This thesis focuses on the theoretical investigation of the electronic structure and magnetic interactions present in 3d and 4d/5d transition metal compounds. We use many-body quantum chemistry methods that provide a theoretical frame for the rigorous construction and systematic improvement of correlated N-electron wave-functions. In Chapter 3 we compute d-d transitions fully ab initio and assign excitation peaks of experimental spectra measured in spin-Peierls TiPO4 compound.
In this material we find that the d1 ground state is composed of an admixture of dz2 and dxz orbital character, which is related to the large positive ionic charge at P sites in the xz plane (defining the shortest Ti-P links) and of Ti nearest-neighbors along the z axis. In addition, the magnitude of the nearest-neighbors Heisenberg magnetic coupling calculated by quantum chemistry methods compares well with resonant inelastic X-ray scattering (RIXS) experimental data. We further demonstrate that the intersite exchange is very sensitive to the Ti-Ti interatomic distance, which is relevant in the context of spin-Peierls physics in TiPO4. In Chapter 4 we have studied the magnetic anisotropy of Fe ions within the Li3N lattice. The calculated magnetic anisotropy splitting of 26.3 meV for Fe2+ d6 ions in D6h symmetry compares favorably to values measured or computed by similar theoretical methods for Fe1+ d7 species with linear coordination. This substantial spin-reversal energy barrier of the Fe2+ ion is associated with a a^1_{1g}e^3_{2g}e^2_{1g} ground-state electron configuration. Our study therefore puts into the spotlight the linearly coordinated Fe2+ d6 ion as candidate for viable single molecule magnet behavior. In Chapter 5 we address the effect of electron-lattice interactions on the magnetic properties of 4d and 5d TM ions with a formally degenerate t^1_{2g} electron configuration in the double-perovskite materials Ba2YMoO6, Ba2LiOsO6 and Ba2NaOsO6. Our analysis indicates that the sizable magnetic moments and g-factors found experimentally
are due to both strong TM d -- ligand p hybridization and dynamic Jahn-Teller effects. Our results also point out that cation charge imbalance in the double-perovskite structure allows a fine tuning of the gap between the t2g and eg levels.
The mechanism has not been explored so far experimentally but seems to hold much potential in the context of orbital engineering in transition metal compounds. In
Chapter 6 we report a study of magnetic exchange interactions in the S=3/2 orthorhombic perovskite NaOsO3. We mapped the ab initio quantum chemistry
results onto model Hamiltonians including both isotropic Heisenberg interactions and anisotropic Dzyaloshinskii-Moriya exchange. We found antiferromagnetic nearest-neighbors Heisenberg exchange interactions of J_ac = 24.4 meV and J_b = 20.9 meV, twice larger than the J extracted from the magnon excitation spectra. The quantum chemistry results motivate further experimental measurements or theoretical analysis to clarify the magnitude of the nearest-neighbors Heisenberg couplings. In Chapter 7 we provide valuable insights on the effective magnetic interactions in 5d and 4d oxides with face-sharing oxygen octahedra, BaIrO3 and BaRhO3, for different bond-angles and bond-lengths. The large antiferromagnetic Heisenberg interactions computed here emphasize the subtle interplay among strong spin-orbit interactions, direct intersite orbital overlap and orbital bonding, and couplings to the lattice degrees of freedom in face-sharing compounds. In Chapter 8 we apply a computational scheme for computing intensities as measured in
X-ray absorption and RIXS experiments. We take into account the readjustment of the charge distribution in the vicinity of an excited electron for the modeling of RIXS.
For L3-edge spectra of Cu2+ 3d9 ions in KCuF3, we discuss the way to consider orbital ordering effects (alternately occupied d_x2-z2 and d_y2-z2 orbitals). For L3-edge spectra of Ni2+ 3d8 ions in La2NiO4, the computed spectra reproduce trends
found experimentally for the incoming-photon incident-angle and polarization dependence.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:35076 |
| Date | 20 August 2019 |
| Creators | Xu, Lei |
| Contributors | van den Brink, Jeroen, Berakdar, Jamal, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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