28 research outputs found
Electromagnon excitations in modulated multiferroics
The phenomenological theory of ferroelectricity in spiral magnets presented
in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe
consistently states with both uniform and modulated-in-space ferroelectric
polarizations. A key point in this description is the symmetric part of the
magnetoelectric coupling since, although being irrelevant for the uniform
component, it plays an essential role for the non-uniform part of the
polarization. We illustrate this importance in generic examples of modulated
magnetic systems: longitudinal and transverse spin-density wave states and
planar cycloidal phase. We show that even in the cases with no uniform
ferroelectricity induced, polarization correlation functions follow to the soft
magnetic behavior of the system due to the magnetoelectric effect. Our results
can be easily generalized for more complicated types of magnetic ordering, and
the applications may concern various natural and artificial systems in
condensed matter physics (e.g., magnon properties could be extracted from
dynamic dielectric response measurements).Comment: 5 page
Origin of magnetoelectric behavior in BiFeO
The magnetoelectric behavior of BiFeO has been explored on the basis of
accurate density functional calculations. The structural, electronic, magnetic,
and ferroelectric properties of BiFeO are predicted correctly without
including strong correlation effect in the calculation. Moreover, the
experimentally-observed elongation of cubic perovskite-like lattice along the
[111] direction is correctly reproduced. At high pressure we predicted a
pressure-induced structural transition and the total energy calculations at
expanded lattice show two lower energy ferroelectric phases, closer in energy
to the ground state phase. Band-structure calculations show that BiFeO will
be an insulator in A- and G-type antiferromagnetic phases and a metal in other
magnetic configurations. Chemical bonding in BiFeO has been analyzed using
various tools and electron localization function analysis shows that
stereochemically active lone-pair electrons at the Bi sites are responsible for
displacements of the Bi atoms from the centro-symmetric to the
noncentrosymmetric structure and hence the ferroelectricity. A large
ferroelectric polarization (88.7 C/cm) is predicted in accordance
with recent experimental findings. The net polarization is found to mainly (
98%) originate from Bi atoms. Moreover the large scatter in experimentally
reported polarization values is due to the large anisotropy in the spontaneous
polarization.Comment: 19 pages, 12 figures, 4 table
High Temperature Emissivity, Reflectivity, and X-ray absorption of BiFeO3
We report on the lattice evolution of BiFeO3 as function of temperature using
far infrared emissivity, reflectivity, and X-ray absorption local structure. A
power law fit to the lowest frequency soft phonon in the magnetic ordered phase
yields an exponent {\beta}=0.25 as for a tricritical point. At about 200 K
below TN~640 K it ceases softening as consequence of BiFeO3 metastability. We
identified this temperature as corresponding to a crossover transition to an
order-disorder regime. Above ~700 K strong band overlapping, merging, and
smearing of modes are consequence of thermal fluctuations and chemical
disorder. Vibrational modes show band splits in the ferroelectric phase as
emerging from triple degenerated species as from a paraelectric cubic phase
above TC~1090 K. Temperature dependent X-ray absorption near edge structure
(XANES) at the Fe K-edge shows that lower temperature Fe3+ turns into Fe2+.
While this matches the FeO w\"ustite XANES profile, the Bi LIII-edge downshift
suggests a high temperature very complex bond configuration at the distorted A
perovskite site. Overall, our local structural measurements reveal high
temperature defect-induced irreversible lattice changes, below, and above the
ferroelectric transition, in an environment lacking of long-range coherence. We
did not find an insulator to metal transition prior to melting.Comment: Accepted for publicatio
Modulated charged defects and conduction behaviour in doped BiFeO 3 thin films
10.1088/0022-3727/42/16/162001Journal of Physics D: Applied Physics4216-JPAP