I. V. Solovyev

Inverse versus normal NiAs structures as high-pressure phases of FeO and MnO

The phase stability of FeO and MnO under high pressure was studied with the first-principles calculations. Our results predict that the high-pressure phase of MnO is a metallic normal NiAs (B8) structure, while that of FeO is the inverse B8 structure (iB8

Magnetic-field control of the electric polarization in BiMnO3

, which can be summa-rized as follows: (1) the ferroelectric polarization is driven by the hidden antiferromagnetic order inthe otherwise centrosymmetric C2/c structure; (2) the relativistic spin-orbit interaction is respon-sible for the canted spin ferromagnetism. Our analysis is supported by numerical calculations ofelectronic polarization using Berry’s phase formalism, which was applied to the low-energy modelof BiMnO

Electronic structure of Bi M O 3 multiferroics and related oxides

We have performed a systematic study of the electronic structures of BiMeO3 (Me = Sc, Cr, Mn, Fe, Co, Ni) series by soft X-ray emission (XES) and absorption (XAS) spectroscopy. The band gap values were estimated for all compounds in the series. For BiFeO3 a band gap of ~0.9 eV was obtained from the alignment of the O Ka XES and O 1s XAS. The O 1s XAS spectrum of BiNiO3 indicates that the formation of holes is due to a Ni2+ valency rather than a Ni3+ valency. We have found that the O Ka XES and O 1s XAS of BiMeO3 probing partially occupied and vacant O 2p states, respectively, are in agreement with the O 2p densities of states obtained from spin-polarized band structure calculations. The O Ka XES spectra show the same degree of Bi 6s--O 2p hybridization for all compounds in the series. We argue herein that the stereochemical activity of Bi 6s lone pairs must be supplemented with inversion symmetry breaking to allow electric polarization. For BiMnO3 and BiFeO3, two cases of multiferroic materials in this series, the former breaks the inversion symmetry due to the antiferromagnetic order induced by particular orbital ordering in the highly distorted perovskite structure and the latter has rhombohedral crystal structure without inversion symmetry.

Orbital polarization in itinerant magnets.

We propose a parameter-free scheme of calculation of the orbital polarization (OP) in metals, which starts with the strong-coupling limit for the screened Coulomb interactions in the random-phase approximation (RPA). For itinerant magnets, RPA can be further improved by restoring the spin polarization of the local-spin-density approximation through the local-field corrections. The OP is then computed as the self-energy correction in the static GW method, which systematically improves the orbital magnetization and the magnetic anisotropy energies in transition-metal and actinide compounds.

Electronic structure of strongly correlated systems emerging from combining path-integral renormalization group with the density-functional approach.

A new scheme of first-principles computation for strongly correlated electron systems is proposed. This scheme starts from the local-density approximation (LDA) at high-energy band structure, while the low-energy effective Hamiltonian is constructed by a downfolding procedure using combinations of the constrained-LDA and the GW method. The obtained low-energy Hamiltonian is solved by the path-integral renormalization-group method, where spatial and dynamical fluctuations are fully considered. An application to Sr2VO4 shows that the scheme is powerful in agreement with experimental results. It further predicts a nontrivial orbital-stripe order.

Orbital ordering and magnetic interactions in BiMnO3

This work is devoted to the analysis of the orbital ordering patterns and associated interatomic magnetic interactions in the centrosymmetric monoclinic structures of BiMnO3, which have been recently determined experimentally. First, we set up an effective lattice fermion model for the manganese 3d bands and derive parameters of this model from first-principles electronic structure calculations. Then, we solve this model in terms of the mean-field Hartree–Fock method and obtain parameters of interatomic magnetic interactions between Mn ions. We argue that although the nearest-neighbor interactions favor the ferromagnetism, they compete with the longer range antiferromagnetic (AFM) interactions, the existence of which is directly related to the peculiar geometry of the orbital ordering pattern realized in BiMnO3 below 474 K. These AFM interactions favor an AFM alignment, which breaks the inversion symmetry. The formation of the AFM phase is additionally assisted by the orbital degrees of freedom, which tend to adjust the nearest-neighbor magnetic interactions in the direction which further stabilizes this phase. We propose that the multiferroelectric behavior, which was observed in BiMnO3, may be related to the emergence of the AFM phase under certain conditions.

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.

Long-Range Magnetic Interactions Induced by the Lattice Distortions and the Origin of the E-Type Antiferromagnetic Phase in the Undoped Orthorhombic Manganites

With the increase of the lattice distortion, the orthorhombic manganites R MnO 3 ( R = La, Pr, Nd, Tb, and Ho) are known to undergo the phase transition from the layered A-type antiferromagnetic (AFM) state to the zigzag E-type AFM state. We consider the microscopic origin of this transition. Our approach consists of the two parts. First, we construct an effective low-energy model for the manganese 3 d -bands and derive parameters of this model from the first-principles electronic structure calculations. Then, we solve this model in the Hartree–Fock approximation (HFA) and analyze the behavior of the interatomic magnetic interactions. We argue that the nearest-neighbor interactions decrease with the increase of the distortion and at certain stage start to compete with the longer range (particularly, second- and third-neighbor) AFM interactions in the orthorhombic ab -plane, which trigger the formation of the E-phase. The origin of these interactions is closely related to the orbital ordering, which takes ...

Spin?orbital superexchange physics emerging from interacting oxygen molecules in KO2

The alkali hyperoxide KO

Lattice distortion and magnetic ground state of YTiO 3 and LaTiO 3

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