878 research outputs found
Television noise-reduction device
System greatly improves signal-to-noise ratio with little or no loss in picture resolution. By storage of luminance component, which is summed with chrominance component, system performs mathematical integration of basically-repetitive television signals. Integration of signals over interval of their repetition causes little change in original signals and eliminates random noise
Thermal stability of metastable magnetic skyrmions: Entropic narrowing and significance of internal eigenmodes
We compute annihilation rates of metastable magnetic skyrmions using a form
of Langer's theory in the intermediate-to-high damping (IHD) regime. For a
N\'eel skyrmion, a Bloch skyrmion, and an antiskyrmion, we look at two possible
paths to annihilation: collapse and escape through a boundary. We also study
the effects of a curved vs. a flat boundary, a second skyrmion and a
non-magnetic defect. We find that the skyrmion's internal modes play a dominant
role in the thermally activated transitions compared to the spin-wave
excitations and that the relative contribution of internal modes depends on the
nature of the transition process. Our calculations for a small skyrmion
stabilized at zero-field show that collapse on a defect is the most probable
path. In the absence of a defect, the annihilation is largely dominated by
escape mechanisms, even though in this case the activation energy is higher
than that of collapse processes. Escape through a flat boundary is found more
probable than through a curved boundary. The potential source of stability of
metastable skyrmions is therefore found not to lie in high activation energies,
nor in the dynamics at the transition state, but comes from entropic narrowing
in the saddle point region which leads to lowered attempt frequencies. This
narrowing effect is found to be primarily associated with the skyrmion's
internal modes.Comment: 14 pages, 9 figure
Exchange bias and training effects in antiferromagnetically coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices
Exchange bias (EB) and the training effects (TE) in an antiferromagnetically
coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices were studied in the temperature
range 1.8 - 150 K. Strong antiferromagnetic (AFM) interlayer coupling is
evidenced from AC - susceptibility measurements. Below 100 K, vertical
magnetization shifts are present due to the two remanent states corresponding
to the two ferromagnetic (FM) layers at FM and AFM coupling condition. After
field cooling (FC), significant decrease in the exchange bias field (HEB) is
observed when cycling the system through several consecutive hysteresis loops.
Quantitative analysis for the variation of HEB vs. number of field cycles (n)
indicates an excellent agreement between the theory, based on triggered
relaxation phenomena, and our experimental observations. Nevertheless, the
crucial fitting parameter K indicates smooth training effect upon repeated
field cycling, in accordance with our observation.Comment: Accepted Europhysics Letter
Path sampling for lifetimes of metastable magnetic skyrmions and direct comparison with Kramers' method
We perform a direct comparison between Kramers' method in many dimensions --
i.e., Langer's theory -- adapted to magnetic spin systems, and a path sampling
method in the form of forward flux sampling, as a means to compute collapse
rates of metastable magnetic skyrmions. We show that a good agreement is
obtained between the two methods. We report variations of the attempt frequency
associated with skyrmion collapse by three to four orders of magnitude when
varying the applied magnetic field by 5 of the exchange strength, which
confirms the existence of a strong entropic contribution to the lifetime of
skyrmions. This demonstrates that in complex systems, the knowledge of the rate
prefactor, in addition to the internal energy barrier, is essential in order to
properly estimate a lifetime.Comment: 5 pages, 5 figures (main text), 8 pages including supplemental
materia
Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers
We demonstrate experimentally dynamic interface binding in a system
consisting of two coupled ferromagnetic layers. While domain walls in each
layer have different velocity-field responses, for two broad ranges of the
driving field, H, walls in the two layers are bound and move at a common
velocity. The bound states have their own velocity-field response and arise
when the isolated wall velocities in each layer are close, a condition which
always occurs as H->0. Several features of the bound states are reproduced
using a one dimensional model, illustrating their general nature.Comment: 5 pages, 4 figures, to be published in Physical Review Letter
Paths to annihilation of first and second-order (anti)skyrmions via (anti)meron nucleation on the frustrated square lattice
We study annihilation mechanisms of small first- and second-order skyrmions
and antiskyrmions on the frustrated square lattice with broken
inversion symmetry (DMI). We find that annihilation happens via the injection
of the opposite topological charge in the form of meron or antimeron
nucleation. Overall, the exchange frustration generates a complex energy
landscape with not only many (meta)stable and unstable local energy solutions,
but also many possible paths connecting them. Whenever possible, we compute the
activation energy and attempt frequency for the annihilation of isolated
topological defects. In particular, we compare the average lifetime of the
antiskyrmion calculated with transition state theory with direct Langevin
simulations, where an excellent agreement is obtained.Comment: 9 pages, 10 figure
Focus on artificial frustrated systems
Frustration in physics is the inability of a system to simultaneously satisfy all the competing pairwise interactions within it. The past decade has seen an explosion of activity involving engineering frustration in artificial systems built using nanotechnology. The most common are the artificial spin ices that comprise arrays of nanomagnets with competing magnetostatic interactions. As well as being physical embodiments of idealized statistical mechanical models in which properties can be tuned by design, artificial spin ices can be studied using magnetic microscopy, allowing all the details of the microstates of these systems to be interrogated, both in equilibrium and when perturbed away from it. This 'focus on' collection brings together reports on the latest results from leading groups around the globe in this fascinating and fast-moving field
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