Public defence in Engineering Physics, M.Sc. Weijia Zhu

Title of the thesis: Control of Spin-Wave Propagation in Multiferroic Heterostructures: Magnetic Domain and Domain-Wall Driven Spin-Wave Guiding

Doctoral student: Weijia Zhu
Opponent: Professor Lasse Laurson, Tampere University, Finland
Custos: Professor Sebastiaan van Dijken, Aalto University School of Science, Department of Applied Physics

Magnonics is a research field that exploits the amplitude and phase of spin waves, collective spin excitations in magnetic systems, for information processing and transport without relying on electronic charge motion. Spin waves offer unique properties, including low-power operation over a broad frequency range (GHz to THz), wavelengths down to the nanometer scale, nonlinear and nonreciprocal dynamics, and compatibility with on-chip integration. These characteristics position magnonics as a promising candidate for next-generation information processing technologies.

The development of efficient magnonic circuits requires precise spin-wave control. In this thesis, I explore the active manipulation of spin waves in strain-coupled multiferroic heterostructures, focusing on two key phenomena: (1) the redirection of spin waves upon transmission across magnetic anisotropy boundaries and (2) reconfigurable spin-wave transport along pinned magnetic domain walls.

The first phenomenon, investigated in a Fe/ BaTiO3 heterostructure, is driven by magnonic refraction arising from abrupt changes in the spin-wave dispersion and phase velocity at magnetic anisotropy boundaries. Experimental and micromagnetic simulation results demonstrate precise spin-wave redirection at zero magnetic field. The second phenomenon, studied in a CoFeB/ BaTiO3 heterostructure, involves the use of pinned magnetic domain walls as nanoscopic channels for spin-wave propagation. By tuning the magnetic microstructure of the domain wall using an external magnetic field, I achieve continuous control over the wavelength of localized spin waves.

The findings of this thesis, combined with the potential for electric-field-driven domain wall motion in multiferroic systems, represent significant steps toward the realization of low-power, reconfigurable magnonic devices. 

Key words: spin waves, magnonics, multiferroics, magnetism, magnetic domains, magnetic domain walls

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact information:

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52