An approach to compute exchange parameters of the Heisenberg model in plane-wave-based methods is presented. This calculation scheme is based on the Green's function method and Wannier function ...projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, Li sub(2)MnO sub(3), and KCuF sub(3). The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and KCuF sub(3). In the case of FeO our calculations explain the pressure dependence of the Neel temperature. Li sub(2)MnO sub(3) turns out to be a Slater insulator with antiferromagnetic nearest-neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling.
Specific features of orbital and spin structure of transition-metal compounds in the case of the face-sharing MO sub(6) octahedra are analyzed. In this geometry, we consider the form of the ...spin-orbital Hamiltonian for transition-metal ions with double (e super()sigma sub(g)) or triple (t sub(2g)) orbital degeneracy. Trigonal distortions typical of the structures with face-sharing octahedra lead to splitting of t sub(2g) orbitals into an a sub(1)g singlet and e super(pi) sub(g) doublet. For both doublets (e super()sigma sub(g) and e super(pi) sub(g)), in the case of one electron or hole per site, we arrive at a symmetric model with the orbital and spin interaction of the Heisenberg type and the Hamiltonian of unexpectedly high symmetry: SU(4). Thus, many real materials with this geometry can serve as a testing ground for checking the prediction of this interesting theoretical model. We also compare general trends in the spin-orbital ("KugelKhomskii") exchange interaction for three typical situations: those of MO sub(6) octahedra with common corner, common edge, and the present case of common face, which has not been considered yet.
In this ultrashort review we consider several ruthenates having layered structure with the Ru ions forming a honeycomb lattice (SrRu
2
O
6
, AgRuO
3
, Li
2
RuO
3
, Na
2
RuO
3
, and α-RuCl
3
). ...Depending on the number of
d-
electrons and Ru–Ru bond length these materials demonstrate very different physical properties.
In the present paper, the electronic, magnetic, and structural properties of a novel system FeZnMo
3
O
8
with a polar crystal structure are investigated using GGA + U (Generalized gradient ...approximation taking into account Coulomb correlations) calculations. It is shown that Fe ions preferably occupy octahedral and Zn ions tetrahedral positions. This structural feature is caused by different ionic radii of these ions and not by the exchange coupling. The calculated exchange constants naturally explain magnetic structure observed in this material.
Rattling phonon modes are known to be the origin of various anomalous physical properties such as superconductivity, suppression of thermal conductivity, enhancement of specific heat etc. By means of ...DFT +
calculations we directly show the presence of the rattling mode in the quadruple perovskites CuCu
3
V
4
O
12
and CuCu
3
Fe
2
Re
2
O
12
and argue that this might appear in others as well. It is demonstrated that Cu ions at
sites vibrate in the center of the icosahedral oxygen O
12
cages and the corresponding potential has a complicated form with many local minima.
Studying of single-molecule magnets has sprung many surprises such as, e.g., quantum tunneling of the magnetization, which is strongly related to the presence of a magnetic anisotropy. Electron spin ...resonance and inelastic neutron scattering measurements of (PPh
4
)
2
ReF
6
⋅2H
2
O complex evidence an unprecedented large single-site magnetic anisotropy of
K in this material. Using state-of-the-art ab initio calculations we found that the single-ion anisotropy is indeed very large (but does not exceed 12 K) and revealed the physical mechanism lying behind this phenomenon.
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