116 research outputs found
Toroidal moments as indicator for magneto-electric coupling: the case of BiFeO_3 versus FeTiO_3
In this paper we present an analysis of the magnetic toroidal moment and its
relation to the various structural modes in R3c-distorted perovskites with
magnetic cations on either the perovskite A or B site. We evaluate the toroidal
moment in the limit of localized magnetic moments and show that the full
magnetic symmetry can be taken into account by considering small induced
magnetic moments on the oxygen sites. Our results give a transparent picture of
the possible coupling between magnetization, electric polarization, and
toroidal moment, thereby highlighting the different roles played by the various
structural distortions in multiferroic BiFeO_3 and in the recently discussed
isostructural material FeTiO_3, which has been predicted to exhibit electric
field-induced magnetization switching.Comment: 6 pages, 2 figure
First-Principles-Based Strain and Temperature Dependent Ferroic Phase Diagram of SrMnO
Perovskite structure SrMnO is a rare example of a multiferroic material
where strain-tuning and/or cation substitution could lead to coinciding
magnetic and ferroelectric ordering temperatures, which would then promise
strong magnetoelectric coupling effects. Here, we establish the temperature and
strain dependent ferroic phase diagram of SrMnO using
first-principles-based effective Hamiltonians. All parameters of these
Hamiltonians are calculated using density functional theory, i.e., no fitting
to experimental data is required. Temperature dependent properties are then
obtained from Monte Carlo and molecular dynamics simulations. We observe a
sequence of several magnetic transitions under increasing tensile strain, with
a moderate variation of the corresponding critical temperatures. In contrast,
the ferroelectric Curie temperature increases strongly after its onset around
2.5\,\% strain, and indeed crosses the magnetic transition temperature just
above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling,
manifested in dramatic changes of the magnetic ordering temperatures and
different magnetic ground states as function of the ferroelectric distortion.
In addition, coexisting ferroelectric and ferromagnetic order is obtained for
strains above 4\,\%. Our calculated phase diagram suggests the possibility to
control the magnetic properties of SrMnO through an applied electric field,
significantly altering the magnetic transition temperatures, or even inducing
transitions between different magnetic states.Comment: 13 pages, 8 figure
Interplay between breathing mode distortion and magnetic order in rare-earth nickelates NiO within DFT+
We present a systematic density functional theory (DFT) plus Hubbard
study of structural trends and the stability of different magnetically ordered
states across the rare-earth nickelate series, NiO, with from Lu to
La. In particular, we investigate how the magnetic order, the change of the
rare-earth ion, and the Hubbard interaction are affecting the bond-length
disproportionation between the nickel sites. Our results show that structural
parameters can be obtained that are in very good agreement with present
experimental data, and that DFT+ is in principle able to capture the most
important structural trends across the nickelate series. However, the amplitude
of the bond-length disproportionation depends very strongly on the specific
value used for the Hubbard parameter and also on the type of magnetic order
imposed in the calculation. Regarding the relative stability of different
magnetic orderings, a realistic antiferromagnetic order, consistent with the
experimental observations, is favored for small values, and becomes more
and more favorable compared to the ferromagnetic state towards the end of the
series (i.e., towards =Pr). Nevertheless, it seems that the stability of the
ferromagnetic state is generally overestimated within the DFT+ calculations.
Our work provides a profound starting point for more detailed experimental
investigations, and also for future studies using more advanced computational
techniques such as, e.g., DFT combined with dynamical mean-field theory.Comment: 13 pages, 11 figures, 1 tabl
Effect of Hubbard U on the construction of low energy Hamiltonians for LaMnO_3 via maximally localized Wannier functions
We use maximally localized Wannier functions to construct tight-binding (TB)
parameterizations for the e_g bands of LaMnO_3 based on first principles
electronic structure calculations. We compare two different ways to represent
the relevant bands around the Fermi level: i) a d-p model that includes
atomic-like orbitals corresponding to both Mn(d) and O(p) states in the TB
basis, and ii) an effective e_g model that includes only two e_g-like Wannier
functions per Mn site. We first establish the effect of the Jahn-Teller
distortion within the d-p model, and then compare the TB representations for
both models obtained from GGA+U calculations with different values of the
Hubbard parameter U. We find that in the case of the d-p model the TB
parameters are rather independent on the specific value of U, if compared with
the mean-field approximation of an appropriate multi-band Hubbard Hamiltonian.
In contrast, the U dependence of the TB parameters for the effective e_g model
cannot easily be related to a corresponding mean-field Hubbard model, and
therefore these parameters depend critically on the specific value of U, and
more generally on the specific exchange-correlation functional, used in the
electronic structure calculation.Comment: 11 pages, 11 figures, 2 table
BaNiF4: an electric field-switchable weak antiferromagnet
We show that in the antiferromagnetic ferroelectric BaNiF4 the
Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic
moments, away from the easy axis, resulting in a noncollinear magnetic
structure. The canting corresponds to an additional "weak" antiferromagnetic
order parameter whose orientation is determined by the polar structural
distortion and can be reversed by switching the ferroelectric polarization with
an electric field. Our results point the way to a more general coupling
mechanism between structural distortions and magnetic order parameters in
magnetoelectric multiferroics which can be exploited in the design of electric
field-switchable magnets.Comment: 4 pages, 4 figure
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