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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Article
  • Peer Reviewed
UC San Diego Previously Published Works (2024)
The spin relaxation of an Er3+ tetranuclear single-molecule magnet, [Er(hdcCOT)I]4, (hdcCOT = hexahydrodicyclopentacyclooctatetraenide dianion), is modeled as a near-tetrahedral arrangement of Ising-type spins. Combining evidence from single-crystal X-ray diffraction, magnetometry, and computational techniques, the slow spin relaxation is interpreted as a consequence of symmetry restrictions imposed on quantum tunneling within the cluster core. The union of spin and spatial symmetries describe a ground state spin-spin coupled manifold wherein 16 eigenvectors generate the 3D quantum spin-space described by the vertices of a rhombic dodecahedron. Analysis of the experimental findings in this context reveals a correlation between the magnetic transitions and edges connecting cubic and octahedral subsets of the eigenspace convex hull. Additionally, the model is shown to map to a theoretically proposed quantum Cayley network, indicating an underexplored synergy between mathematical descriptions of molecular spin interactions and quantum computing configuration spaces.
    Cover page: Designing Quantum Spaces of Higher Dimensionality from a Tetranuclear Erbium-Based Single-Molecule Magnet.
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2024)

    The focus of this dissertation is the thorough analysis of the effects of dipolar coupling on magnetic relaxation behavior within erbium-based molecular magnets. Utilizing the Er-COT unit as the starting point and building block, we investigate intra- and inter-molecular dipolar coupling motifs generated with analogous highly anisotropic building block units. Each chapter holds a specific focus, organized as follows: Chapter 1 offers a brief introduction to molecular magnetism, followed by an overview of topics necessary towards understanding magnetic relaxation in the scope of this work, including energy perturbations, relaxation dynamics, and anisotropy; an introduction to the Er-COT unit and coupling schemes in lanthanide-based molecular magnetism, and the motivation for investigating dipolar coupled systems. The chapter concludes with extended chapter summaries for the remainder of this work. Chapter 2 focuses on the role and effects of intramolecular dipolar coupling in a series of compounds of increasing nuclearity. This work demonstrates the ability of intramolecular dipolar coupling to control quantum tunnelling of magnetization, and thus the rate and mechanism of magnetic relaxation. This chapter utilizes an expanded frequency space magnetometry technique to garner new insights into magnetic relaxation by visualizing, fitting, and analyzing multiple relaxation regimes. The chapter concludes with an intuitive model of thought based on a simple vector addition model, within which spin interactions can be estimated directly from a crystal structure. Chapter 3 discusses the effects of intermolecular dipolar coupling within a series of identical single-ion magnets within varied crystal packing environments. This work depicts the propensity of intermolecular dipolar coupling to drive dramatic differences in resulting magnetic behavior and seeks to shed light on the relationship between single-ion magnetism and solid-state magnetism. This chapter applies a novel fitting methodology to quantify additional parameters from isothermal magnetization data for downstream analysis. Chapter 4 presents the analysis of a highly symmetric, near-tetrahedral tetranuclear single-molecule magnet to discuss the effects of crystallographic symmetry on the resulting dipole-coupled spin-space symmetry. This chapter discusses the ensuing available rhombic dodecahedral quantum space of this molecule, composed of octahedral and cubic subspaces, and makes connections to theoretically proposed quantum Cayley networks upon hypercubes.

      Cover page: Manifestations and Effects of Dipolar Coupling in Erbium-Based Molecular Magnets
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2023)
      We present a wide-ranging interrogation of the border between single-molecule and solid-state magnetism through a study of erbium-based Ising-type magnetic compounds with a fixed magnetic unit, using three different charge-balancing cations as the means to modulate the crystal packing environment. Properties rooted in the isolated spin Hamiltonian remain fixed, yet careful observation of the dynamics reveals the breakdown of this approximation in a number of interesting ways. First, differences in crystal packing lead to a striking 3 orders of magnitude suppression in magnetic relaxation rates, indicating a rich interplay between intermolecular interactions governed by the anisotropic Ising lattice stabilization and localized slow magnetic relaxation driven by the spin-forbidden nature of quantum tunneling of the f-electron-based magnetization. By means of diverse and rigorous physical methods, including temperature-dependent X-ray crystallography, field, temperature, and time-dependent magnetometry, and the application of a new magnetization fitting technique to quantify the magnetic susceptibility peakshape, we are able to construct a more nuanced view of the role nonzero-dimensional interactions can play in what are predominantly considered zero-dimensional magnetic materials. Specifically, we use low field susceptibility and virgin-curve analysis to isolate metamagnetic spin-flip transitions in each system with a field strength corresponding to the expected strength of the internal dipole-dipole lattice. This behavior is vital to a complete interpretation of the dynamics and is likely common for systems with such high anisotropy. This collective interactivity opens a new realm of possibility for molecular magnetic materials, where their unprecedented localized anisotropy is the determining factor in building higher dimensionality.
        Cover page: Molecular Network Approach to Anisotropic Ising Lattices: Parsing Magnetization Dynamics in Er3+ Systems with 0-3-Dimensional Spin Interactivity.
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2024)
        The design of molecular magnets has progressed greatly by taking advantage of the ability to impart successive perturbations and control vibronic transitions in 4fn systems through the careful manipulation of the crystal field. Herein, we control the orientation and rigidity of two dinuclear ErCOT-based molecular magnets: the inversion-symmetric bridged [ErCOT(μ-Me)(THF)]2 (2) and the nearly linear Li[(ErCOT)2(μ-Me)3] (3). The conserved anisotropy of the ErCOT synthetic unit facilitates the direction of the arrangement of its magnetic anisotropy for the purposes of generating controlled internal magnetic fields, improving control of the energetics and transition probabilities of the electronic angular momentum states with exchange biasing via dipolar coupling. This control is evidenced through the introduction of a second thermal barrier to relaxation operant at low temperatures that is twice as large in 3 as in 2. This barrier acts to suppress through-barrier relaxation by protecting the ground state from interacting with stray local fields while operating at an energy scale an order of magnitude smaller than the crystal field term. These properties are highlighted when contrasted against the mononuclear structure ErCOT(Bn)(THF)2 (1), in which quantum tunneling of the magnetization processes dominate, as demonstrated by magnetometry and ab initio computational methods. Furthermore, far-infrared magnetospectroscopy measurements reveal that the increased rigidity imparted by successive removal of solvent ligands when adding bridging methyl groups, along with the increased excited state purity, severely limits local spin-vibrational interactions that facilitate magnetic relaxation, manifesting as longer relaxation times in 3 relative to those in 2 as temperature is increased.
          Cover page: Dipolar Coupling as a Mechanism for Fine Control of Magnetic States in ErCOT-Alkyl Molecular Magnets.
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