390 research outputs found
Analysis of optical magnetoelectric effect in GaFeO_3
We study the optical absorption spectra in a polar ferrimagnet GaFeO_3. We
consider the E1, E2 and M1 processes on Fe atoms. It is shown that the
magnetoelectric effect on the absorption spectra arises from the E1-M1
interference process through the hybridization between the 4p and 3d states in
the noncentrosymmetry environment of Fe atoms. We perform a microscopic
calculation of the spectra on a cluster model of FeO_6 consisting of an
octahedron of O atoms and an Fe atom displaced from the center with reasonable
values for Coulomb interaction and hybridization. We obtain the magnetoelectric
spectra, which depend on the direction of magnetization, as a function of
photon energy in the optical region 1.0-2.5 eV, in agreement with the
experiment.Comment: 18 pages, 5 figure
Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5
Three independent contributions to the magnetically induced spontaneous
polarization of multiferroic TbMn2O5 are uniquely separated by optical second
harmonic generation and an analysis in terms of Landau theory. Two of them are
related to the magnetic Mn3+/4+ order and are independent of applied fields of
up to 7 T. The third contribution is related to the long-range
antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application
of a magnetic field and mediates the change of sign of the spontaneous electric
polarization in TbMn2O5. The close relationship between the rare-earth
long-range order and the non-linear optical properties points to isotropic
Tb-Tb exchange and oxygen spin polarization as mechanism for this rare-earth
induced ferroelectricity.Comment: 8 pages, 5 figure
Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets
The impulsive generation of two-magnon modes in antiferromagnets by
femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent
longitudinal oscillations of the antiferromagnetic order parameter that cannot
be described by a thermodynamic Landau-Lifshitz approach. We argue that this
dynamics is triggered as a result of a laser-induced modification of the
exchange interaction. In order to describe the oscillations we have formulated
a quantum mechanical description in terms of magnon pair operators and coherent
states. Such an approach allowed us to} derive an effective macroscopic
equation of motion for the temporal evolution of the antiferromagnetic order
parameter. An implication of the latter is that the photo-induced spin dynamics
represents a macroscopic entanglement of pairs of magnons with femtosecond
period and nanometer wavelength. By performing magneto-optical pump-probe
experiments with 10 femtosecond resolution in the cubic KNiF and the
uniaxial KNiF collinear Heisenberg antiferromagnets, we observed
coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The
detected frequencies as a function of the temperature ideally fit the
two-magnon excitation up to the N\'eel point. The experimental signals are
described as dynamics of magnetic linear dichroism due to longitudinal
oscillations of the antiferromagnetic vector.Comment: 25 pages, 10 figure
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