Merging analogue gravity and magnonics

Spin waves (SWs), the collective excitations of magnetization in magnetic materials, offer a promising alternative to conventional electronics by enabling wave-based computing with potentially lower power consumption. This thesis investigates spin-wave manipulation using spin-transfer torque (STT), with the broader goal of realizing analogue gravity phenomena in magnonic systems. Inspired by theoretical proposals for black-hole analogues based on spin-drift velocity gradients, this work bridges fundamental physics and functional device applications in metallic ferromagnets. The thesis is structured in four main experimental Chapters. Chapter 2 and 3 focuses on the development and optimization of experimental techniques and materials to enable current-driven control of spin waves. A general description of propagating spin-wave spectroscopy (PSWS) is given, together with key nanofabrication improvements, including a single-step lithography approach and optimized ion beam etching protocols. These innovations were crucial to reliably fabricate ferromagnetic strips with sub-micrometer constrictions, enabling spatial control over current density. The magnetic materials were characterized using ferromagnetic resonance and PSWS, revealing a direct relationship between the magnetic damping parameter (a) and spin polarization (P) in CoFeB alloys. This result informs material selection strategies for efficient spin-current interactions and strengthens the theoretical understanding of spin transport. Chapter 4 investigates spin-wave propagation in non-uniform current density landscapes, using constrictions to introduce spatial gradients in the spin-drift velocity. Through PSWS measurements, supported by micromagnetic simulations and analytical modeling, it is shown that these current gradients induce both phase shifts and group velocity modulation. Chapter 5 explores the application of STT-based control in a magnonic multiplexer, where spin waves are selectively routed between output ports depending on the applied current. This demonstrates the versatility of current-induced spin-wave modulation beyond the analogue gravity context, showing its relevance for reconfigurable wave-based logic devices. The final chapter outlines future directions toward the experimental realization of magnonic analogue horizons. Strategies include the use of backward volume spin-wave modes, which naturally exhibit lower group velocities, and the implementation of tapered geometries to spatially shape the current density. These approaches aim to bring the spin-drift velocity (v_d) closer to the spin-wave group velocity (v_g)—a prerequisite for forming black-hole and white-hole analogues in magnetic systems. The thesis also proposes operating at cryogenic temperatures, where increased spin polarization and reduced resistivity can enhance STT efficiency. In summary, this thesis provides experimental evidence and theoretical insight into how inhomogeneous spin-transfer torques can be used to control spin-wave dynamics in patterned ferromagnetic systems. It opens new possibilities for both functional magnonic devices and the experimental exploration of analogue gravity, establishing a solid foundation for future research at the intersection of spintronics and fundamental physics. Finally, this work serves as a testament to how bold and unconventional ideas -such as merging analogue gravity with magnonics- can inspire new directions in condensed matter research and lead to results that might otherwise have remained unexplored.