| dc.description.abstract | An emerging field of research in recent years has been magnonics, the manipulation of coherent spin excitations, spin-waves, in magnetically ordered materials. Recent advances in experimental techniques for high-frequency magnetisation dynamics and the advent of micromagnetic simulations has led to the propositions of functional magnetic devices based upon the control of spin-waves. This thesis presents work for characterisation and future development of high-speed magnonic devices derived from micromagnetic simulations, and numerical techniques for the solution of the Landau-Lifshitz equation for micromagnetic simulations in the finite-difference time-domain approach.
In chapter 3, spin-waves were controlled in the propagation along a thin film magnonic waveguide via resonant scattering from a mesoscale chiral magnetic resonator, in the backwards volume, forwards volume and Damon-Eshbach geometries. The scattering interaction demonstrated non-reciprocity associated with devices acting as spin-wave diodes. Additionally, such devices demonstrated the possibility of phase-shifting. The results obtained were numerically fit and interpreted in terms of a phenomenological model of resonant chiral scattering. The origin of the chiral coupling was discussed in terms of the stray field.
In chapter 4, the phenomenon of spin-wave confinement, wavelength conversion and Möbius mode formation was demonstrated in the backwards volume configuration of thin-film magnetic waveguides. The presence of magnetic field gradients or thickness gradients modified the position of the Γ-point of the dispersion relation for Backwards Volume Dipolar-Exchange Spin-Waves (BVDESW), such that back-scattering and wavelength conversion occurred from the field/thickness gradients due to the “valleys” of the spin-wave dispersion. This work highlights a basis for not only experimental observation of such phenomena, but the potential for devices based upon valleytronics, an exploitation of the valley degree-of-freedom due to the spin-wave dispersion.
In chapter 5, motivated by numerical error encountered in previous work in the thesis, the validity of implicit methods formulated for the numerical solution of the Landau-Lifshitz equation for finite-difference time-domain micromagnetic simulations were demonstrated. The implicit methods were tested for single spin precession in an external field, the μMAG standard problems and additional test cases. A source of numerical instability in explicit integration methods, numerical stiffness in systems of differential equations, was demonstrated to occur in existing explicit numerical methods, applied to the Landau-Lifshitz equation, common to popular micromagnetic software. The stability of implicit methods was demonstrated to be advantageous over explicit methods in micromagnetic scenarios where numerical stiffness could occur. Additionally, it was demonstrated that the quality of the numerical results was improved compared to explicit methods when the implicit method possessed L-stability, a damping of stiff, high wave number spin waves in the simulation. | en_GB |
