1,101 research outputs found
Low-Prandtl-number B\'enard-Marangoni convection in a vertical magnetic field
The effect of a homogeneous magnetic field on surface-tension-driven
B\'{e}nard convection is studied by means of direct numerical simulations. The
flow is computed in a rectangular domain with periodic horizontal boundary
conditions and the free-slip condition on the bottom wall using a
pseudospectral Fourier-Chebyshev discretization. Deformations of the free
surface are neglected. Two- and three-dimensional flows are computed for either
vanishing or small Prandtl number, which are typical of liquid metals. The main
focus of the paper is on a qualitative comparison of the flow states with the
non-magnetic case, and on the effects associated with the possible
near-cancellation of the nonlinear and pressure terms in the momentum equations
for two-dimensional rolls. In the three-dimensional case, the transition from a
stationary hexagonal pattern at the onset of convection to three-dimensional
time-dependent convection is explored by a series of simulations at zero
Prandtl number.Comment: 26 pages, 9 figure
Highly efficient room temperature spin injection in a metal-insulator-semiconductor light emitting diode
We demonstrate highly efficient spin injection at low and room temperature in
an AlGaAs/GaAs semiconductor heterostructure from a CoFe/AlOx tunnel spin
injector. We use a double-step oxide deposition for the fabrication of a
pinhole-free AlOx tunnel barrier. The measurements of the circular polarization
of the electroluminescence in the Oblique Hanle Effect geometry reveal injected
spin polarizations of at least 24% at 80K and 12% at room temperature
Electrical Spin Injection in a Ferromagnetic / Tunnel Barrier/ Semiconductor Heterostructure
We demonstrate experimentally the electrical ballistic electron spin
injection from a ferromagnetic metal / tunnel barrier contact into a
semiconductor III-V heterostructure. We introduce the Oblique Hanle Effect
technique for reliable optical measurement of the degree of injected spin
polarization. In a CoFe / Al2O3 / GaAs / (Al,Ga)As heterostructure we observed
injected spin polarization in excess of 8 % at 80K.Comment: 5 pages, 4 figure
Non-Volatile Memory Characteristics of Submicrometre Hall Structures Fabricated in Epitaxial Ferromagnetic MnAl Films on GaAs
Hall-effect structures with submicrometre linewidths (<0.3pm) have been fabricated in ferromagnetic thin films of Mn[sub 0.60]Al[sub 0.40] which are epitaxially grown on a GaAs substrate. The MnAl thin films exhibit a perpendicular remanent magnetisation and an extraordinary Hall effect with square hysteretic behaviour. The presence of two distinct stable readout states demonstrates the potential of using ultrasmall ferromagnetic volumes for electrically addressable, nonvolatile storage of digital information
The extraordinary Hall effect in coherent epitaxial tau (Mn,Ni)Al thin films on GaAs
Ultrathin coherent epitaxial films of ferromagnetic tau(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal tau unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3-7.1 muOMEGA-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and -7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, tau(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information
Epitaxial-tau(Mn,Ni)Al/(Al,Ga)As heterostructures: Magnetic and magneto-optic properties
Ferromagnetic Perpendicularly magnetized epitaxial thin films of tau (Mn,Ni)AI have been successfully grown on AlAs/GaAs heterostructures by molecular beam epitaxy. We have investigated the polar Kerr rotation and magnetization of tau MnAl and (Mn,Ni) Al as a function of Mn and Ni concentration. The largest polar Kerr rotation and remnant magnetization were obtained for Mn0.5Al0.5 thin films with values of 0.16-degrees and 224 emu/cm3, respectively. We observed that the Kerr rotation and magnetization remained constant with Ni additions up to about 12 at. % and subsequently decreased with further Ni additions. We discuss these results and one possible method of enhancing the Kerr rotation
Asymptotic analysis of a secondary bifurcation of the one-dimensional Ginzburg-Landau equations of superconductivity
The bifurcation of asymmetric superconducting solutions from the normal solution is considered for the one-dimensional Ginzburg--Landau equations by the methods of formal asymptotics. The behavior of the bifurcating branch depends on the parameters d, the size of the superconducting slab, and , the Ginzburg--Landau parameter. The secondary bifurcation in which the asymmetric solution branches reconnect with the symmetric solution branch is studied for values of for which it is close to the primary bifurcation from the normal state. These values of form a curve in the -plane, which is determined. At one point on this curve, called the quintuple point, the primary bifurcations switch from being subcritical to supercritical, requiring a separate analysis. The results answer some of the conjectures of [A. Aftalion and W. C. Troy, Phys. D, 132 (1999), pp. 214--232]
Structural and magnetic properties of the (001) and (111) surfaces of the half-metal NiMnSb
Using the full potential linearised augmented planewave method we study the
electronic and magnetic properties of the (001) and (111) surfaces of the
half-metallic Heusler alloy NiMnSb from first-principles. We take into account
all possible surface terminations including relaxations of these surfaces.
Special attention is paid to the spin-polarization at the Fermi level which
governs the spin-injection from such a metal into a semiconductor. In general,
these surfaces lose the half-metallic character of the bulk NiMnSb, but for the
(111) surfaces this loss is more pronounced. Although structural optimization
does not change these features qualitatively, specifically for the (111)
surfaces relaxations can compensate much of the spin-polarization at the Fermi
surface that has been lost upon formation of the surface.Comment: 18 pages, 8 figure
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