Abstract
Quantum magnetic materials are widely studied because of their relevance to spin-based information processing, energy-efficient memory technologies, and emerging quantum spintronic devices. They provide a rich platform from which collective excitations such as magnons, spin-orbit excitons (SOEs), magnetic frustration, and topological states can be explained. The ilmenite titanate system ATiO3 (A = Co, Ni, Mn) exhibit unique quantum magnetic properties. They share a common rhombohedral structure and a honeycomb antiferromagnetic (AFM) lattice, yet their magnetic ground states and excitation spectra differ significantly. Their distinct magnetic responses make ilmenites an ideal platform for investigating how spin structure, crystal field effects, magnetic exchange interactions, and spin-orbit coupling (SOC) govern quantum magnetism. In this work, elastic and inelastic neutron scattering (INS) techniques were employed on CoTiO3 (CTO), NiTiO3 (NTO), Co0.5Ni0.5TiO3 (CNTO), and MnTiO3 (MTO) to study distinct magnon excitations, magnetic anisotropies, the interplay between magnetic and orbital degree of freedom, and the possibility of tunable quantum magnetic excitations. This has shed light into how spin-orbit coupling, orbital quenching, magnetic frustration, and exchange anisotropy collectively shape the magnetic ground states and excitation spectra across the ilmenite family. The A-site ions occupy a trigonal prismatic oxygen environment that either preserves or quenches the orbital moment. When the orbital moment remains active, SOC mixes spin and orbital degrees of freedom and produces a pseudospin ground state with strong anisotropy. The excitation spectrum resulting from the strong coupling between spin and orbital degrees of freedom consists of low energy magnons and high-energy excitations in spin-orbit split crystal field levels, resulting in SOEs and crystal field excitations. The SOC driven anisotropy of the systems controls the dimensionality of the system and results in topological properties. The energies and intensities of the magnetic excitations evolve with temperature, reflecting changes in the ordered moment and responding to this magnetic fluctuation. When the orbital moment is quenched, the SOC-driven anisotropy is suppressed, the effective Hamiltonian becomes nearly isotropic, and the excitation spectrum simplifies to low energy magnons without observable SOEs. This was demonstrated by investigating NTO. Its contrasting magnetic behavior demonstrates that the presence or absence of an orbital moment is central to the magnetic response and the entire excitation spectrum. The evolution of the magnon bandwidth, anisotropy, and SOEs energy levels was further explained by investigating solid solution of Co and Ni. The changes in magnetic dynamics demonstrated that the ilmenites can form a tunable platform in which SOC strength, exchange interactions, and crystal-field splitting can be controlled through substitution. The ability to modify both low-energy magnons and high-energy SOEs within a single structural family highlights the usefulness of ilmenites for studying coupled spin–orbital physics. The honeycomb lattice of the ilmenite provides additional opportunities to explore orbital and anisotropy effects in systems where SOC is not a dominant factor, such as in MTO. Orbital and structural driven distortions of the exchange network modify the magnetic ground state and generate additional excitation branches that emerge due to the symmetry lowering. These behaviors demonstrate that orbital degrees of freedom influence magnetism not only through SOC but also through lattice and symmetry-mediated mechanisms. The neutron scattering results presented in this thesis provide new insights into the magnetic behavior of the ATiO3 ilmenites. In CTO, strong SOC produces pronounced exchange anisotropy, gapless Dirac magnons, and well-defined SOEs. In NTO, the absence of an orbital moment leads to nearly isotropic exchange, competing in-plane and out-of-plane interactions, and weak magnetic frustration, with no evidence of Dirac crossings. In CNTO, Co–Ni disorder modifies exchange pathways, softens both magnons and SOEs, and generates magnetic dynamics that are more complex than a simple average of the parent compounds. In MTO, an intrinsic 42 K transition was identified. The transition is associated with a spin-canted magnetic phase characterized by a second propagation vector, and the strong buckling of the Mn honeycomb layers introduces bond-anisotropic exchange that gives rise to additional magnetic excitation modes. Together, these results demonstrate how variations in electronic configuration, spin and orbital degrees of freedom, disorder, frustration, and lattice geometry produce distinct quantum magnetic properties and excitation spectra across
the ilmenite family.
