295 research outputs found
Formation of guided spin-wave bullets in ferrimagnetic film stripes
The formation of quasi-2D nonlinear spin-wave eigenmodes in longitudinally
magnetized stripes of a ferrimagnetic film, so-called guided spin-wave bullets,
was experimentally observed by using time- and space-resolved Brillouin light
scattering spectroscopy and confirmed by numerical simulation. They represent
stable spin-wave packets propagating along a waveguide structure, for which
both transversal instability and interaction with the side edges of the
waveguide are important. The experiments and the numerical simulation of the
evolution of the spin-wave excitations show that the shape of the formed
packets and their behavior are strongly influenced by the confinement
conditions. The discovery of these modes demonstrates the existence of
quasi-stable nonlinear solutions in the transition regime between
one-dimensional and two-dimensional wave packet propagation.Comment: 4 pages, 3 figure
Field-induced Polar Order at the N\'eel Temperature of Chromium in Rare-earth Orthochromites: Interplay of Rare-earth and Cr Magnetism
We report field-induced switchable polarization (P = 0.2 ~ 0.8 microC/cm2)
below the N\'eel temperature of chromium (TN Cr) in weakly ferromagnetic
rareearth orthochromites, RCrO3 (R=rareearth) but only when the rareearth ion
is magnetic. Intriguingly, the polarization in ErCrO3 (TC ~ 133 K) disappears
at a spin reorientation (Morin) transition (TSR ~ 22 K) below which the weak
ferromagnetism associated with the Cr sublattice also disappears, demonstrating
the crucial role of weak ferromagnetism in inducing the polar order. Further,
the polarization (P) is strongly influenced by applied magnetic field,
indicating a strong magneto electric effect. We suggest that the polar order
occurs in RCrO3, due to the combined effect of poling field that breaks the
symmetry and the exchange field on R ion from Cr sublattice stabilizes the
polar state. We propose that a similar mechanism could work in the
isostructural rareearth orthoferrites, RFeO3 as well.Comment: 31 pages (Manuscript(6 figures)+supplemental information(8 figures)
Giant Keplerate molecule Fe30 - the first octopole magnet
The multipole expansion technique is applied to one of the largest magnetic
molecules, Fe30. The molecule's dipole, toroid and quadrupole magnetic moments
are equal to zero (in the absence of magnetic field) so the multipole expansion
starts from the octopole moment. Probably the Fe30 molecule is the most
symmetrical magnetic body synthesized so far. The magnetization process is
considered theoretically in different geometries. Some components of the
octopole moment experience a jump while the magnetization rises linearly up to
its saturation value. An elementary octopole moment consisting of four magnetic
dipoles is proposed as a hint for designing of an experiment for measurement of
octopole magnetic moment components.Comment: 7 pages, 9 figure
Generalized scattering-matrix approach for magneto-optics in periodically patterned multilayer systems
We present here a generalization of the scattering-matrix approach for the
description of the propagation of electromagnetic waves in nanostructured
magneto-optical systems. Our formalism allows us to describe all the key
magneto-optical effects in any configuration in periodically patterned
multilayer structures. The method can also be applied to describe periodic
multilayer systems comprising materials with any type of optical anisotropy. We
illustrate the method with the analysis of a recent experiment in which the
transverse magneto-optical Kerr effect was measured in a Fe film with a
periodic array of subwavelength circular holes. We show, in agreement with the
experiments, that the excitation of surface plasmon polaritons in this system
leads to a resonant enhancement of the transverse magneto-optical Kerr effect.Comment: 12 pages, 6 figures, submitted to Physical Review
Magnetoelectric Effect and Spontaneous Polarization in HoFe(BO) and HoNdFe(BO)
The thermodynamic, magnetic, dielectric, and magnetoelectric properties of
HoFe(BO) and HoNdFe(BO) are
investigated. Both compounds show a second order Ne\'{e}l transition above 30 K
and a first order spin reorientation transition below 10 K.
HoFe(BO) develops a spontaneous electrical polarization below the
Ne\'{e}l temperature (T) which is diminished in external magnetic fields.
