430 research outputs found
A high-temperature expansion method for calculating paramagnetic exchange interactions
The method for calculating the isotropic exchange interactions in the
paramagnetic phase is proposed. It is based on the mapping of the
high-temperature expansion of the spin-spin correlation function calculated for
the Heisenberg model onto Hubbard Hamiltonian one. The resulting expression for
the exchange interaction has a compact and transparent formulation. The quality
of the calculated exchange interactions is estimated by comparing the
eigenvalue spectra of the Heisenberg model and low-energy magnetic part of the
Hubbard model. By the example of quantum rings with different hopping setups we
analyze the contributions from the different part of the Hubbard model spectrum
to the resulting exchange interaction.Comment: 8 pages, 8 figure
Magnetism of sodium superoxide
By combining first-principles electronic-structure calculations with the
model Hamiltonian approach, we systematically study the magnetic properties of
sodium superoxide (NaO2), originating from interacting superoxide molecules. We
show that NaO2 exhibits a rich variety of magnetic properties, which are
controlled by relative alignment of the superoxide molecules as well as the
state of partially filled antibonding molecular \pi_g-orbitals. The orbital
degeneracy and disorder in the high-temperature pyrite phase gives rise to weak
isotropic antiferromagnetic (AFM) interactions between the molecules. The
transition to the low-temperature marcasite phase lifts the degeneracy, leading
to the orbital order and formation of the quasi-one-dimensional AFM spin
chains. Both tendencies are consistent with the behavior of experimental
magnetic susceptibility data. Furthermore, we evaluate the magnetic transition
temperature and type of the long-range magnetic order in the marcasite phase.
We argue that this magnetic order depends on the behavior of weak isotropic as
well as anisotropic and Dzyaloshinskii-Moriya exchange interactions between the
molecules. Finally, we predict the existence of a multiferroic phase, where the
inversion symmetry is broken by the long-range magnetic order, giving rise to
substantial ferroelectric polarization.Comment: 10 pages, 7 figure
Magnetic structure and ferroelectric activity in orthorhombic YMnO3: relative roles of magnetic symmetry breaking and atomic displacements
We discuss relative roles played by the magnetic inversion symmetry breaking
and the ferroelectric (FE) atomic displacements in the multiferroic state of
YMnO3. For these purposes we derive a realistic low-energy model, using results
of first-principles calculations and experimental parameters of the crystal
structure. Then, we solve this model in the Hartree-Fock approximation. We
argue that the multiferroic state in YMnO3 has a magnetic origin, and the
centrosymmetric Pbnm structure is formally sufficient for explaining details of
the noncentrosymmetric magnetic ground state. The relativistic spin-orbit
interaction lifts the degeneracy, caused by the frustration of isotropic
interactions, and stabilizes a twofold periodic magnetic state, which is
similar to the E-state apart from the spin canting. The noncentrosymmetric
atomic displacements in the P2_1nm phase reduce the spin canting, but do not
change the symmetry of the magnetic state. The effect of the P2_1nm distortion
on the FE polarization P_a is twofold: (i) it gives rise to ionic
contributions, associated with the Y and O sites; (ii) it affects the
electronic polarization, through the change of the spin canting. The relatively
small value of P_a, observed in the experiment, is caused by a partial
cancelation of the electronic and ionic contributions in the experimental
P2_1nm structure. Finally, we theoretically optimize the crystal structure, by
using the LSDA+U approach and assuming the collinear E-type alignment. We have
found that the agreement with the experimental data in this case is less
satisfactory and P_a is largely overestimated. Although the magnetic structure
can be formally tuned by varying the Coulomb repulsion U as a parameter,
apparently LSDA+U fails to reproduce some fine details of the experimental
structure, and the cancelation of different contributions in P_a does not
occur.Comment: 24 pages, 5 figures, 5 table
Magnetic structure of hexagonal YMnO3 and LuMnO3 from a microscopic point of view
The aim of this work is to unravel a basic microscopic picture behind complex
magnetic properties of hexagonal manganites. For these purposes, we consider
two characteristic compounds: YMnO3 and LuMnO3, which form different magnetic
structures in the ground state. First, we establish an electronic low-energy
model, which describes the behavior of the Mn 3d bands of YMnO3 and LuMnO3, and
derive parameters of this model from the first-principles calculations. From
the solution of this model, we conclude that, despite strong frustration
effects in the hexagonal lattice, the relativistic spin-orbit interactions lift
