  The magnetic resonance of iron garnets is at the heart of rich physics and
provides a crucial technology used inside microwave components. A barrier still
stands in the way of integrated cryogenic devices with iron garnets, because
their epitaxy requires paramagnetic substrates that cause large microwave
losses at low temperatures. Finding alternative combinations of substrates and
magnetic garnets is thus essential to enabling cryogenic integrated magnonics.
With this motivation, we develop a new approach to the synthesis of epitaxial
garnet films, based on the co-sputtering of binary oxides, enabling a precise
and continuous tuning of the iron garnet composition. By substituting a
controlled proportion of the iron with additional yttrium, we obtain
Y$_{3}$(Y$_{x}$Fe$_{5-x}$)O$_{12}$ films of high crystalline quality that can
be lattice-matched to yttrium scandium gallium garnet (YSGG), a diamagnetic
substrate suitable for low-temperature applications. Ferromagnetic resonance
performed at low temperatures confirms the elimination of substrate
paramagnetism losses on YSGG. We also show that the
Y$_{3}$(Y$_{x}$Fe$_{5-x}$)O$_{12}$ system can be matched to other substrates,
such as gadolinium yttrium scandium gallium garnet and gadolinium scandium
gallium garnet. Additionally, we demonstrate that Bi-substituted films of
(Bi$_{0.8}$Y$_{2.2}$)Fe$_{5}$O$_{12}$ also offer ideal lattice matching to YSGG
for low-temperature magnetic resonance. The present approach of controlled
substitutions in iron garnets to obtain ideal lattice matching with diamagnetic
substrates opens a very promising pathway to magnonic devices operating in
low-temperature environments.
