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Magnetoelastic Interactions And Spin-Transfer Torque Driven Dynamics In Ultrathin Ferromagnetic Nanostructures
This dissertation can be divided into two separate sections. Chapters 2-5 constitute the first section and reflect my general interest in magnetoelastic interactions in ultra-thin film structures. I became interested in both the fundamental structure and origin of magnetoelastic interactions in ultra-thin films as well as the various applications that this interaction can be used in. Chapters 2 and 3 focus on the fundamentals of the magnetoelastic interaction and how magnetoelastic interactions can be affected by surfaces. In particular, we conduct experiments in Chapter 3 that demonstrate the presence of strong surface magnetoelastic coupling in the technologically relevant Ta|CoFeB|MgO interface. While the subject of surface magnetoelasticity is not new, we argue for its relevance in certain quandaries that are currently riddling the community - in particular: a) the origin/behavior of the surface anisotropy in the CoFeB|MgO system and the optimization of the perpendicular magnetic anisotropy in this technologically relevant system. b) the modification of the anisotropy by electric field. Chapter 4 focuses on using magnetoelastic interactions to drive spin wave resonance in ultra-thin film ferromagnetic films. We use surface acoustic waves i (SAWs) in combination with the magnetoelastic interaction to drive these resonances in Ni|Pt bilayers and use a combination of acoustical loss measurements (via Vector Network Analyzer) and spin pumping + inverse spin-Hall measurements to characterize the properties of the spin wave resonance and the SAW-generated magnetoelastic pump field. We argue that GHz-frequency SAWs in tandem with the magnetoelastic interaction can be used as a truly novel tool for studying spin-wave physics and manipulating nano-scale ferromagnets. Chapter 5 describes modeling work that we conducted in order to understand the various limitations involved in using the magnetoelastic interaction in giant magnetostrictive materials to switch nanomagnets for ultra-low power memory applications. We discuss a variety of switching modes, geometries, and materials and conclude that implementation of giant magnetostrictive memories for ultra-high density, ultra-low power consumption data storage is not as straightforward as it would seem at first glance. We propose a few schemes that have not been considered that are better than others (e.g. giant magnetostrictive nanomagnets with perpendicular anisotropy) and discuss the limitations of othe other schemes. The second section of our work is concerned with Spin-Transfer Torque (STT) Driven Dynamics in MgO-based Magnetic Tunnel Junctions (MTJs). We have observed that upon high voltage stressing of the junction and subsequent partial breakdown of the barrier, the time-coherence of STT-induced auto-oscillations are vastly improved. We discuss the origin of this effect both from the point of view of a generalized nonlinear auto-oscillator dynamical system in the presence of Langevin ii noise (discussed and elucidated thoroughly by Andrei Slavin), but also from the point of view of spatial changes in the magnetic structure of the MTJ. Our conclusion is that the formation of magnetic nanocontacts between the polarizer and free layers during voltages stressing are crucial to all of the observed phenomena including the increase in the auto-oscillator time coherence. iii
magnetoelastic; surface anisotropy; surface acoustic waves
Buhrman, Robert A
Ralph, Daniel C; Van Dover, Robert B.
Ph.D. of Applied Physics
Doctor of Philosophy
dissertation or thesis