Nonreciprocal Spin Wave Propagation and Spectroscopy in Yittrium Iron Garnet in the Presence of Metals and Spin-Orbit Coupling

Trossman, Jonathan Scott

doi:https://doi.org/10.21985/n2-q0bt-w817

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Nonreciprocal Spin Wave Propagation and Spectroscopy in Yittrium Iron Garnet in the Presence of Metals and Spin-Orbit Coupling

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Understanding how spin waves propagate through ferromagnets is an important component of a wider international effort to create spintronic devices. In this thesis, we develop a propagating spin wave spectroscopy to make a variety of measurements on thin films of Yttrium Iron Garnet (YIG), a material known for having long spin wave lifetimes. We rederive the spin wave dispersion relations using the method of Damon and Eshbach and review the resulting mode structure, which consists of a surface wave, the non-reciprocal Damon-Eshbach (DE) mode, and multiple bulk modes, termed backward volume (BV) modes. Two experimental methods are described, one to resonantly couple to minimally-propagating modes via a meander line, and the other to study propagation across the film by launching and detecting waves from opposing edges. The first was used to study the exchange split backward volume modes to determine the exchange stiffness parameter in several samples. The second, the propagating spin wave spectroscopy, was used to to determine the dispersion relations of the DE and BV modes in the small wavevector regime as a function of the angle between the magnetic field and wavevector. With the availability of these techniques we then move on to study the effects of an adjacent metal and/ or some material with spin-orbit coupling on an underlying YIG film; spin-orbit coupling is known to manifest itself through the Dzyaloshinskii-Moriya interaction (DMI). In particular, we study the differences between forward and backward propagating spin waves in a variety of configurations: GGG/YIG, GGG/YIG/Air/Cu, GGG/YIG/Cu, and GGG/YIG/Pt. Through these measurements we put a limit on the DM interaction at a GGG/YIG interface of $5.69\times 10^{-4} \pm .05\times 10^{-4}$ mJ/m$^2$.

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