Part I - Experimental Studies of Spin Waves in a Micrometer-Thick Yttrium Iron Garnet filmAn investigation of spin wave dispersion in a micrometer-thick ferrimagnetic yttrium iron garnet (YIG) film has been conducted in various regimes. In the linear excitation regime, the dispersion of spin waves was characterized in an yttrium iron garnet film at submicron wavelengths using a set of wave-vector-specific multielement antennas. The dispersion relations of multiple backward volume (BV) modes have been resolved, particularly in the region of their minima. The techniques developed now facilitate the characterization of spin waves at length scales limited only by available lithography and at a spectral resolution that generally exceeds that of Brillouin scattering. The data obtained are in excellent agreement with theoretical predictions based on a model Hamiltonian. In the non-linear excitation regime, parametric excitation was used to study the minimum frequency associated with the backward volume spin wave branch for the magnetic field lying both in-plane and parallel to the wave vector as well as for out-of-plane field angles. We find that there is a drastic change in the efficiency of parametric excitation between two different pumping-frequency regimes. Part II - Micromagnetic Simulations of Microwave Assisted Magnetic Reversal of Nanosized MagnetsUsing a many spin micromagnetic simulation tool that directly integrates the Landau-Lifshitz equation, we demonstrated that by applying an r.f. pulse, generally referred to as a Pi pulse, it is possible to near-perfectly reverse the direction of the magnetization in a ferromagnet, provided that the sample is sufficiently small and the angular dependence of the precession frequency is continuously matched using an appropriately “chirped” r.f. pulse of the proper length. Simulations were carried out for “prolate” uniaxially symmetric cylindrical samples in the presence of the dipole and exchange interactions. Such reversals can be performed in the presence of a static external magnetic field or, importantly, at zero field under the sample’s own internal demagnetization field. However, the ability to perform near-perfect Pi or two-Pi rotations is lost for samples above certain dimensions for which additional internal degrees of freedom are excited, particularly at higher static fields. In such larger samples the magnetization may still be reversed by utilizing damping, provided it can be rotated past a critical angle. Also, as an alternative way, magnetization reversals in cylindrical Yttrium Iron Garnet nanomagnets are simulated by applying a fixed-frequency transverse microwave field and a time-varying longitudinal applied field along the direction of the static field so as to continually match the precession frequency with the microwave frequency. The ideal form of microwave field is circularly polarized, but we also studied linearly polarized microwave fields since they are simpler to implement. Inhomogeneous modes nucleate in larger samples with dimensions several times larger than an exchange length which leads to incomplete switching. We also analyzed the magnetic mode structure of axially-magnetized, finite-length, nanoscopic cylinders of yttrium iron garnet in a regime where the exchange interaction dominates, along with simulations of the mode frequencies. For the bulk modes we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized “cap modes” that are “trapped” at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semi-quantitative explanations are given for some of the modes in terms of a one-dimensional Schrodinger equation which is valid in the exchange dominant case. The assignment of the azimuthal mode number is carefully discussed and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

Creator

• Lim, Jinho

DOI

• https://doi.org/10.21985/n2-scqe-qm36

Subject

• Condensed matter physics
• Computational physics

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:15798
• etdadmin_upload_851303

Keyword

• Microwave assisted reversal
• Micromagnetic simulation
• YIG film
• FMR
• Spin wave instability
• Nano-size magnet

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright