46 research outputs found
Electric-field control of magnetic ordering in the tetragonal BiFeO3
We propose a way to use electric-field to control the magnetic ordering of
the tetragonal BiFeO3. Based on systematic first-principles studies of the
epitaxial strain effect on the ferroelectric and magnetic properties of the
tetragonal BiFeO3, we find that there exists a transition from C-type to G-type
antiferromagnetic (AFM) phase at in-plane constant a ~ 3.905 {\AA} when the
ferroelectric polarization is along [001] direction. Such magnetic phase
transition can be explained by the competition between the Heisenberg exchange
constant J1c and J2c under the influence of biaxial strain. Interestingly, when
the in-plane lattice constant enlarges, the preferred ferroelectric
polarization tends to be canted and eventually lies in the plane (along [110]
direction). It is found that the orientation change of ferroelectric
polarization, which can be realized by applying external electric-field, has
significant impact on the Heisenberg exchange parameters and therefore the
magnetic orderings of tetragonal BiFeO3. For example, at a ~ 3.79 {\AA}, an
electric field along [111] direction with magnitude of 2 MV/cm could change the
magnetic ordering from C-AFM to G-AFM. As the magnetic ordering affects many
physical properties of the magnetic material, e.g. magnetoresistance, we expect
such strategy would provide a new avenue to the application of multiferroic
materials.Comment: 4 pages, 4 figure
Finite-size effects on the magnetoelectric response of field-driven ferroelectric/ferromagnetic chains
We study theoretically the coupled multiferroic dynamics of one-dimensional
ferroelectric/ferromagnet chains driven by harmonic magnetic and electric
fields as a function of the chain length. A linear magnetoelectric coupling is
dominated by the spin-polarized screening charge at the interface. We performed
Monte-Carlo simulations and calculations based on the coupled
Landau-Lifshitz-Gilbert and Landau-Khalatnikov equations showing that the net
magnetization and the total polarization of thin heterostructures, i.e. with up
to ten ferroelectric and ferromagnetic sites counted from the interface, can be
completely reversed by external electric and magnetic fields, respectively.
However, for larger system solely a certain magnetoelectrical control can be
achieved.Comment: J. Phys.: Conf. Series. (2011) (to be published
Temperature Driven Structural Phase Transition in Tetragonal-Like BiFeO3
Highly-strained BiFeO3 exhibits a "tetragonal-like, monoclinic" crystal
structure found only in epitaxial films (with an out-of-plane lattice parameter
exceeding the in-plane value by >20%). Previous work has shown that this phase
is properly described as a M monoclinic structure at room temperature
[with a (010) symmetry plane, which contains the ferroelectric
polarization]. Here we show detailed temperature-dependent x-ray diffraction
data that evidence a structural phase transition at ~100C to a high-temperature
M phase ["tetragonal-like" but with a (1-10) symmetry plane].
These results indicate that the ferroelectric properties and domain structures
of strained BiFeO will be strongly temperature dependent.Comment: 10 pages, 3 figure
Piezoresponse Force Spectroscopy of Ferroelectric Materials
Piezoresponse Force Spectroscopy (PFS) has emerged as a powerful technique
for probing highly localized switching behavior and the role of microstructure
and defects on switching. The application of a dc bias to a scanning probe
microscope tip in contact with a ferroelectric surface results in the
nucleation and growth of a ferroelectric domain below the tip, resulting in
changes in local electromechanical response. Resulting hysteresis loops
contains information on local ferroelectric switching behavior. The signal in
PFS is the convolution of the volume of the nascent domain and the probing
volume of the tip. Here, we analyze the signal formation mechanism in PFS by
deriving the main parameters of domain nucleation in a semi-infinite material
and establishing the relationships between domain parameters and PFM signal
using a linear Greens function theory. The effect of surface screening and
finite Debye length on the switching behavior is established. In particular, we
predict that the critical nucleus size in PFM is controlled by the surface
screening mechanism and in the absence of screening, tip-induced switching is
impossible. Future prospects of PFS to study domain nucleation in the vicinity
of defects, local switching centers in ferroelectrics, and unusual polarization
states in low-dimensional ferroelectrics are discussed.Comment: 74 pages, 18 figures, 3 appendices, sent to Phys. Rev.
Phase transition close to room temperature in BiFeO3 thin films
BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be
mapped by appropriately substrate-induced strain in epitaxial films. By using
Raman spectroscopy, we conclusively show that films of the so-called
supertetragonal T-BFO phase, stabilized under compressive strain, displays a
reversible temperature-induced phase transition at about 100\circ, thus close
to room temperature.Comment: accepted in J. Phys.: Condens. Matter (Fast Track Communication
Test of the Kolmogorov-Johnson-Mehl-Avrami picture of metastable decay in a model with microscopic dynamics
The Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the time evolution of
the order parameter in systems undergoing first-order phase transformations has
been extended by Sekimoto to the level of two-point correlation functions.
Here, this extended KJMA theory is applied to a kinetic Ising lattice-gas
model, in which the elementary kinetic processes act on microscopic length and
time scales. The theoretical framework is used to analyze data from extensive
Monte Carlo simulations. The theory is inherently a mesoscopic continuum
picture, and in principle it requires a large separation between the
microscopic scales and the mesoscopic scales characteristic of the evolving
two-phase structure. Nevertheless, we find excellent quantitative agreement
with the simulations in a large parameter regime, extending remarkably far
towards strong fields (large supersaturations) and correspondingly small
nucleation barriers. The original KJMA theory permits direct measurement of the
order parameter in the metastable phase, and using the extension to correlation
functions one can also perform separate measurements of the nucleation rate and
the average velocity of the convoluted interface between the metastable and
stable phase regions. The values obtained for all three quantities are verified
by other theoretical and computational methods. As these quantities are often
difficult to measure directly during a process of phase transformation, data
analysis using the extended KJMA theory may provide a useful experimental
alternative.Comment: RevTex, 21 pages including 14 ps figures. Submitted to Phys. Rev. B.
One misprint corrected in Eq.(C1