Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy

The field of magnon spintronics is experiencing an increasing interest in the development of solutions for spin-wave-based data transport and processing technologies that are complementary or alternative to modern complementary metal-oxide semiconductor architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the gold standard for insulator-based spintronics to date, but a potential process technology that can deliver perfect, homogeneous large-diameter films is still lacking. We report that liquid phase epitaxy (LPE) enables the deposition of nanometer-thin YIG films with low ferromagnetic resonance losses and consistently high magnetic quality down to a thickness of 20 nm. The obtained epitaxial films are characterized by an ideal stoichiometry and perfect film lattices, which show neither significant compositional strain nor geometric mosaicity, but sharp interfaces. Their magnetostatic and dynamic behavior is similar to that of single crystalline bulk YIG. We found that the in-plane Gilbert damping coefficient ${\ensuremath{\alpha}}_{||}$ is independent of the film thickness and close to $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, while the out-of-plane coefficient ${\ensuremath{\alpha}}_{\ensuremath{\perp}}$ increases with decreasing thickness. Together with an inhomogeneous peak-to-peak linewidth broadening of $\mathrm{\ensuremath{\Delta}}{H}_{0||}=0.4\phantom{\rule{0.16em}{0ex}}\mathrm{G}$, these values are among the lowest ever reported for YIG films with a thickness smaller than 40 nm. These results suggest that nanometer-thin LPE films can be used to fabricate nano- and microscaled circuits with the required quality for magnonic devices. The LPE technique is easily scalable to YIG sample diameters of several inches.