No magnetoelectric effect could be observed in HoFe(BO). In
contrast, the solid solution HoNdFe(BO) exhibits
both, a spontaneous polarization below T and a magnetoelectric effect at
higher fields that extends to high temperatures. The superposition of
spontaneous polarization, induced by the internal magnetic field in the ordered
state, and the magnetoelectric polarizations due to the external field results
in a complex behavior of the total polarization measured as a function of
temperature and field.Comment: 12 pages, 15 figure
Theory for a dissipative droplet soliton excited by a spin torque nanocontact
A novel type of solitary wave is predicted to form in spin torque oscillators
when the free layer has a sufficiently large perpendicular anisotropy. In this
structure, which is a dissipative version of the conservative droplet soliton
originally studied in 1977 by Ivanov and Kosevich, spin torque counteracts the
damping that would otherwise destroy the mode. Asymptotic methods are used to
derive conditions on perpendicular anisotropy strength and applied current
under which a dissipative droplet can be nucleated and sustained. Numerical
methods are used to confirm the stability of the droplet against various
perturbations that are likely in experiments, including tilting of the applied
field, non-zero spin torque asymmetry, and non-trivial Oersted fields. Under
certain conditions, the droplet experiences a drift instability in which it
propagates away from the nanocontact and is then destroyed by damping.Comment: 15 pages, 12 figure
The Ginzburg-Landau model of Bose-Einstein condensation of magnons
We introduce a system of phenomenological equations for Bose-Einstein
condensates of magnons in the one-dimensional setting. The nonlinearly coupled
equations, written for amplitudes of the right-and left-traveling waves,
combine basic features of the Gross-Pitaevskii and complex Ginzburg-Landau
models. They include localized source terms, to represent the microwave
magnon-pumping field. With the source represented by the -functions,
we find analytical solutions for symmetric localized states of the magnon
condensates. We also predict the existence of asymmetric states with unequal
amplitudes of the two components. Numerical simulations demonstrate that all
analytically found solutions are stable. With the -function terms
replaced by broader sources, the simulations reveal a transition from the
single-peak stationary symmetric states to multi-peak ones, generated by the
modulational instability of extended nonlinear-wave patterns. In the
simulations, symmetric initial conditions always converge to symmetric
stationary patterns. On the other hand, asymmetric inputs may generate
nonstationary asymmetric localized solutions, in the form of traveling or
standing waves. Comparison with experimental results demonstrates that the
phenomenological equations provide for a reasonably good model for the
description of the spatiotemporal dynamics of magnon condensates.Comment: Physical Review B, in pres
Radiation of caustic beams from a collapsing bullet
Collapse of an intense (2+1)-dimensional wave packet in a medium with cubic
nonlinearity and a two-dimensional dispersion of an order higher than parabolic
is studied both theoretically and experimentally. The carrier waves are
microwave backward volume spin waves which propagate in a stripe made from a
thin ferrimagnetic film and the packet is a spin-wave bullet. We show that
before being self-destroyed the bullet irradiates untrapped dispersive waves,
which is in agreement with a previous theoretical prediction. Since, in
addition, the ferromagnetic medium is characterized by an induced uniaxial
anisotropy, this radiation takes the form of narrow beams of continuous waves
at very specific angles to its propagation direction. Based on our theoretical
calculations we find that these beams are caustic beams and the angles are the
characteristic spin-wave caustic angles modified by the motion of the source.Comment: 4 pages, 4 figure
Quantum Fluctuations in Large-Spin Molecules
A new type of mesoscopic quantum effect in large-spin molecules possessing
easy-axis anisotropy, such as Mn12, is predicted. The response of such a system
to an external field applied perpendicular to the easy axis is considered. It
is shown that the susceptibility of this system exhibits a peculiar peak of
purely quantum origin. This effect arises from very general properties of
quantum fluctuations in spin systems. We demonstrate that the effect is
entirely accessible for contemporary experimental techniques. Our studies show
that the many-spin nature of the Mn12 clusters is important for a correct
description of this quantum peak.Comment: REVTeX, 3 pages, 6 figures (postscript
Tunnelling series in terms of perturbation theory for quantum spin systems
Considered is quantum tunnelling in anisotropic spin systems in a magnetic
field perpendicular to the anisotropy axis. In the domain of small field the
problem of calculating tunnelling splitting of energy levels is reduced to
constructing the perturbatio n series with degeneracy, the order of degeneracy
being proportional to a spin value. Partial summation of this series taking
into account ''dangerous terms'' with small denominators is performed and the
value of tunnelling splitting is calculated with allowance for the first
correction with respect to a magnetic field.Comment: 7 pages, REVTeX 3.
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