1,032 research outputs found
Exchange coupled perpendicular media
The potential of exchange spring bilayers and graded media is reviewed. An
analytical model for the optimization of graded media gives an optimal value of
the magnetic polarization of Js = 0.8 T. The optimum design allows for
thermally stable grains with grain diameters in the order of 3.3 nm, which
supports ultra high density up to 5 to 10 Tbit per inch2. The switching field
distribution is significantly reduced in bilayer media and graded media
compared to single phase media. For the graded media the switching field
distribution is reduced by about a factor of two. For bilayer media the minimum
switching field distribution is obtained for soft layer anisotropies about one
fifth of the hard layer anisotropy. The influence of precessional switching on
the reversal time and the reversal field is investigated in detail for magnetic
bilayers. Exchange spring bilayers can be reversed with field pulses of 20 ps.Comment: submitted to JMMM, 'Current Perspectives; Perpendicular recording
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
High-Speed, Photon Counting CCD Cameras for Astronomy
The design of electron multiplying CCD cameras require a very different
approach from that appropriate for slow scan CCD operation. This paper
describes the main problems in using electron multiplying CCDs for high-speed,
photon counting applications in astronomy and how these may be substantially
overcome. With careful design it is possible to operate the E2V Technologies
L3CCDs at rates well in excess of that claimed by the manufacturer, and that
levels of clock induced charge dramatically lower than those experienced with
commercial cameras that need to operate at unity gain. Measurements of the
performance of the E2V Technologies CCD201 operating at 26 MHz will be
presented together with a guide to the effective reduction of clock induced
charge levels. Examples of astronomical results obtained with our cameras are
presented.Comment: 11 pages, 5 figure
High-resolution imaging and spectroscopy in the visible from large ground-based telescopes with natural guide stars
Near-diffraction limited imaging and spectroscopy in the visible on large
(8-10 meter) class telescopes has proved to be beyond the capabilities of
current adaptive optics technologies, even when using laser guide stars. The
need for high resolution visible imaging in any part of the sky suggests that a
rather different approach is needed. This paper describes the results of
simulations, experiments and astronomical observations that show that a
combination of low order adaptive optic correction using a 4-field curvature
sensor and fast Lucky Imaging strategies with a photon counting CCD camera
systems should deliver 20-25 milliarcsecond resolution in the visible with
reference stars as faint as 18.5 magnitude in I band on large telescopes. Such
an instrument may be used to feed an integral field spectrograph efficiently
using configurations that will also be described.Comment: 8 pages, 3 figure
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
Coupling of dynamical micromagnetism and a stationary spin drift-diffusion equation: A step towards a fully self-consistent spintronics framework
This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.We consider the coupling of the Landau-Lifshitz-Gilbert equation with a quasilinear diffusion equation to describe the interplay of magnetization and spin accumulation in magnetic-nonmagnetic multilayer structures. For this problem, we propose and analyze a convergent finite element integrator, where, in contrast to prior work, we consider the stationary limit for the spin diffusion. Numerical experiments underline that the new approach is more effective, since it leads to the same experimental results as for the model with time-dependent spin diffusion, but allows for larger time-steps of the numerical integrator.The authors acknowledge support from the Vienna Science and Technology Fund (WWTF) under grant MA14-44 (GH, DP, DS), from the Austrian Science Fund (FWF) under grant W1245 (DP, MR), from TU Wien through the innovative projects initiative (DP, MR), from the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development (CA, DS), through the EPSRC grant EP/K008412/1 (GH), from the Royal Society under grant UF080837 (GH)
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