Principles, Modeling, and Measurement Towards Efficient Microwave Spin-Wave Circuits

Microwave spin-wave circuits have the potential to vastly outperform the current state-of-the-art RF front-end devices. Efficient spin-wave circuits are necessary to enable a host of on-chip microwave components, including tunable filters at millimeterwaves for radio front-ends, true-time delays, attenuators, non-reciprocal devices, and frequency selective limiters (FSLs). Microwave spin-wave circuits also hold promise for wave-based computing and Boolean computing applications because of their miniature wavelengths at microwave frequencies.

Spin waves are propagating modes of precessing magnetic dipole moments driven by Maxwellian fields and quantum mechanical interactions. These waves can be excited by RF currents at frequencies up to 100 GHz, propagate with velocities on the order of 1000 m/s, and exhibit non-linear behavior.

Development of these devices has been stymied by inefficient conversion of electromagnetic (EM) waves to spin waves. Fundamentally, efficient transduction requires matching the periodicity of EM fields to the periodicity of spin wave fields, a difficult task considering their respective wavelengths.

This challenge is compounded by a lack of fully-coupled electromagnetic-micromagnetic simulation tools. Existing numerical methods and theoretical formulations typically rely on numerous simplifying assumptions, predominantly valid in the Maxwellian (classical) regime, to approximately predict the energy conversion between domains. However, there has been a significant hiatus in rigorous, quantitative transducer analysis since the 1990s, prior to which most of the foundational theoretical work was published.

This dissertation studies the principles behind efficient EM-to-spinwave transduction by utilizing existing magnetostatic and exchange regime theory along with the finite element method (FEM). A major focus of this work includes providing a design methodology that accurately and rapidly predicts a transducer's performance. Various transducer designs, some of which are ubiquitous but perform poorly and some that have been mathematically but not practically evaluated for their potential, are examined for fundamental efficiency and bandwidth limitations. This dissertation also considers system-level ramifications imposed by transducers on microwave circuits, such as the need for and complexity of matching networks. An end-to-end microwave/spin-wave transducer model is proposed that permits the use of classical microwave network analysis and matching theory towards analyzing and designing efficient transduction systems. This research culminates in calibrated, on-chip measurements which agree well with the theoretical and numerical modeling methodology.