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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2024)

Artificial spin ices (ASIs) are lithographically-patterned arrays of interacting magnetic nanoislands which have traditionally been designed such that each nanoisland possesses an Ising-like magnetic configuration (i.e., behaves as a bar magnet). These systems were originally introduced as an engineered analog of spin-ice crystals whose atomic-level magnetic ordering could be emulated and visualized with conventional magnetic microscopy techniques. Since this initial study, ASIs have served as a platform for designing and investigating phenomena including phase transitions between different global magnetic ordering states, magnetically-reconfigurable spin wave dynamics, and computation in nanomagnet-based devices.

Many ASI studies have used Ni80Fe20-based nanoislands whose material and geometric properties enforce the formation of uniform, Ising-like magnetizations in each nanoisland. While these systems have facilitated the investigation of magnetic ordering phenomena in various array geometries, ASIs fabricated from ferromagnetic complex oxides offer opportunities to explore how the nanoisland material properties and magnetostatic interactions can influence the behavior of the individual nanoislands and ASI as a whole. Understanding how these magnetic parameters can tailor the nanoisland functionalities will be essential in the development of nanomagnet-based computing devices.

This dissertation focuses on ASIs fabricated from the ferromagnetic perovskite oxide La0.7Sr0.3MnO3 (LSMO) to understand how the interaction between magnetostatically-coupled nanostructures can influence the formation of spin textures. Soft x-ray photoemission electron microscopy (X-PEEM) was used to perform magnetic domain imaging of LSMO-based ASIs. Through these imaging studies, these systems were observed to allow the formation of both Ising states, observed in traditional ASI systems, as well as complex spin textures (CSTs), comprised of single and double vortices. The formation of these different spin texture states were found to be sensitive to the ASI geometry as well as the magnetization of the neighboring nanoislands. Through the use of micromagnetic simulations, an energetic analysis of the system was performed which revealed how spin texture formation arises from a competition between effects intrinsic to each nanoisland as well as the magnetostatic interaction between adjacent nanoislands. This competition is facilitated by the unique combination of magnetic material properties of LSMO.

The unusual domains stabilized in LSMO-based ASIs also raises the question of the relaxation kinetics of CST-bearing ASI systems. Insights into the thermal relaxation behavior of LSMO-based ASIs was provided using X-PEEM and an in-situ pulsed annealing protocol which allows for “snapshots” of the time-dependent relaxation to be acquired. By tracking the domain changes in each nanoisland as a function of the annealing time, several types of apparent domain conversion were observed whose relative transformation rates evolved over time. These results suggest that the domain formation kinetics of each individual nanoisland is sensitive to the evolving magnetizations of the neighboring nanoislands.

These static and dynamic studies of LSMO-based ASIs demonstrate how magnetostatic interactions can be used to tailor spin texture formations in these systems. Moreover, this work highlights the potential for material-driven ASI studies to provide deeper insights into ASI and domain formation physics.

