Graduate Thesis Or Dissertation
 

Field-Free Precessional Magnetization Switching by Focused Surface Acoustic Waves

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sj139822n

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  • In this research, the possibility of the precessional magnetization switching by focused surface acoustic waves, without external magnetic field, has been demonstrated. The technique of generating an effective magnetic field by applying stress to a magnetostrictive material via a surface acoustic wave is applied to perform field-free magnetization switching. It is shown that focused interdigital transducers can provide sufficient surface acoustic wave amplitude to achieve switching. For the purpose of controlling the timing and direction of acoustic waves, this research proposes a device containing two perpendicular channels of the interdigital transducers making ±45° angles with the initial magnetization direction of a ferromagnetic thin film at the focal point of the focused interdigital transducers. At first, one of the interdigital transducer channels is excited to generate surface acoustic waves, which will change the magnetization direction in the magnetostrictive material as a consequence of the Villari effect, in combination with precessional magnetization dynamics. Next, the other channel is excited when the magnetization is furthest away from its initial position. This second surface acoustic wave will finalize the magnetization reversal, and thus, field-free switching can be accomplished. To perform the magnetization switching, a high strain amplitude in the magnetostrictive material is required and hence surface acoustic wave focused interdigital transducers are necessary. Focused interdigital transducers are design for Y-cut lithium niobate substrate. Due to the crystalline structure of Y-cut lithium niobate, the surface acoustic wave velocity in this material depends on propagation direction. To design focused interdigital transducers that will generate maximum strain amplitude in the magnetostrictive material, the difference between phase velocity and group velocity, as well as the power flow angle, of the acoustic wave on this crystalline structure must be considered. The research design of a surface acoustic wave focused interdigital transducer device has been experimentally verified by laser interferometry. The surface acoustic wave is focused with 400 pm vibrational amplitude at the focal point. Finally, the magnetization dynamics with the experimental strain value has been simulated to demonstrate the possibility of the field-free magnetization switching.
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file details LinHan-Ting2020.pdf 2020-03-20 Pubblico Scaricare