing domain walls, namely
in triangular, square, circular or elliptic magnetic dots
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