    Cover page: Spin Texture Formation in Perovskite-Based Artificial Spin Ice Structures
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2024)
    Antiferromagnetic (AFM) spintronics offer several benefits compared to their ferromagnetic (FM) counterparts, such as high storage capacity and faster processing speed, however, difficulties in manipulating and detecting the AFM moments impede their implementation. Spin-flop coupling, the interfacial perpendicular coupling between FM and AFM moments, can be utilized to control the orientation of AFM moments with the application of moderate magnetic fields on the scale of tenths of a Tesla. In this work, epitaxial bilayers of AFM La0.5Sr0.5FeO3 (LSFO)/FM La0.7Sr0.3MnO3 (LSMO) with fixed LSMO thickness (∼85 u.c.) and LSFO thicknesses varying from 10 to 50 u.c. were investigated to determine the effect of Sr doping and La1-xSrxFeO3 magnetocrystalline anisotropy on the strength of spin-flop coupling. X-ray magnetic linear dichroism demonstrated that the spin-flop coupling strength decreased with increasing LSFO layer thickness, persisting at a thickness of 50 u.c. (∼20 nm). Furthermore, photoemission electron microscopy revealed a domain-by-domain correlation between the FM and AFM domains consistent with the perpendicular orientation dictated by spin-flop coupling. These results demonstrate that LSFO/LSMO bilayers have the potential to serve as a model materials system for AFM spin transport measurements.
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    Cover page: Spin-flop coupling at La0.5Sr0.5FeO3/La0.7Sr0.3MnO3 interfaces
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is achieved using a global voltage bias applied to the whole device.
      Cover page: Transverse barrier formation by electrical triggering of a metal-to-insulator transition
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2024)
      Metal-insulator transitions (MITs) in resistive switching materials can be triggered by an electric stimulus that produces significant changes in the electrical response. When these phases have distinct magnetic characteristics, dramatic changes in the spin excitations are also expected. The transition metal oxide La0.7Sr0.3MnO3 (LSMO) is a ferromagnetic metal at low temperatures and a paramagnetic insulator above room temperature. When LSMO is in its metallic phase, a critical electrical bias has been shown to lead to an MIT that results in the formation of a paramagnetic resistive barrier transverse to the applied electric field. Using spin-transfer ferromagnetic resonance spectroscopy, we show that even for electrical biases less than the critical value that triggers the MIT, there is magnetic phase separation, with the spin-excitation resonances varying systematically with applied bias. Therefore, voltage-triggered MITs in LSMO can alter magnetic resonance characteristics, offering an effective method for tuning synaptic weights in neuromorphic circuits.
        Cover page: Electrical Control of Magnetic Resonance in Phase Change Materials.
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2022)
        Magnetic properties and interfacial phenomena of epitaxial perovskite oxides depend sensitively on parameters such as film thickness and strain state. In this work, epitaxial La0.67Sr0.33CoO3 (LSCO)/La0.67Sr0.33MnO3 (LSMO) bilayers were grown on NdGaO3 (NGO) and LaAlO3 (LAO) substrates with a fixed LSMO thickness of 6 nm, and LSCO thickness (tLSCO) varying from 2 to 10 nm. Soft x-ray magnetic spectroscopy revealed that magnetically active Co2+ ions that strongly coupled to the LSMO layer were observed below a critical tLSCO for bilayers grown on both substrates. On LAO substrates, this critical thickness was 2 nm, above which the formation of Co2+ ions was quickly suppressed leaving only a soft LSCO layer with mixed valence Co3+/Co4+ ions. The magnetic properties of both LSCO and LSMO layers displayed strong tLSCO dependence. This critical tLSCO increased to 4 nm on NGO substrates, and the magnetic properties of only the LSCO layer displayed tLSCO dependence. A non-magnetic layer characterized by Co3+ ions and with a thickness below 2 nm exists at the LSCO/substrate interface for both substrates. The results contribute to the understanding of interfacial exchange spring behavior needed for applications in next generation spintronic and magnetic memory devices.
          Cover page: Strain- and thickness-dependent magnetic properties of epitaxial La0.67Sr0.33CoO3/La0.67Sr0.33MnO3 bilayers
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2023)
          Controlling the in-plane magnetocrystalline anisotropy and interfacial exchange coupling between ferromagnetic (FM) layers plays a key role in next-generation spintronic and magnetic memory devices. In this work, we explored the effect of tuning the magnetocrystalline anisotropy of La2/3Sr1/3CoO3 (LSCO) and La2/3Sr1/3MnO3 (LSMO) layers and the corresponding effect on interfacial exchange coupling by adjusting the thickness of the LSCO layer (tLSCO). The epitaxial LSCO/LSMO bilayers were grown on (110)o-oriented NdGaO3 (NGO) substrates with a fixed LSMO (top layer) thickness of 6 nm and LSCO (bottom layer) thicknesses varying from 1 to 10 nm. Despite the small difference (∼0.2%) in lattice mismatch between the two in-plane directions, [001]o and [11̅0]o, a pronounced in-plane magnetic anisotropy was observed. Soft X-ray magnetic circular dichroism hysteresis loops revealed that for tLSCO ≤ 4 nm, the easy axes for both LSCO and LSMO layers were along the [001]o direction, and the LSCO layer was characterized by magnetically active Co2+ ions that strongly coupled to the LSMO layer. No exchange bias effect was observed in the hysteresis loops. In contrast, along the [11̅0]o direction, the LSCO and LSMO layers displayed a small difference in their coercivity values, and a small exchange bias shift was observed. As tLSCO increased above 4 nm, the easy axis for the LSCO layer remained along the [100]o direction, but it gradually rotated to the [11̅0]o direction for the LSMO layer, resulting in a large negative exchange bias shift. Therefore, we provide a way to control the magnetocrystalline anisotropy and exchange bias by tuning the interfacial exchange coupling between the two FM layers.
            Creative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2024)
            Electrical triggering of a metal-insulator transition (MIT) often results in the formation of characteristic spatial patterns such as a metallic filament percolating through an insulating matrix or an insulating barrier splitting a conducting matrix. When MIT triggering is driven by electrothermal effects, the temperature of the filament or barrier can be substantially higher than the rest of the material. Using X-ray microdiffraction and dark-field X-ray microscopy, we show that electrothermal MIT triggering leads to the development of an inhomogeneous strain profile across the switching device, even when the material does not undergo a pronounced, discontinuous structural transition coinciding with the MIT. Diffraction measurements further reveal evidence of unique features associated with MIT triggering including lattice distortions, tilting, and twinning, which indicate structural nonuniformity of both low- and high-resistance regions inside the switching device. Such lattice deformations do not occur under equilibrium, zero-voltage conditions, highlighting the qualitative difference between states achieved through increasing temperature and applying voltage in nonlinear electrothermal materials. Electrically induced strain, lattice distortions, and twinning could have important contributions in the MIT triggering process and drive the material into nonequilibrium states, providing an unconventional pathway to explore the phase space in strongly correlated electronic systems.
              Cover page: Local strain inhomogeneities during electrical triggering of a metal-insulator transition revealed by X-ray microscopy.
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