1,333 research outputs found
Dynamical signature of a domain phase transition in a perpendicularly-magnetized ultrathin film
Domain phases in ultrathin Fe/Ni/W(110) films with perpendicular anisotropy
have been studied using the ac magnetic susceptibility. Dynamics on time scales
of minutes to hours were probed by quenching the system from high temperature
to the stripe phase region, and varying the constant rate of temperature
increase as the susceptibility traces were measured. The entire susceptibility
peak is observed to relax slowly along the temperature axis, with the peak
temperature increasing as the rate of heating is decreased. This is precisely
opposite to what would happen if this slow relaxation was driven by changes in
the domain density within the stripe phase. The data are instead consistent
with a simple model for the removal of a significant density of pattern defects
and curvature trapped in the quench from high temperature. A quantitative
analysis confirms that the relaxation dynamics are consistent with the
mesoscopic rearrangement of domains required to remove pattern defects, and
that the experiment constitutes a "dynamical" observation of the phase
transition from a high temperature, positionally disordered phase to the low
temperature, ordered stripe phase.Comment: 8 two column pages, 5 figures, full article with extra data figure
Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules
We argue that to explain recent resonant tunneling experiments on crystals of
Mn and Fe, particularly in the low-T limit, one must invoke dynamic
nuclear spin and dipolar interactions. We show the low-, short-time
relaxation will then have a form, where depends on the
nuclear , on the tunneling matrix element between the two
lowest levels, and on the initial distribution of internal fields in the
sample, which depends very strongly on sample shape. The results are directly
applicable to the system. We also give some results for the long-time
relaxation.Comment: 4 pages, 3 PostScript figures, LaTe
Quantum walks of correlated particles
Quantum walks of correlated particles offer the possibility to study
large-scale quantum interference, simulate biological, chemical and physical
systems, and a route to universal quantum computation. Here we demonstrate
quantum walks of two identical photons in an array of 21 continuously
evanescently-coupled waveguides in a SiOxNy chip. We observe quantum
correlations, violating a classical limit by 76 standard deviations, and find
that they depend critically on the input state of the quantum walk. These
results open the way to a powerful approach to quantum walks using correlated
particles to encode information in an exponentially larger state space
Dipolar interaction between two-dimensional magnetic particles
We determine the effective dipolar interaction between single domain
two-dimensional ferromagnetic particles (islands or dots), taking into account
their finite size. The first correction term decays as 1/D^5, where D is the
distance between particles. If the particles are arranged in a regular
two-dimensional array and are magnetized in plane, we show that the correction
term reinforces the antiferromagnetic character of the ground state in a square
lattice, and the ferromagnetic one in a triangular lattice. We also determine
the dipolar spin-wave spectrum and evaluate how the Curie temperature of an
ensemble of magnetic particles scales with the parameters defining the particle
array: height and size of each particle, and interparticle distance. Our
results show that dipolar coupling between particles might induce ferromagnetic
long range order at experimentally relevant temperatures. However, depending on
the size of the particles, such a collective phenomenon may be disguised by
superparamagnetism.Comment: 11 pages, 5 figure
Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature
The field-driven reorientation transition of an anisotropic ferromagnetic
monolayer is studied within the context of a finite-temperature Green's
function theory. The equilibrium state and the field dependence of the magnon
energy gap are calculated for static magnetic field applied in plane
along an easy or a hard axis. In the latter case, the in-plane reorientation of
the magnetization is shown to be continuous at T=0, in agreement with free spin
wave theory, and discontinuous at finite temperature , in contrast with
the prediction of mean field theory. The discontinuity in the orientation angle
creates a jump in the magnon energy gap, and it is the reason why, for ,
the energy does not go to zero at the reorientation field. Above the Curie
temperature , the magnon energy gap vanishes for H=0 both in the
easy and in the hard case. As is increased, the gap is found to increase
almost linearly with , but with different slopes depending on the field
orientation. In particular, the slope is smaller when is along the hard
axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist
well above ().Comment: Final version accepted for publication in Physical Review B (with
three figures
Structure of characteristic Lyapunov vectors in spatiotemporal chaos
We study Lyapunov vectors (LVs) corresponding to the largest Lyapunov
exponents in systems with spatiotemporal chaos. We focus on characteristic LVs
and compare the results with backward LVs obtained via successive Gram-Schmidt
orthonormalizations. Systems of a very different nature such as coupled-map
lattices and the (continuous-time) Lorenz `96 model exhibit the same features
in quantitative and qualitative terms. Additionally we propose a minimal
stochastic model that reproduces the results for chaotic systems. Our work
supports the claims about universality of our earlier results [I. G. Szendro et
al., Phys. Rev. E 76, 025202(R) (2007)] for a specific coupled-map lattice.Comment: 9 page
The influence of diffusion- and relaxation-related factors on signal intensity: An introductive guide to magnetic resonance diffusion-weighted Imaging studies
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