23 research outputs found
The effect of transverse magnetic correlations on a coupled order parameter: shifted transition temperatures and thermal hysteresis
We use a Green's function method with Random Phase Approximation to show how
magnetic correlations may affect electric polarization in multiferroic
materials with magnetic-exchange-type magnetoelectric coupling. We use a model
spin 1/2 ferromagnetic ferroelectric system but our results are expected to
apply to multiferroic materials with more complex magnetic structures. In
particular, we find that transverse magnetic correlations result in a change in
the free energy of the ferroelectric solutions leading to the possibility for
thermal hysteresis of the electric polarization above the magnetic Curie
temperature. Although we are motivated by multiferroic materials, this problem
represents a more general calculation of the effect of fluctuations on coupled
order parameters
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.Comment: subm. to PR
Phenomenological Landau analysis of predicted magnetoelectric fluorides: KMnFeF and BaNiF
Recently, we predicted based on symmetry considerations that KMnFeF and
BaNiF are likely magnetoelectric multiferroic materials. In
this contribution, we investigate with Landau theory and crystal structure
considerations the polarization and the linear magnetoelectric effect in these
materials. Based on these two examples, we show that any magnetoferroelectric
will display additional electrical polarization below its magnetic ordering
temperature. This additional electrical polarization is not related to the
linear magnetoelectric effect. Its magnitude depends on the dielectric
susceptibility.Comment: 11 pages, accepted for publication in Journal of Physics: Condensed
Matte
Energy in Lossless and Low-loss Networks, and Foster\u27s Reactance Theorem
Energy stored in a lossless network driven by a voltage (current) source is related to a frequency derivative of the network susceptance (reactance). This relation is proven to be valid also for Low-loss networks at frequencies far from the network resonance frequencies. Close to the resonance frequency, the stored energy is expressed in terms of the network susceptance (reactance) and the bandwidth of the resonance. A simple and lucid proof of Foster\u27s Reactance theorem follows from this consideration. It is also proven that Foster\u27s theorem is a direct consequence of causality. © 1989 IEE
Arrival time inversion and traveltime function
© 1989 Society of Exploration Geophysicists. All rights reserved. The traveltime inversion techniques usually involve modeling the earth above the reflector. The paper discusses an alternative approach, vhich deals with the modeling of a traveltime function (TTF) and does not require the earth modeling. The migration equations are formulated in such a way that they relate the problem to the TTF. Since the TTF is needed only in the limited range of its variables, it may be modeled by a simple function with a few adjustable parameters. The deeper the reflector, the smaller the number of parameters required. Therefore, the method may be attractive for deep reflector data. Some restrictions applied to the modeled TTF are discussed