143 research outputs found
Temperature-Dependent Magnetoelectric Effect from First Principles
We show that nonrelativistic exchange interactions and spin fluctuations can give rise to a linear magnetoelectric effect in collinear antiferromagnets at elevated temperatures that can exceed relativistic magnetoelectric responses by more than 1 order of magnitude. We show how symmetry arguments, ab initio methods, and Monte Carlo simulations can be combined to calculate temperature-dependent magnetoelectric susceptibilities entirely from first principles. The application of our method to Cr2O3 gives quantitative agreement with experiment.
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
Theoretical prediction of multiferroicity in double perovskite YNiMnO
We put forward double perovskites of the RNiMnO family (with a
rare-earth atom) as a new class of multiferroics on the basis of {\it ab
initio} density functional calculations. We show that changing from La to Y
drives the ground-state from ferromagnetic to antiferromagnetic with
spin patterns. This E-type ordering
breaks inversion symmetry and generates a ferroelectric polarization of few
. By analyzing a model Hamiltonian we understand the microscopic
origin of this transition and show that an external electric field can be used
to tune the transition, thus allowing electrical control of the magnetization.Comment: 4 pages, 3 figure
Relativistic nature of a magnetoelectric modulus of Cr_2O_3-crystals: a new 4-dimensional pseudoscalar and its measurement
Earlier, the magnetoelectric effect of chromium sesquioxide Cr_2O_3 has been
determined experimentally as a function of temperature. One measures the
electric field-induced magnetization on Cr_2O_3 crystals or the magnetic
field-induced polarization. From the magnetoelectric moduli of Cr_2O_3 we
extract a 4-dimensional relativistic invariant pseudoscalar
. It is temperature dependent and of the order of
10^{-4}/Z_0, with Z_0 as vacuum impedance. We show that the new pseudoscalar is
odd under parity transformation and odd under time inversion. Moreover,
is for Cr_2O_3 what Tellegen's gyrator is for two port
theory, the axion field for axion electrodynamics, and the PEMC (perfect
electromagnetic conductor) for electrical engineering.Comment: Revtex, 36 pages, 9 figures (submitted in low resolution, better
quality figures are available from the authors
Terahertz spectroscopy of electromagnons in Eu_{1-x}Y_xMnO_3
Dielectric permittivity spectra of yttrium-doped EuMnO in the composition
range 0 =< x =< 0.5 have been investigated in the terahertz frequency range.
Magnetoelectric contributions to the permittivity were observed in all
compositions for ac electric fields parallel to the crystallographic a-axis.
Well defined electromagnons exist for x >= 0.2 close to \nu ~ 20 cm^{-1} and
with dielectric strength strongly increasing on doping. In addition to
electromagnons, a broad contribution of magnetoelectric origin is observed for
all compositions. For Eu_{0.8}Y_{0.2}MnO_3 the electromagnons can be suppressed
by external magnetic fields which induce a canted antiferromagnetic phase.
Magnetoelectric effects in the different doping regimes are discussed in
detail.Comment: 7 pages, 9 figures include
Neel state of antiferromagnet as a result of a local measurement in the distributed quantum system
Single-site measurement in a distributed macroscopic antiferromagnet is
considered; we show that it can create antiferromagnetic sublattices at
macroscopic scale. We demonstrate that the result of measurement depends on the
symmetry of the ground state: for the easy-axis case the Neel state is formed,
while for the easy-plane case unusual ``fan'' sublattices appear with unbroken
rotational symmetry, and a decoherence wave is generated. For the latter case,
a macroscopically large number of measurements is needed to pin down the
orientation of the sublattices, in spite of the high degeneracy of the ground
state. We note that the type of the final state and the appearance of the
decoherence wave are governed by the degree of entanglement of spins in the
system.Comment: 4 REVTeX pages, 1 figure in PostScrip
Magnetic control of large room-temperature polarization
Numerous authors have referred to room-temperature magnetic switching of
large electric polarizations as The Holy Grail of magnetoelectricity.We report
this long-sought effect using a new physical process of coupling between
magnetic and ferroelectric relaxor nano-regions. Here we report magnetic
switching between the normal ferroelectric state and the ferroelectric relaxor
state. This gives both a new room-temperature, single-phase, multiferroic
magnetoelectric, PbZr0.46Ti0.34Fe0.13W0.07O3, with polarization, loss (<4%),
and resistivity (typically 108 -109 ohm.cm) equal to or superior to BiFeO3, and
also a new and very large magnetoelectric effect: switching not from +Pr to
negative Pr with applied H, but from Pr to zero with applied H of less than a
Tesla. This switching of the polarization occurs not because of a conventional
magnetically induced phase transition, but because of dynamic effects:
Increasing H lengthens the relaxation time by x500 from 100 ?s, and
it couples strongly the polarization relaxation and spin relaxations. The
diverging polarization relaxation time accurately fits a modified Vogel-Fulcher
Equation in which the freezing temperature Tf is replaced by a critical
freezing field Hf that is 0.92 positive/negative 0.07 Tesla. This field
dependence and the critical field Hc are derived analytically from the
spherical random bond random field (SRBRF) model with no adjustable parameters
and an E2H2 coupling. This device permits 3-state logic (+Pr,0,negative Pr) and
a condenser with >5000% magnetic field change in its capacitance.Comment: 20 pages, 5 figure
Superexchange-Driven Magnetoelectricity in Magnetic Vortices
We demonstrate that magnetic vortices in which spins are coupled to polar
lattice distortions via superexchange exhibit an unusually large linear
magnetoelectric response. We show that the periodic arrays of vortices formed
by frustrated spins on Kagom\'e lattices provide a realization of this concept;
our {\it ab initio} calculations for such a model structure yield a
magnetoelectric coefficient that is 30 times larger than that of prototypical
single phase magnetoelectrics. Finally, we identify the design rules required
to obtain such a response in a practical material.Comment: 5 pages, 6 figure
Transition from Townsend to glow discharge: subcritical, mixed or supercritical
The full parameter space of the transition from Townsend to glow discharge is
investigated numerically in one space dimension in the classical model: with
electrons and positive ions drifting in the local electric field, impact
ionization by electrons ( process), secondary electron emission from
the cathode ( process) and space charge effects. We also perform a
systematic analytical small current expansion about the Townsend limit up to
third order in the total current that fits our numerical data very well.
Depending on and system size pd, the transition from Townsend to glow
discharge can show the textbook subcritical behavior, but for smaller values of
pd, we also find supercritical or some intermediate ``mixed'' behavior. The
analysis in particular lays the basis for understanding the complex
spatio-temporal patterns in planar barrier discharge systems.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
Interface driven magnetoelectric effects in granular CrO2
Antiferromagnetic and magnetoelectric Cr2O3-surfaces strongly affect the
electronic properties in half metallic CrO2. We show the presence of a Cr2O3
surface layer on CrO3 grains by high-resolution transmission electron
microscopy. The effect of these surface layers is demonstrated by measurements
of the temperature variation of the magnetoelectric susceptibility. A major
observation is a sign change at about 100 K followed by a monotonic rise as a
function of temperature. These electric field induced moments in CrO3 are
correlated with the magnetoelectric susceptibility of pure Cr2O3. This study
indicates that it is important to take into account the magnetoelectric
character of thin surface layers of Cr2O3 in granular CrO2 for better
understanding the transport mechanism in this system. The observation of a
finite magnetoelectric susceptibility near room temperature may find utility in
device applications.Comment: Figure 1 with strongly reduced resolutio
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