193 research outputs found
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
Interplay between strain, defect charge state and functionality in complex oxides
We use first-principles calculations to investigate the interplay between
strain and the charge state of point defect impurities in complex oxides. Our
work is motivated by recent interest in using defects as active elements to
provide novel functionality in coherent epitaxial films. Using oxygen vacancies
as model point defects, and CaMnO and MnO as model materials, we calculate
the changes in internal strain caused by changing the charge state of the
vacancies, and conversely the effect of strain on charge-state stability. Our
results show that the charge state is a degree of freedom that can be used to
control the interaction of defects with strain and hence the concentration and
location of defects in epitaxial films. We propose the use of field-effect
gating to reversibly change the charge state of defects and hence the internal
strain and corresponding strain-induced functionalities.Comment: 4 pages, 4 figure
High-temperature multiferroicity and strong magnetocrystalline anisotropy in 3d-5d double perovskites
Using density functional calculations we explore the properties of
as-yet-unsynthesized ordered double perovskites (O) with
highly polarizable Bi ions on the site. We find that the
BiNiReO and BiMnReO compounds are insulating and exhibit a
robust net magnetization that persists above room temperature. When the
in-plane lattice vectors of the pseudocubic unit cell are constrained to be
orthogonal (for example, by coherent heteroepitaxy), the ground states are
ferroelectric with large polarization and a very large uniaxial
magnetocrystalline anisotropy with easy axis along the ferroelectric
polarization direction. Our results suggest a route to multiferroism and
electrically controlled magnetization orientation at room temperature
Self-interaction correction with Wannier functions
We describe the behavior of the Perdew-Zunger self-interaction-corrected
local density approximation (SIC-LDA) functional when implemented in a
plane-wave pseudopotential formalism with Wannier functions. Prototypical
semiconductors and wide-bandgap oxides show a large overcorrection of the LDA
bandgap. Application to transition-metal oxides and elements with d electrons
is hindered by a serious breaking of the spherical symmetry, which appears even
in a closed shell free atom. Our results indicate that, when all spherical
approximations are lifted, the general applicability of orbital-dependent
potentials is very limited and should be reconsidered in favor of rotationally
invariant functionals.Comment: 10 pages, 5 figure
On the relationship between topological and geometric defects
The study of topology in solids is undergoing a renaissance following renewed
interest in the properties of ferroic domain walls as well as recent
discoveries regarding topological insulators and skyrmionic lattices. Each of
these systems possess a property that is `protected' in a symmetry sense, and
is defined rigorously using a branch of mathematics known as topology. In this
article we review the formal definition of topological defects as they are
classified in terms of homotopy theory, and discuss the precise
symmetry-breaking conditions that lead to their formation. We distinguish
topological defects from geometric defects, which arise from the details of the
stacking or structure of the material but are not protected by symmetry. We
provide simple material examples of both topological and geometric defect
types, and discuss the implications of the classification on the resulting
material properties
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