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Browsing by Author "Zadrozny, Joe, committee member"

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    Exotic phenomena in rare-earth based geometrically frustrated magnets
    (Colorado State University. Libraries, 2022) Yahne, Danielle Rose, author; Ross, Kate A., advisor; Bradley, Mark, committee member; Buchanan, Kristen, committee member; Zadrozny, Joe, committee member
    Rare-earth (RE) based frustrated magnets are ideal systems to explore quantum effects in materials, which are paramount for the development of quantum computers, MRAM, and other next-generation technology. RE based materials are of specific interest due to the strong spin-orbit coupling and crystal electric field effects, which split the degenerate 4f angular momentum states, often leading to an effective spin-1/2 doublet with anisotropic effective exchange models. For this reason, RE materials are paramount to investigating the effects of anisotropic exchange on exotic ground states or quantum phases. Exchange frustration refers to when a system cannot simultaneously satisfy competing interactions, which can lead to a macroscopic degeneracy in the ground state of the system. Materials with geometric frustration, where competing interactions occur due to the crystal geometry alone, have been shown to host a wealth of exotic phenomena, including spin ice phases, quasi-particle excitations, order-by-disorder, and the highly entangled quantum spin liquid (QSL) state, to name a few. In this thesis, we will discuss three RE systems that exhibit geometric frustration in addition to exchange frustration: two RE pyrochlore oxides (RE2TM2O7) and a 2D isosceles triangular lattice material K3Er(VO4)2. Spin-1/2 antiferromagnetic (AFM) 2D triangular lattice magnets are an archetype of geometric frustration. While these materials are theorized to host a variety of different ground states and exotic phases depending on the anisotropies of the system, only a handful of RE material examples have been explored. We report the first deep dive into one such system, K3Er(VO4)2. We have determined the ordered magnetic structure of K3Er(VO4)2, finding an unusual structure with alternating layers comprised of AFM aligned and zero moment. We theorize this unique structure is due to the strong XY single-ion anisotropy, suggested from magnetometry measurements, which acts to suppress (to the point of vanishing completely) the out-of-plane pseudo-spin-1/2 magnetic moments. Next, we explored the effects of phase competition in a well-studied effective spin-1/2 RE pyrochlore oxide, Er2Sn2O7. Previous polycrystalline work has found Er2Sn2O7 to possess a suppressed critical temperature and an AFM Palmer-Chalker ground state. The determined exchange and single-ion anisotropy of Er2Sn2O7 find the ground state lies in close proximity to a competing AFM phase. Through extensive single crystal heat capacity measurements, we discovered a reentrant field vs. temperature phase diagram, where a system that has developed order returns to the original, less ordered (paramagnetic) state as some external parameter (field) is tuned continuously. We investigated the underlying mechanisms behind the reentrance by utilizing Monte Carlo simulations, mean field theory, and classical linear spin-wave calculations. This theory suggests that reentrance is linked to soft modes arising from phase competition, either from enhanced competition of the proximal AFM phase or from competing T=0 field-evolved ground states, depending on the specific applied field direction. In both cases, the soft modes enhance thermal fluctuations which cause the specific ordered phase to be entropically stabilized, thus forming a reentrant phase diagram. Finally, we report recent elastic neutron diffraction results on a RE pyrochlore oxide and candidate octupolar spin-ice, Ce2Sn2O7. The pseudo-spin-1/2 moments in Ce2Sn2O7 are known to possess dipolar-octupolar character and a large parameter space within the phase diagram is theorized to host novel QSL states. Previous powder neutron diffraction found diffuse scattering at high scattering vectors associated with magnetic octupoles. However, our undertaking of a similar measurement on nominally the same sample, found strikingly different results. Our neutron diffraction resulted in a broad, diffuse signal at low scattering vectors, reminiscent of a dipolar spin-ice. Neutron diffraction and atomic PDF measurements have not turned up obvious sample deformities or evidence of oxidation that could explain the differences in the diffuse signals. Further atomic studies and significant theory work is necessary to fully understand the results of this measurements, but the similarities to sister compound Ce2Zr2O7 suggest that Ce2Sn2O7 could lie on a phase boundary that is sensitive to minor distortions.
