589 research outputs found
Field-Driven Domain-Wall Dynamics in GaMnAs Films with Perpendicular Anisotropy
We combine magneto-optical imaging and a magnetic field pulse technique to
study domain wall dynamics in a ferromagnetic (Ga,Mn)As layer with
perpendicular easy axis. Contrary to ultrathin metallic layers, the depinning
field is found to be smaller than the Walker field, thereby allowing for the
observation of the steady and precessional flow regimes. The domain wall width
and damping parameters are determined self-consistently. The damping, 30 times
larger than the one deduced from ferromagnetic resonance, is shown to
essentially originate from the non-conservation of the magnetization modulus.
An unpredicted damping resonance and a dissipation regime associated with the
existence of horizontal Bloch lines are also revealed
Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio
Field-induced domain wall propagation: beyond the one-dimensional model
We have investigated numerically the field-driven propagation of
perpendicularly magnetized ferromagnetic layers. It was then compared to the
historical one-dimensional domain wall (DW) propagation model widely used in
spintronics studies of magnetic nanostructures. In the particular regime of
layer thickness (h) of the order of the exchange length, anomalous velocity
peaks appear in the precessional regime, their shape and position shifting with
h. This has also been observed experimentally. Analyses of the simulations show
a distinct correlation between the curvature of the DW and the twist of the
magnetization vector within it, and the velocity peak. Associating a
phenomenological description of this twist with a four-coordinate DW
propagation model, we reproduce very well these kinks and show that they result
from the torque exerted by the stray field created by the domains on the
twisted magnetization. The position of the peaks is well predicted from the
DW's first flexural mode frequency, and depends strongly on the layer
thickness. Comparison of the proposed model to DW propagation data obtained on
dilute semiconductor ferromagnets GaMnAs and GaMnAsP sheds light on the origin
of the measured peaks
Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory
The magnetic domain structure and magnetic properties of a ferromagnetic
(Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We
show that, despite strong hysteresis, domain theory at thermodynamical
equilibrium can be used to determine the micromagnetic parameters. Combining
magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance
measurements, we obtain the characteristic parameter for magnetic domains
, the domain wall width and specific energy, and the spin stiffness
constant as a function of temperature. The nucleation barrier for magnetization
reversal and the Walker breakdown velocity for field-driven domain wall
propagation are also estimated
Enhancing industrial vertical falling film evaporation through modification of heat transfer surfaces – an experimental study
In this study we introduce and experimentally study modifications of heat transfer surfaces as a means to improve the heat transfer in evaporative vertical falling film units. We look at two modified tubular surfaces and compare the obtained results to those acquired using a similar smooth surface. We carry out experiments with two fluids of significantly different viscosities and work with a broad range of industrially relevant operating conditions, with Kapitza (Ka) and Reynolds (Re) numbers in the range of Ka = 500–10 000 and Re = 100–2 000, respectively. We find that the heat transfer rate in a pilot-scale unit can be improved by 50% to more than 100%, depending on geometrical features (e.g. sharpness) of the modified surface. Our results pave the way for increasing the overall efficiency of industrial evaporation plants by increasing the energy intensity of the evaporator per unit area through carefully tailored modifications of heat transfer surfaces
Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)
The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of
(Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect
(MOKE). A transient MOKE signal followed by low amplitude oscillations was
evidenced, with a strong dependence on applied magnetic field, temperature and
ps-ASP amplitude. Careful interferometric measurement of the layer's thickness
variation induced by the ps-ASP allowed us to model very accurately the
resulting signal, and interpret it as the strain modulated reflectivity
(differing for probe polarizations), independently from dynamic
magnetization effects.Comment: 6 pages, 5 figure
Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films
The phase transition between the intermediate and normal states in type-I
superconducting films is investigated using magneto-optical imaging. Magnetic
hysteresis with different transition fields for collapse and nucleation of
superconducting domains is found. This is accompanied by topological hysteresis
characterized by the collapse of circular domains and the appearance of
lamellar domains. Magnetic hysteresis is shown to arise from supercooled and
superheated states. Domain-shape instability resulting from long-range magnetic
interaction accounts well for topological hysteresis. Connection with similar
effects in systems with long-range magnetic interactions is emphasized
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
Scale invariant correlations and the distribution of prime numbers
Negative correlations in the distribution of prime numbers are found to
display a scale invariance. This occurs in conjunction with a nonstationary
behavior. We compare the prime number series to a type of fractional Brownian
motion which incorporates both the scale invariance and the nonstationary
behavior. Interesting discrepancies remain. The scale invariance also appears
to imply the Riemann hypothesis and we study the use of the former as a test of
the latter.Comment: 13 pages, 8 figures, version to appear in J. Phys.
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