341 research outputs found
Tributyltin in de Belgische kustwateren en -havens
The objective of this article is to relate the emissions of tributyltin (TBT) used in antifouling paints for ships to the concentrations and the effects found in the marine environment. After a survey of the quantity of TBT used in antifouling paints for ships and the emissions in the marine environment, this article reviews the study on TBT in sediments and mussels along the Belgian coast and in the marine harbours and the imposex-survey with the dogwhelk Nucella lapillus. The results show that the concentration of TBT in the sediments of the Belgian harbours (especially Zeebrugge) are high and contrast with the lower concentration measured in the open sea. The imposex-survey confirms these higher concentrations. As a follow-up of the regulations of the Paris Commission and the CEE a new legislation for the use of organotin containing paints on ships smaller than 25 meters is in force in Belgium. On the understanding that this legislation is strictly applied, it can be expected that the TBT-concentrations in the marine environment will fall
Spin-transfer torque induced reversal in magnetic domains
Using the complex stereographic variable representation for the macrospin,
from a study of the nonlinear dynamics underlying the generalized
Landau-Lifshitz(LL) equation with Gilbert damping, we show that the
spin-transfer torque is effectively equivalent to an applied magnetic field. We
study the macrospin switching on a Stoner particle due to spin-transfer torque
on application of a spin polarized current. We find that the switching due to
spin-transfer torque is a more effective alternative to switching by an applied
external field in the presence of damping. We demonstrate numerically that a
spin-polarized current in the form of a short pulse can be effectively employed
to achieve the desired macro-spin switching.Comment: 16 pages, 6 figure
Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators
We study the magnetization dynamics in a nanocontact magnetic vortex
oscillators as function of temperature. Low temperature experiments reveal that
the dynamics at low and high currents differ qualitatively. At low currents, we
excite a temperature independent standard oscillation mode, consisting in the
gyrotropic motion of a free layer vortex about the nanocontact. Above a
critical current, a sudden jump of the frequency is observed, concomitant with
a substantial increase of the frequency versus current slope factor. Using
micromagnetic simulation and analytical modeling, we associate this new regime
to the creation of a vortex-antivortex pair in the pinned layer of the spin
valve. The vortex-antivortex distance depends on the Oersted field which favors
a separation, and on the exchange bias field, which favors pair merging. The
pair in the pinned layer provides an additional spin torque altering the
dynamics of the free layer vortex, which can be quantitatively accounted for by
an analytical model
Auto-oscillation threshold, narrow spectral lines, and line jitter in spin-torque oscillators based on MgO magnetic tunnel junctions
We demonstrate spin torque induced auto-oscillation in MgO-based magnetic
tunnel junctions. At the generation threshold, we observe a strong line
narrowing down to 6 MHz at 300K and a dramatic increase in oscillator power,
yielding spectrally pure oscillations free of flicker noise. Setting the
synthetic antiferromagnet into autooscillation requires the same current
polarity as the one needed to switch the free layer magnetization. The induced
auto-oscillations are observed even at zero applied field, which is believed to
be the acoustic mode of the synthetic antiferromagnet. While the phase
coherence of the auto-oscillation is of the order of microseconds, the power
autocorrelation time is of the order of milliseconds and can be strongly
influenced by the free layer dynamics
Probing the Dzyaloshinskii-Moriya interaction in CoFeB ultrathin films using domain wall creep and Brillouin light spectroscopy
We have characterized the strength of the interfacial Dyzaloshinskii-Moriya
interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films,
grown on different underlayers of W, TaN, and Hf, using two experimental
methods. First, we determined the effective DMI field from measurements of
field-driven domain wall motion in the creep regime, where applied in-plane
magnetic fields induce an anisotropy in the wall propagation that is correlated
with the DMI strength. Second, Brillouin light spectroscopy was employed to
quantify the frequency non-reciprocity of spin waves in the CoFeB layers, which
yielded an independent measurement of the DMI. By combining these results, we
show that DMI estimates from the different techniques only yield qualitative
agreement, which suggests that open questions remain on the underlying models
used to interpret these results.Comment: 8 page
Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars
We study the current and temperature dependences of the microwave voltage
emission of spin-valve nanopillars subjected to an in-plane magnetic field and
a perpendicular-to-plane current. Despite the complex multilayer geometry,
clear microwave emission is shown to be possible and spectral lines as narrow
as 3.8 MHz (at 150 K) are observed.Comment: To appear in Applied Physics Letter
Quantized spin wave modes in magnetic tunnel junction nanopillars
We present an experimental and theoretical study of the magnetic field
dependence of the mode frequency of thermally excited spin waves in rectangular
shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned
from MgO-based magnetic tunnel junctions. The spin wave frequencies were
measured using spectrally resolved electrical noise measurements. In all
spectra, several independent quantized spin wave modes have been observed and
could be identified as eigenexcitations of the free layer and of the synthetic
antiferromagnet of the junction. Using a theoretical approach based on the
diagonalization of the dynamical matrix of a system of three coupled, spatially
confined magnetic layers, we have modeled the spectra for the smallest pillar
and have extracted its material parameters. The magnetization and exchange
stiffness constant of the CoFeB free layer are thereby found to be
substantially reduced compared to the corresponding thin film values. Moreover,
we could infer that the pinning of the magnetization at the lateral boundaries
must be weak. Finally, the interlayer dipolar coupling between the free layer
and the synthetic antiferromagnet causes mode anticrossings with gap openings
up to 2 GHz. At low fields and in the larger pillars, there is clear evidence
for strong non-uniformities of the layer magnetizations. In particular, at zero
field the lowest mode is not the fundamental mode, but a mode most likely
localized near the layer edges.Comment: 16 pages, 4 figures, (re)submitted to PR
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