the degeneracy of the magnetic ground state so that the experimentally observed
magnetic structures are successfully reproduced by the low-energy model. Then,
we analyze this result in terms of interatomic magnetic interactions, which
were computed using different approximations (starting from the model
Hamiltonian as well as directly from the first-principles electronic structure
calculations in the local-spin-density approximation). We argue that the main
reason why YMnO3 and LuMnO3 tend to form different magnetic structures is
related to the behavior of the single-ion anisotropy, which reflects the
directional dependence of the lattice distortion: namely, the expansion and
contraction of the Mn-trimers, which take place in YMnO3 and LuMnO3,
respectively. On the other hand, the magnetic coupling between the haxagonal
planes is controlled by the next-nearest-neighbor interactions, which are less
sensitive to the direction of the trimerization. Finally, using the Berry-phase
formalism, we evaluate the magnetic-state dependence of the ferroelectric
polarization, and discuss potential applications of the latter in
magnetoelectric switching phenomena.Comment: 22 pages, 2 figures, 4 table
Electronic Structure and Exchange Interactions of NaVO
We have performed first-principle calculations of the electronic structure
and exchange couplings for the nanotube compound NaVO using
the LDA+U approach. Our results show that while the intra-ring exchange
interactions are mainly antiferromagnetic, the inter-ring couplings are {\it
ferromagnetic}. We argue that this is a consequence of the strong hybridization
between filled and vacant 3d vanadium orbitals due to the low symmetry of
NaVO, which results into strong - and often dominant -
ferromagnetic contributions to the total exchange interaction between vanadium
atoms. A comparison with results of previous works is included.Comment: 6 pages, 5 figure
Validity and limitations of the superexchange model for the magnetic properties of Sr2IrO4 and Ba2IrO4 mediated by the strong spin-orbit coupling
Layered perovskites Sr2IrO4 and Ba2IrO4 are regarded as the key materials for
understanding the properties of magnetic relativistic insulators, mediated by
the strong spin-orbit (SO) coupling. One of the most fundamental issues is to
which extent these properties can be described by the superexchange (SE) model,
formulated in the limit of the large Coulomb repulsion. In the present work we
address this issue by deriving the relevant models and extracting parameters of
these models from the first-principles calculations. First, we construct the
effective Hubbard-type model for the t2g bands, by recasting the problem in the
language of Wannier orbitals. Then, we map the obtained electron model onto the
pseudospin model by applying the theory of SE interactions. We discuss the
microscopic origin of anisotropic SE interactions, inherent to the compass
Heisenberg model, and the appearance of the antisymmetric Dzyaloshinskii-Moriya
term, associated with the additional rotation of the IrO6 octahedra in Sr2IrO4.
In order to evaluate the Neel temperature (TN), we employ the non-linear sigma
model. While for Sr2IrO4 our value of TN agrees with the experimental one, for
Ba2IrO4 it is overestimated by a factor two. We argue that this discrepancy is
related to limitations of the SE model: while for more localized t2g states in
Sr2IrO4 it works reasonably well, the higher-order terms, beyond the SE model,
play a more important role in the more "itinerant" Ba2IrO4, giving rise to the
new type of isotropic and anisotropic exchange interactions. This conclusion is
supported by unrestricted Hartree-Fock calculations for the same electron
model, where in the case of Ba2IrO4, already on the mean-field level, we were
able to reproduce the experimentally observed magnetic ground state, while for
Sr2IrO4 the main results are essentially the same as in the SE model.Comment: 37 pages, 9 figure
Microscopic analysis of the magnetic form factor in low-dimensional cuprates
We analyze the magnetic form factor of Cu in low-dimensional quantum
magnets by taking the metal-ligand hybridization into account explicitly. In
this analysis we use the form of magnetic Wannier orbitals, derived from the
first-principles calculations, and identify the contributions of different
atomic sites. Having performed local density approximation calculations for
cuprates with different types of ligand atoms, we discuss the influence of the
on-site Coulomb correlations on the structure of the magnetic orbital. The
typical composition of Wannier functions for copper oxides, chlorides and
bromides is defined and related to features of the magnetic form factor. We
propose easy-to-use approximations of the partial orbital contributions to the
magnetic form factor in order to give a microscopic explanation for the results
obtained in previous first-principles studies.Comment: 5 pages, 4 figure
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