Emergent spin dynamics in magnetic nanostructures and magnonic hybrid systems
Date
2024
Authors
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Publisher
University of Delaware
Abstract
One of the issues regarding current electronic devices is that they are based on electron movements to control and transmit information. As a result, much power is wasted in Joule heating. The field of magnonics is concerned with utilizing magnons, the elementary quanta of spin waves, as energy-efficient alternatives to charge-based electronics. Engineered arrays of 2D or 3D nanomagnetic structures offer interesting opportunities for low-power magnonics. Artificial spin ice, which are 2D or 3D magnonic crystals, in which the band structure can be controlled by external parameters, have attracted increasing attention in recent years. Due to their rich magnetic microstates, they could be used for various magnonic applications including wave-based computing and tunable microwave filters. However, artificial spin ice requires more investigations on lattice design and controlling the magnetization dynamics to make it practical for computing and storage devices. ☐ As a part of this thesis project, magnetization dynamics in different types of artificial spin ice (ASI) lattices were studied using two experimental techniques, microwave (MW) absorption, and Brillouin light scattering spectroscopy (BLS) techniques, along with micromagnetic simulation technique (using mumax3). First, an ASI lattice in the form of submicron Ni81Fe19 nanodisk arrays closely packed on a honeycomb lattice was studied. Rich mode spectra related to saturated states at high-frequency/high-field and low-frequency/ low-field dynamics related to vortex creation and annihilation were reported. Controllable spin-wave dynamics and spin-wave channel formation using experimental conditions were shown via micromagnetic simulation. This result was confirmed using the micro-focused BLS technique, which provided the first experimental visualization of magnetization dynamics in ASI. Second, a bicomponent square ASI made of two dissimilar magnetic materials, i.e., Ni81Fe19 and Co90Fe10, was introduced. Unique dynamic spectra related to each sublattice were discovered, and intra- and inter-lattice dynamics originating from different magnetization properties of each material were observed. It was found that the dynamics of the entire lattice are affected by the interaction of the sublattices, and proper choice of materials gives one more degree of freedom to finely tune dynamics in ASI besides other controllable experimental conditions. This result was complemented with a different bicomponent structure defined on a honeycomb lattice. My study demonstrates the capability to achieve an innovative class of 2D magnonic crystals, introducing new concepts in the field of nanomagnonics. ☐ Magnons are highly tunable and can be coupled to different types of excitations, such as photons, phonons, etc. A coherent conversion of magnons to photons requires strong coupling between subsystems. This type of strong magnon-photon (MP) coupling can be used for coherent microwave to optical down- and up-conversion, a prerequisite for large-scale quantum information transfer applications. Most works on MP coupling have focused on microwave cavity resonators due to their high-quality factors and bulk yttrium iron garnet (YIG) samples due to their high number of spins. This large size makes them unsuitable for on-chip solutions. Within the framework of this thesis project, direct probing of MP coupling in a planar geometry, a split-ring resonator (SRR), and thin YIG was observed using the BLS technique. It was complemented by the MW absorption technique. Two YIG films with thicknesses of 200 nm and 2.46 μm were investigated, and a strong coupling regime was observed for the thicker sample, while a magnetically induced transparency (MIT) regime was observed for the thinner sample. It was shown that BLS is advantageous in probing the magnonic characteristics of MP coupling, while the MW absorption technique is advantageous in detecting the photonic characteristic. Furthermore, MP coupling detection using the BLS technique demonstrates MW to optical upconversion. The planar geometry studied here provides spatially-resolved observation of MP polaritons and can serve as a foundation for studying magnons strongly coupled to MWs. ☐ Lastly, this dissertation discusses the possibility of studying magnetization dynamics in a few arrays of magnetic microstructures with a few spins by introducing mi- microresonators, specifically, R-type microresonators (RTM). RTMs are compatible with nanofabricated devices, and successful detection of the mode spectra for a few arrays of Ni81Fe19 microstructures was demonstrated. This study establishes the groundwork for future investigations into MP coupling based on RTMs in magnetic microstructures.
Description
Keywords
Artificial spin ice, Magnon-photon coupling, Magnonics, Nanostructures, Spin dynamics