Date of Award
Doctor of Philosophy (PhD)
Professor Thao T. Tran
Professor William T. Pennington Jr.
Professor Joseph S. Thrasher
Professor Byoungmoo Kim
Noncentrosymmetric (NCS) materials with crystal lattices lacking spatial inversion symmetry display a wide range of exciting functionalities. This dissertation covers two classes of functional NCS materials: magnetic skyrmion-host compounds and multifunctional materials. Magnetic skyrmion and multifunctional materials combining optical and magnetic responses are providing avenues for developing and optimizing the performance of electronic devices that can have uses in memory storage, laser technology, medicine, sensors, etc. The formation of skyrmions is driven by asymmetric Dzyaloshinskii–Moriya (DM) interaction facilitated by broken spatial inversion symmetry and large spin-orbit coupling (SOC), while multiple functionalities arise when spin carriers and optical chromophores are optimally placed in an ideal lattice framework. However, designing and creating materials featuring these unique spin configurations and harmonized magneto-optical properties remain a significant challenge. To address this, new strategies based on fundamental chemistry concepts and electronic structure calculations was developed. For the design of skyrmion-host compounds, we combine magnetic spins, asymmetric ligands with stereochemically active lone-pair electrons, and polar lattice symmetry to enhance the asymmetric DM interaction in new systems. For multifunctional compounds, we integrate lanthanide f spin carriers and optical chromophores with mixed asymmetric ligands in a NCS chiral crystal structure. Based on the first design strategy, we synthesized NCS polar magnets Fe(IO3)3, α-Cu(IO3)2, Mn(IO3)2, and Fe2(SeO3)3(H2O)3 using low-temperature chemistry and characterized their crystal structures, magnetic structures, and magnetic spin evolutions. We found evidence of a possible skyrmion phase in Fe(IO3)3 driven by sizeable DM interaction and facilitated by the 3-fold symmetry and the stereochemically active lone pair electrons. This is supported by an incommensurate spiral antiferromagnetic ground state and field-induced first-order magnetic transition. The compound α-Cu(IO3)2 exhibits incommensurate antiferromagnetic order similar to that in Fe(IO3)3 via enhanced DM interaction stabilized by large orbital contribution. In contrast, the magnetic order in Mn(IO3)2 is commensurate antiferromagnetic with zero asymmetric DM exchange. Furthermore, we uncovered a complex magnetic structure in Fe2(SeO3)3(H2O)3 featuring two inequivalent Fe3+ cation sites Fe(1) and Fe(2). The ground state magnetic order of this material is described as a collinear antiparallel arrangement of ferrimagnetic Fe(1)-Fe(2) dimers. Utilizing the second design approach, we created new chiral compounds Ln2(SeO3)2(SO4)(H2O)2 (Ln = Sm, Eu, Dy, Yb) featuring multifunctional nonlinear optical, photoluminescence (PL), and magnetic anisotropy properties. The second harmonic generation (SHG) intensities of these materials are approximately that of α-SiO2, and they display type-I phase matching behavior. The magnetic and electronic properties of these materials are largely influenced by large spin-orbit coupling. The Eu material shows excellent red PL emission comparable to commercial red phosphors. The results herein connect basic chemical principles to novel physical phenomena, enabling a worthwhile pathway for navigating the journey of materials design and development.
Oyeka, Ebube, "Designing Noncentrosymmetric Multifunctional Materials" (2023). All Dissertations. 3307.
Available for download on Friday, May 31, 2024