1,703 research outputs found
Magnetoelectric response of multiferroic BiFeO3 and related materials
We present a first-principles scheme for computing the magnetoelectric
response of multiferroics. We apply our method to BiFeO3 (BFO) and related
compounds in which Fe is substituted by other magnetic species. We show that
under certain relevant conditions -- i.e., in absence of incommensurate spin
modulation, as in BFO thin films and some BFO-based solid solutions -- these
materials display a large linear magnetoelectric response. Our calculations
reveal the atomistic origin of the coupling and allow us to identify the most
promising strategies to enhance it.Comment: 4 pages with 1 figure embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Negative high-frequency differential conductivity in semiconductor superlattices
We examine the high-frequency differential conductivity response properties
of semiconductor superlattices having various miniband dispersion laws. Our
analysis shows that the anharmonicity of Bloch oscillations (beyond
tight-binding approximation) leads to the occurrence of negative high-frequency
differential conductivity at frequency multiples of the Bloch frequency. This
effect can arise even in regions of positive static differential conductivity.
The influence of strong electron scattering by optic phonons is analyzed. We
propose an optimal superlattice miniband dispersion law to achieve
high-frequency field amplification
Superlattice with hot electron injection: an approach to a Bloch oscillator
A semiconductor superlattice with hot electron injection into the miniband is
considered. The injection changes the stationary distribution function and
results in a qualitative change of the frequency behaviour of the differential
conductivity. In the regime with Bloch oscillating electrons and injection into
the upper part of the miniband the region of negative differential conductivity
is shifted from low frequencies to higher frequencies. We find that the dc
differential conductivity can be made positive and thus the domain instability
can be suppressed. At the same time the high-frequency differential
conductivity is negative above the Bloch frequency. This opens a new way to
make a Bloch oscillator operating at THz frequencies.Comment: RevTeX, 8 pages, 2 figures, to be published in Phys. Rev. B, 15
Januar 200
Field dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy change
The field dependence of the magnetic entropy change can be expressed as SM Hn. For soft
magnetic amorphous alloys n=1 well below the Curie temperature TC , n=2 in the paramagnetic
range, and n 0.75 for T=TC. The first value can be explained with simple arguments, n=2 is a
consequence of the Curie-Weiss law, but n TC deviates from mean field predictions. From the
Arrott-Noakes equation of state, a relation between n TC and the critical exponents has been
obtained, showing remarkable agreement with experimental data for an example alloy, predicted
n=0.72 versus experimental n=0.73 . A master curve behavior for the temperature dependence of
SM measured for different maximum fields is proposed
Continuous Neel to Bloch Transition as Thickness Increases: Statics and Dynamics
We analyze the properties of Neel and Bloch domain walls as a function of
film thickness h, for systems where, in addition to exchange, the dipole-dipole
interaction must be included. The Neel to Bloch phase transition is found to be
a second order transition at hc, mediated by a single unstable mode that
corresponds to oscillatory motion of the domain wall center. A uniform
out-of-plane rf-field couples strongly to this critical mode only in the Neel
phase. An analytical Landau theory shows that the critical mode frequency
varies as the square root of (hc - h) just below the transition, as found
numerically.Comment: 4 pages, 4 figure
Magnetocaloric response of FeCrB amorphous alloys: Predicting the magnetic entropy change from the ArrottâNoakes equation of state.
The magnetic entropy change in Fe92âxCr8Bx x=12,15 amorphous alloys has been studied.
Increasing the B content, both the peak entropy change and the Curie temperature of the alloy
increase. This is in agreement with an increase in the average magnetic moment per iron atom. The
thermal and field dependences of the magnetic entropy change curves have been analyzed with the
use of the ArrottâNoakes equation of state. It is shown that determining the parameters in this
equation of state through fitting the magnetization data allows prediction of the field and
temperature dependences of the magnetic entropy change curves in a broad temperature range
around the Curie temperature
Dynamical Symmetry Breaking in Models with the Yukawa Interaction
We discuss models with a massless fermion and a self-interacting massive
scalar field with the Yukawa interaction. The chiral condensate and the fermion
mass are calculated analytically. It is shown that the models have a phase
transition as a function of the squared mass of the scalar field.Comment: 7 pages, no figures, in Eqs. (7) and (11) one coefficient was change
The magnetocaloric effect in materials with a second order phase transition: Are T C and T peak necessarily coincident?
Using the ArrottâNoakes equation of state it is shown that the Curie point TC and the temperature
where the magnetic entropy change is maximum Tpeak coincide only in the mean field
approximation, but the Heisenberg model implies that Tpeak TC even for homogeneous materials.
The distance between Tpeak and TC increases with applied magnetic field following a power law. In
both cases, TC corresponds to a singular point in the temperature dependence of the magnetic
entropy change. The field dependence of the magnetic entropy change is exactly the same at the
Curie temperature and at the temperature of the peak
Luminescent properties of Bi-doped polycrystalline KAlCl4
We observed an intensive near-infrared luminescence in Bi-doped KAlCl4
polycrystalline material. Luminescence dependence on the excitation wavelength
and temperature of the sample was studied. Our experimental results allow
asserting that the luminescence peaked near 1 um belongs solely to Bi+ ion
which isomorphically substitutes potassium in the crystal. It was also
demonstrated that Bi+ luminescence features strongly depend on the local ion
surroundings
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