Collective spin and heat transport through magnetic systems

In this Thesis, the transport properties of spin superfluids, and the interplay between magnons and phonons, or electrons, play a key role. Chapter 1 is intended to give the reader an overview of the state-of-the-art, and also introduces the experimental techniques to which we refer in the following chapters. In Chapter 2, we focus on the interactions between spin superfluid and thermally excited magnons. We derive a two-fluid model describing the coupled dynamics of condensed and thermal magnons, and we identify a possible experimental signature of spin superfluidity. In Chapter 3, we develop a general phenomenology describing the coupling between the coherent dynamics of the magnetic order parameter and the incoherent dynamics of thermal magnons in ferromagnetic insulators. Specifically, we show that the interactions between thermal and condensed magnons open up the possibility of mediating - via low dissipation spin superfluid current - nonlocal communication between an incoherent spin source and topological solitons.
In Chapter 4, we investigate how magnetoelastic coupling affects thermal spin transport. Motivated by recent spin transport measurements, we develop a transport theory for magnon-polaron modes, and we show that the experimental observations can be explained by invoking magneto-elastic coupling. Ultimately, in Chapter 5, we turn our focus onto magnon-electron interactions, and we address the magnon-drag to the thermopower arising in ferromagnetic metals subject to temperature gradient. Our findings unveil a novel contribution to the already predicted magnon-drag, opening interesting prospects for engineering new thermoelectric devices based on metallic ferromagnetic materials.