150 research outputs found
Optimizing magneto-dipolar interactions for synchronizing vortex based spin-torque nano-oscillators
We report on a theoretical study about the magneto-dipolar coupling and
synchronization between two vortex-based spin-torque nano-oscillators. In this
work we study the dependence of the coupling efficiency on the relative
magnetization parameters of the vortices in the system. For that purpose, we
combine micromagnetic simulations, Thiele equation approach, and analytical
macro-dipole approximation model to identify the optimized configuration for
achieving phase-locking between neighboring oscillators. Notably, we compare
vortices configurations with parallel (P) polarities and with opposite (AP)
polarities. We demonstrate that the AP core configuration exhibits a coupling
strength about three times larger than in the P core configuration.Comment: 8 pages, 11 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
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.
Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets
The Q dependence of the inelastic neutron scattering (INS) intensity of
transitions within the ground-state spin multiplet of single-molecule magnets
(SMMs) is considered. For these transitions, the Q dependence is related to the
spin density map in the ground state, which in turn is governed by the
Heisenberg exchange interactions in the cluster. This provides the possibility
to infer the exchange-coupling constants from the Q dependence of the INS
transitions within the spin ground state. The potential of this strategy is
explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the
molecule Mn12 as an example. The Q dependence is calculated for powder as well
as single-crystal Mn12 samples for various exchange-coupling situations
discussed in the literature. The results are compared to literature data on a
powder sample of Mn12 and to measurements on an oriented array of about 500
single-crystals of Mn12. The calculated Q dependence exhibits significant
variation with the exchange-coupling constants, in particular for a
single-crystal sample, but the experimental findings did not permit an
unambiguous determination. However, although challenging, suitable experiments
are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR
Stable Magnetostatic Solitons in Yttrium Iron Garnet Film Waveguides for Tilted in-Plane Magnetic Fields
The possibility of nonlinear pulses generation in Yttrium Iron Garnet thin
films for arbitrary direction between waveguide and applied static in-plane
magnetic field is considered. Up to now only the cases of in-plane magnetic
fields either perpendicular or parallel to waveguide direction have been
studied both experimentally and theoretically. In the present paper it is shown
that also for other angles (besides 0 or 90 degrees) between a waveguide and
static in-plane magnetic field the stable bright or dark (depending on
magnitude of magnetic field) solitons could be created.Comment: Phys. Rev. B (accepted, April 1, 2002
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
First order valence transition in YbInCu_4 in the (B,T) - plane
The puzzling properties of the first order phase transition in YbInCu and
its alloys in the wide range of magnetic fields and temperatures are perfectly
described in terms of a simple entropy transition for free Yb ions. In
particular, it turns out that the transition line in the -plane is very
close to the elliptic shape, as it has been observed experimentally. Similar
calculations are done, and the experiments are proposed for the
phase transition in Ce in Megagauss fields. We speculate,
that in case of YbInCu the first order transition is a Mott transition
between a higher temperature phase in which localized moments are stabilized by
the entropy terms in the free energy, and a band-like non-magnetic ground state
of the -electrons.Comment: RevTeX, 5 pages, 2 figure
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