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Defence of dissertation in the field of engineering physics, Sampo Hämäläinen, M.Sc.(Tech.)

Increasing tension and exciting spin waves

Sampo Hämäläinen, M.Sc.(Tech.), will defend the dissertation "Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers" on 12 October 2018 at 12 noon at the Aalto University School of Science, lecture hall U1, Otakaari 1, Espoo. In this dissertation, results on spin-wave excitation, filtering and confinement in multiferroic heterostructures and YIG/CoFeB bilayers are presented.

Spin waves are collective magnetic excitations in magnetic materials that can exhibit long coherence lengths, high group velocities, and wavelengths down to the nanometer scale. These waves propagate without motion of electrons as in conventional electronics and thus, power dissipation by Joule heating can be avoided. The properties of spin waves could be exploited in low-power wave-like computing and in other devices such as microwave filters and data storage elements.

A key factor to utilize spin waves in applications is energy efficient generation of short wavelength spin waves. In this thesis, I present results on spin-wave generation, filtering and confinement in strain-coupled multiferroic heterostructures and YIG/CoFeB bilayers. I show that firmly pinned magnetic domain walls in multiferroic heterostructures can be used to excite short-wavelength spin waves by driving the domain wall into oscillatory motion or by applying an uniform oscillating magnetic field when the system is at magnetic saturation. Furthermore, I demonstrate that pinned domain walls can be exploited as a switchable spin wave filter that can be reversibly re-programmed by modifying the domain wall width. Such programmable device could be used as a building block for magnonic conduits, filters or logic gates. Moreover, the domain wall state is non-volatile and thus, the state can be used to store information.

Opponent: Prof. Dr.-Ing. Jeffrey McCord, Kiel University, Germany

Custos: Professor Sebastiaan van Dijken, Aalto University School of Science, Department of Applied Physics