Investigating spin wave dynamics in YIG microstrips using micromagnetic simulations

Abstract

The goal of this thesis is spin wave propagation in magnetic microstrips with non-standard magnetic field directions using micromagnetic simulations. Spin waves, quasi-particles known as magnons, are waves that are generated when the electron spins oscillate collectively in a magnetic lattice. These waves can be used to transport energy. Spin waves have potential applications for microwave signal processing, logic operations, filtering, and biomedical applications. Spin waves are often launched using microstrip antennas in rectangular magnetic strips that serve as waveguides, and understanding how spin waves behave in these waveguides is critical to developing magnonics devices. The existing analytical theories that describe spin-wave behavior in microstrips are, however, limited to high-symmetry configurations (i.e., for the standard case, typically the static magnetic field direction is either parallel or perpendicular to spin waves' propagation direction) and fail to address spin-wave behavior in practical, lower-symmetry scenarios. In this thesis, field directions of 70° and 80° are examined and compared to the surface spin wave configurations. The angle is measured with respect to the long axis (i.e., x-axis) of the microstrip antenna, and the magnetic fields are applied in the plane of the microstrip. The lower-symmetry scenarios show complex, titled diamond patterns because anisotropic dispersion relations in magnetic thin films govern spin-wave propagation. In this study, micromagnetic simulations were used to model spin wave dynamics in a series of rectangular Yttrium Iron Garnet (YIG) microstrips in low-symmetry configurations. YIG is an ideal material for spin wave investigations because it has exceptionally low damping. The spin wave behavior was analyzed in a series of simulations in which the static magnetic field direction, driving frequency, excitation type (stripline antenna or point source), and microstrip dimensions (i.e., ranges of widths 1.28 μm to 2252.8 μm) were systematically altered. For stripline antennas, diamond-shaped spin wave propagation patterns are observed when the field is applied in-plane at 90° with respect to the long axis of the magnetic thin film, and this diamond pattern can be understood by considering the interference of width-quantized modes. As the static magnetic field direction is reduced, the diamond pattern tilts, and the outline angles are similar to what is seen for point source excitations. The micromagnetic investigations explain the discrepancy and fill the gap between the experimental observations and analytical calculations. These findings enhance our understanding of spin wave dynamics and will be useful for the development of advanced magnonics devices and information processing technologies.