Neutron Scattering Investigations of Three-Dimensional Topological States

Abstract

Topological magnets represent a unique class of quantum materials in which a nontrivial Berry curvature in real- or momentum-space couples to the magnetic properties of the topological electronic or spin system. Magnetic skyrmions constitute one such class of topological magnets, characterized by real space topological swirling spin-textures which manifest as localized nanometric excitations in the magnetization field. These protected quasi-particle objects possess a helical chiral structure which supports a diverse landscape of states and defects, whose interactions with spins and electrons produce novel transport properties and emergent dynamics controllable over a wide range of parameter space. This spectrum of phenomena has inspired magnetic skyrmions as the forerunners for novel spintronic high-density memory and ultra-low power logic device applications. As quasiparticles, skyrmions may condense into crystalline orders, typically forming periodic lattice arrangements which extend three-dimensionally in bulk materials. This enhanced dimensionality opens the door to new stabilization pathways, configurational degrees of freedom, and dynamical modes which offer unique functionalities to those of thin systems. For practical applications, understanding skyrmion nucleation, annihilation, transition, and organizational pathways is critical to realizing controllable dynamics and manipulation in future devices.

In this thesis, we explore the development and application of various neutron scattering tomography and structured neutron beam techniques for three-dimensional investigations of bulk magnetic topological materials and their defect-mediated dynamical phenomena. A combination of X-ray, magnetometry, and neutron scattering techniques are used to first identify and characterize the disordered phase of an above room-temperature bulk skyrmion material, Co8Zn8Mn4. Detailed small angle neutron scattering (SANS) measurements are then performed over the entire temperature-magnetic field phase diagram of the material as a function of a dynamic skyrmion ordering sequence. 2D SANS images in combination with micromagnetic simulations reveal a novel disordered-to-ordered skyrmion square lattice transition pathway which represents a new type of non-charge conserving topological transition. This transition is characterized by a novel promotion of four-fold order in SANS and a violation of the conservation of total skyrmion number. Dynamical skyrmion responses in the metastable skyrmion triangular lattice phase showed an exotic memory phase, with an ordered skyrmion signal persisting in spite of hysteresis protocols involving field-induced saturation into the ferromagnetic phase.

Further studies of skyrmion stabilization mechanisms and their dynamical defect pathways were performed through the development of a novel SANS tomography algorithm, applied to the ordered thermal equilibrium skyrmion triangular lattice phase of the bulk Co8Zn8Mn4 sample. Multi-projection neutron scattering datasets collected from the sample were used to generate the first three-dimensional visualizations of a bulk skyrmion lattice. The reconstructions unveiled a host of exotic skyrmion features, such as branching, segmented, twisting, and filament structures, mediated by three-dimensional topological transitions through two different emergent monopole (MP)-antimonopole (AMP) defect pathways.

Methods for the direct identification and determination of topological features and defects of bulk micromagnetic materials, without a priori knowledge of the sample, can be achieved through the incorporation of structured neutron beam methods to neutron scattering experiments. Holographic approaches similar to those used in the development of optical structured waves were implemented with neutrons to generate a method for the selective tuning of single-valued neutron orbital angular momentum (OAM) states. A conventional SANS setup was used to explore the diffraction of linear neutron waves input on a microfabricated grating which consists of arrays of phase-gratings with q-fold fork dislocations and nanometric spatial dimensions comparable to those of magnetic skyrmion lattice periodicities. Far-field scattering images exhibit doughnut intensity profiles centered on the first diffraction orders, with q-dependent radii, thereby demonstrating the tunable generation of topological neutron states for phase- and topology-matched studies of quantum materials.

Together, these studies demonstrate the development and application of novel tools for direct investigations of bulk topological magnetic materials, while uncovering a diverse collection of skyrmion energetics, disorder-dependent dynamics, and three-dimensional topological transition defect pathways. Future works are proposed which explore the threedimensional formation and evolution of bulk skyrmion tubes under various temperaturemagnetic field trajectories and degrees of skyrmion order, using both tomographic, structured neutron beam approaches, and combinations thereof. In doing so, we may provide the first standalone method of characterizing bulk magnetic sample topologies, defect densities, and their correlations. These methods open the door to a new generation of neutron scattering techniques for probing exotic topological interactions and the complete standalone characterization of quantum materials.