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    Investigating the origins of slow magnetic relaxation of S = ½ Ni(III) cyclams
    (Colorado State University. Libraries, 2023) Morrison, Thomas L., author; Shores, Matthew P., advisor; Zadrozny, Joe, committee member; Kennan, Alan, committee member; Gelfand, Martin, committee member
    This dissertation describes the syntheses and characterizations of several Ni(III) and Ni(II) complexes in an attempt to better understand the origin of slow magnetic relaxation, or spin reversal, in S = ½ systems by utilizing Ni(III) cyclam (1,4,8,11-tetraazacyclotetradecane) as a toy model system. The content is organized as follows: Chapter 1 provides the historical context and theory surrounding the class of materials called single molecule magnets (SMMs). Therein I describe the prototypical SMM and its primary figures of merit and characteristics, such as S and D, followed by the observation of how S = ½ systems, which have previously been shown to act as SMMs, do not fit within the context currently provided by the literature. The choice of using the Ni(III) cyclam system is then elaborated upon, along with its quirks and foibles. In Chapter 2 I describe the synthesis and magnetic characterization of three Ni(III) cyclams. The first two contain halides in the axial positions, which are 100% abundant in isotopes containing nuclear spin, and the third complex has perchlorate bound in the axial position, where oxygen is nearly nuclear spin free. Neither halide systems showed slow magnetic relaxation, but it was not clear whether it was due to the superhyperfine coupling between the nuclear and electronic spins or due to the antiferromagnetic interactions present at low temperatures. The perchlorate containing complex did show slow magnetic relaxation, consistent with the literature and our predictions. Chapter three describes the crystallographic tuning tools and corresponding magnetic properties of novel S = ½ Ni(III) cyclam complex salts: strong antiferromagnetic coupling in sulfate-bridged chain {[Ni(cyclam)(µ2-SO4)]ClO4·H2O}n and field-, temperature-, and size-dependent slow magnetic relaxation in molecular [Ni(cyclam)(HSO4)2]HSO4. I have reported two methods of manipulating the dynamic magnetic response of these coordination molecules: particle size selection and deuteration. I find that particle size dependency, which I attribute to the phonon bottleneck effect, for the magnetic dynamics in the parent protiated compound is removed in deuterated isotopologue, revealing only the faster molecular relaxation mode(s). Chapter 4 describes the synthesis and characterization of four novel Ni(III) cyclams utilizing neutral ligands in the axial positions as opposed to the anionic ones considered previously, namely [Ni(cyclam)(acetonitrile)2]X3 (X = OTf, ClO4, BF4) and [Ni(cyclam)(butyronitrile)2]OTf3. Through these complexes we probe the role of ligand charge, identity, and subtle differences in the hydrogen-bonding network on the slow magnetic relaxation of the Ni(III) ion. Chapter 5 describes the solution phase studies of [Ni(cyclam)(MeCN)2]OTf3 and [Ni(cyclam)(butyronitrile)2]OTf3 in glassy and non-glassy solvents, as well as their suitability for studying other novel species in situ that may not be able to be synthesized and measured traditionally. We find that there are significant differences in the magnetic relaxation of the Ni(III) cyclams between glassy and non-glassy solutions and discuss the possibilities these findings present. In Chapter 6 I summarize the key findings from Chapters 2-5 and propose new avenues of research for further investigating this phenomenon. Finally, in Chapter 7 I describe a different ligand involving intra-ligand π-π interactions and explore the feasibility of using such interactions for intelligently controlling and tuning the first coordination sphere geometry and electronic structure. By introducing new substituents, changes to the aromaticity, and oxidation of the ligand we are able to exhibit rational control over the crystallographic and electronic structure of the metal center.