708 research outputs found
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Electric circuit networks equivalent to chaotic quantum billiards
We formulate two types of electric RLC resonance network equivalent to
quantum billiards. In the network of inductors grounded by capacitors squared
resonant frequencies are eigenvalues of the quantum billiard. In the network of
capacitors grounded by inductors squared resonant frequencies are given by
inverse eigen values of the billiard. In both cases local voltages play role of
the wave function of the quantum billiard. However as different from quantum
billiards there is a heat power because of resistance of the inductors. In the
equivalent chaotic billiards we derive the distribution of the heat power which
well describes numerical statistics.Comment: 9 pages, 7 figure
Magnetoresistance Anomalies in (Ga,Mn)As Epilayers with Perpendicular Magnetic Anisotropy
We report the observation of anomalies in the longitudinal magnetoresistance
of tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy.
Magnetoresistance measurements carried out in the planar geometry (magnetic
field parallel to the current density) reveal "spikes" that are antisymmetric
with respect to the direction of the magnetic field. These anomalies always
occur during magnetization reversal, as indicated by a simultaneous change in
sign of the anomalous Hall effect. The data suggest that the antisymmetric
anomalies originate in anomalous Hall effect contributions to the longitudinal
resistance when domain walls are located between the voltage probes. This
interpretation is reinforced by carrying out angular sweeps of ,
revealing an antisymmetric dependence on the helicity of the field sweep.Comment: Submitted to Phys. Rev.
Computer modeling of coupled electromagnetic, temperature and magnetohydrodynamic fields in the induction heating and melting devices
Computer modeling is necessary part of design new induction heating and melting devices [1]. One of the complicated technologies when it is necessary to simulate coupled electromagnetic, temperature and magnetohydrodynamic fields is heating and melting of titanium alloys in the alternating electromagnetic field. Thermal processing of titanium alloys in the inductor has some features that it is necessary to take into account on the designing of the advanced technology and equipment. Low thermal conductivity and high temperature losses at the surface result in maximum temperature inside of the billet that could under appropriate conditions exceed melting point. In this way it is possible to obtain liquid phase of titanium alloy inside of the billet and protect it from the contact with surrounding atmosphere. To get this it is necessary to choose the right regime of processing, frequency of current, power and thermal conditions. At the same time precise heating with very strong execution of the temperature profile during the heating time are essential for thermal processing of titanium alloys in this technology [2]. Mathematical model comprising computation of electromagnetic, temperature, MHD fields after getting melt zone and dynamic of its growth was developed. The calculation of the melting process has been carried out by the method âenthalpy-porosityâ with application of models of turbulent currents k-Ï SST in a non-static setting. Electromagnetic forces and heat sources have been defined by solving a harmonic task by the method of finite elements on a vector magnetic potential in the system âinductor â loadâ for each iteration of the hydrodynamic task. Experiments confirmed need in simulation of MHD fields to receive good coincidence. Using of the developed models for simulation of electromagnetic processing billets make it easy to develop and implement optimal heat processing systems for the crucibleless induction melting of titanium alloys.The calculations on the basis of the model and the analysis of physical processes with non-crucible melting of titanium alloys have also been carried out
Geometrical edge barriers and magnetization in superconducting strips with slits
We theoretically investigate the magnetic-field and current distributions for
coplanar superconducting strips with slits in an applied magnetic field H_a. We
consider ideal strips with no bulk pinning and calculate the hysteretic
behavior of the magnetic moment m_y as a function of H_a due solely to
geometrical edge barriers. We find that the m_y-H_a curves are strongly
affected by the slits. In an ascending field, the m_y-H_a curves exhibit kink
or peak structures, because the slits prevent penetration of magnetic flux. In
a descending field, m_y becomes positive, because magnetic flux is trapped in
the slits, in contrast to the behavior of a single strip without slits, for
which m_y =0.Comment: 11 pages, 5 figures, revtex
Origin of Low-Energy Excitations in Charge-Ordered Manganites
The low-energy excitations in the charge-ordered phase of polycrystalline
La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In
the frequency range from 4 cm^-1 to 700 cm^-1 (energies 0.4 meV to 90 meV) and
at temperatures down to 5 K, we do not detect any feature that can be
associated with the collective response of the spatially modulated charge
continuum. In the antiferromagnetically ordered phase, broad absorption bands
appear in the conductivity and permittivity spectra around 30 cm^-1 and 100
cm^-1 which are assigned to former acoustic phonons optically activated due to
a fourfold superstructure in the crystal lattice. Our results indicate that
characteristic energies of collective excitations of the charge-ordered phase
in La0:25Ca0:75MnO3, if any, lie below 1 meV. At our lowest frequencies of only
few wavenumbers a strong relaxation is observed above 100 K connected to the
formation of the charge-ordered state.Comment: 5 pages, 3 figure
Magnetic-field dependence of the critical currents in a periodic coplanar array of narrow superconducting strip
We calculate the magnetic-field dependence of the critical current due to
both geometrical edge barriers and bulk pinning in a periodic coplanar array of
narrow superconducting strips. We find that in zero or low applied magnetic
fields the critical current can be considerably enhanced by the edge barriers,
but in modest applied magnetic fields the critical current reduces to that due
to bulk pinning alone.Comment: 23 pages, 7 figure
Exchange Biasing of the Ferromagnetic Semiconductor Ga1-xMnxAs
We demonstrate the exchange coupling of a ferromagnetic semiconductor
(Ga1-xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional
exchange biased systems, the blocking temperature of the antiferromagnet (T_B =
48 +- 2 K) and the Curie temperature of the ferromagnet (T_C = 55.1 +- 0.2 K)
are comparable. The resulting exchange bias manifests itself as a clear shift
in the magnetization hysteresis loop when the bilayer is cooled in the presence
of an applied magnetic field and an enhancement of the coercive field.Comment: pdf file only; submitted to Applied Physics Letter
Strong interconversion of non-polar phonons and Josephson plasma oscillations induced by equilibrium Josephson currents in high T_c superconductors
We analyze consequences of dynamical modulations of Josephson current by
non-polar lattice mode in the Josephson junction barrier. In the high
junctions, the effect of such modulations can be anomalously strong due to the
proximity of the insulating barrier to the superconducting state. Accordingly,
the interconversion of sound (as well as other non-polar phonons) and the
Josephson plasma oscillations mediated by stationary Josephson currents, which
may be present in the junction due to various reasons, becomes possible. We
suggest that this effect can be employed for imaging of the stationary
Josephson currents. Estimates of the effect are given.Comment: 11 RevTeX pages, no figure
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