1,550 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
THE GENDER GAP: 100 M TO 42 KM
Gender differences in running performance have attracted considerable interest in the last thirty years. Projections that, over time, women would run as fast or faster than men has fueled a controversy about the abilities of the sexes. The purpose of this study was to examine data from the 2000 Olympic Games to determine if there is evidence of convergence of men’s and women’s times over standard race distances from 100 m to 42 km. Gender differences were found to fall in a relatively small band over these distances. The mean difference over all events was 10.59% when only first place finishers were considered and 11.06% when the first seven finishers were used. Results of this study supports previous work that suggests gender differences have plateaued
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
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