294 research outputs found
Coupled Spin-Phonon Excitations in Helical Multiferroics
Both the Dzyaloshiskii-Moriya interaction and the exchange-striction are
shown to affect dynamically the magnetoelectric excitations in the perovskite
multiferroic RMnO3. The exchange-striction results in a biquadratic interaction
between the spins and the transverse phonons, giving rise to quantum
fluctuations of the ferroelectric polarization P. This leads to low-lying
phonon modes that are perpendicular to P and to the helical spins at small wave
vector but are parallel to P at a wave vector close to the magnetic modulation
vector. For spin-1/2 helimagnet, the local polarization can be completely
reversed by the spin fluctuation, and so does the direction of the on-site spin
chirality, which allows for a finite differential scattering intensity of
polarized neutrons from a cycloidal magnet.Comment: 7 page
Internal magnetic field effect on magnetoelectricity in orthorhombic crystals
We have investigated the role of the 4 moment on the magnetoelectric (ME)
effect of orthorhombic MnO (=rare earth ions). In order to clarify
the role of the 4 moment, we prepared three samples: (Eu,Y)MnO without
the 4 moment, TbMnO with the anisotropic 4 moment, and
(Gd,Y)MnO with the isotropic 4 moment. The ferroelectric behaviors of
these samples are different from each other in a zero magnetic field.
(Eu,Y)MnO and (Gd,Y)MnO show the ferroelectric polarization along
the a axis in the ground state, while TbMnO shows it along the c axis.
Such difference may arise from the influence of the anisotropic Tb 4
moment. The direction of the ferroelectric polarization of MnO is
determined by the internal magnetic field arising from the 4 moment.Comment: 2 pages, 1 figure, the proceeding of International Conference of
Magnetism, to be published in the Journal of Magnetism and Magnetic Material
Electric-dipole active two-magnon excitation in {\textit{ab}} spiral spin phase of a ferroelectric magnet GdTbMnO
A broad continuum-like spin excitation (1--10 meV) with a peak structure
around 2.4 meV has been observed in the ferroelectric spiral spin phase of
GdTbMnO by using terahertz (THz) time-domain spectroscopy.
Based on a complete set of light-polarization measurements, we identify the
spin excitation active for the light vector only along the a-axis, which
grows in intensity with lowering temperature even from above the magnetic
ordering temperature but disappears upon the transition to the -type
antiferromagnetic phase. Such an electric-dipole active spin excitation as
observed at THz frequencies can be ascribed to the two-magnon excitation in
terms of the unique polarization selection rule in a variety of the
magnetically ordered phases.Comment: 11 pages including 3 figure
Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites
Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and
Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic
model, and then analyze the model using the Monte-Carlo method. We reproduce
the diagrams, which contain two different multiferroic states, i.e., the
ab-plane spin cycloid with electric polarization P//a and the bc-plane spin
cycloid with P//c. We reveal that their competition originates from a conflict
between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction,
which is controlled by the second-neighbor spin exchanges enhanced by the
GdFeO3-type distortion. This leads to a P flop from a to c with increasing x in
agreement with the experiments.Comment: 5 pages, 5 figures. Recalculated results after correcting errors in
the assignment of DM vectors. The conclusion is not affecte
Spin chirality and electric polarization in multiferroic compounds MnO (Ho, Er)
Polarized neutron diffraction experiments have been performed on multiferroic
materials MnO (Ho, Er) under electric fields in the
ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM)
phases, where the former has the highest electric polarization and the latter
has reduced polarization. It is found that, after cooling in electric fields
down to the CM phase, the magnetic chirality is proportional to the electric
polarization. Also we confirmed that the magnetic chirality can be switched by
the polarity of the electric polarization in both the CM and LT-ICM phases.
These facts suggest an intimate coupling between the magnetic chirality and the
electric polarization. However, upon the transition from the CM to LT-ICM
phase, the reduction of the electric polarization is not accompanied by any
reduction of the magnetic chirality, implying that the CM and LT-ICM phases
contain different mechanisms of the magnetoelectric coupling.Comment: 4 pages, 2 figures. Proceedings of PNCMI2008/QuBS200
Roles of Bond Alternation in Magnetic Phase Diagram of RMnO3
In order to investigate nature of the antiferromagnetic structures in
perovskite RMnO3, we study a Heisenberg J1-J2 model with bond alternation using
analytical and numerical approaches. The magnetic phase diagram which includes
incommensurate spiral states and commensurate collinear states is reproduced.
We discuss that the magnetic structure with up-up-down-down spin configuration
(E-type structure) and the ferroelectricity emerge cooperatively to stabilize
this phase. Magnetoelastic couplings are crucial to understand the magnetic and
electric phase diagram of RMnO3.Comment: 5 pages, 6 figure
Flexomagnetoelectric effect in bismuth ferrite
There is a profound analogy between inhomogeneous magnetoelectric effect in
multiferroics and flexoelectric effect in liquid crystals. This similarity
gives rise to the flexomagnetoelectric polarization induced by spin modulation.
The theoretical estimations of flexomagnetoelectric polarization agree with the
value of jumps of polarization in magnetoelectric dependences (~20muC/m^2)
observed at spin cycloid suppression at critical magnetic field 200kOe.Comment: 6 pages,2 figure
CuBr2-A New Multiferroic Material with High Critical Temperature
A new multiferroic material, CuBr2, is reported for the first time. CuBr2 has
not only a high transition temperature (close to liquid nitrogen temperature)
but also low dielectric loss and strong magnetoelectric coupling. These
findings reveal the importance of anion effects in the search for the high
temperature multiferroics materials among these low-dimensional spin systems.Comment: 3 figures, accepted by Advanced Material
Anomalous Coexistence of Ferroelectric Phases ( and ) in Orthorhombic EuYMnO () Crystals
We have investigated the magnetic and dielectric properties of orthorhombic
EuYMnO () single crystals without the presence
of the 4 magnetic moments of the rare-earth ions. In , the
magnetic-structure driven ferroelectricity is observed. The ferroelectric
transition temperature is steeply reducing with increasing . In , two ferroelectric phases ( and ) are
coexistent at low temperatures. In these phases, ferroelectricity has different
origin, which is evidenced by the distinctive poling-electric-field dependence
of electric polarization. Namely, the electric polarization along the c axis
() is easily saturated by a poling electric field, therefore is
caused by the spiral antiferromagnetic order. On the other hand, the
electric polarization along the a axis () is probably attributed to the
collinear -type antiferromagnetic order, because is unsaturated even
in a poling field of V/m.Comment: 10 pages, 4figures, to be published in Journal of the Physical
Society of Japa
Theory of magnetoelectric resonance in two-dimensional antiferromagnet via spin-dependent metal-ligand hybridization mechanism
We investigate magnetic excitations in an Heisenberg model
representing two-dimensional antiferromagnet . In
terahertz absorption experiment of the compound, Goldstone mode as well as
novel magnetic excitations, conventional magnetic resonance at 2 meV and both
electric- and magnetic-active excitation at 4 meV, have been observed. By
introducing a hard uniaxial anisotropy term , three modes can
be explained naturally. We also indicate that, via the spin-dependent
metal-ligand hybridization mechanism, the 4 meV excitation is an
electric-active mode through the coupling between spin and electric-dipole.
Moreover, at 4 meV excitation, an interference between magnetic and electric
responses emerges as a cross correlated effect. Such cross correlation effects
explain the non-reciprocal linear directional dichroism observed in .Comment: 5 pages, 3 figure
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