Low damping at few-K temperatures in Y3Fe5O12 epitaxial films isolated from Gd3Ga5O12 substrate using a diamagnetic Y3Sc2.5Al2.5O12 spacer

•Growth of epitaxial Y3Fe5O12 thin films on diamagnetic Y3Sc2.5Al2.5O12 buffer layer on Gd3Ga5O12 substrates.•Characterization of magnetic damping of Y3Fe5O12/Y3Sc2.5Al2.5O12 bilayers and a Y3Fe5O12 single layer at various temperatures and frequencies.•Observation of significantly narrower ferromagnetic resonance linewidths and lower damping in the Y3Fe5O12/Y3Sc2.5Al2.5O12 bilayers on Gd3Ga5O12 at low temperatures, as compared to those of Y3Fe5O12 films directly grown on Gd3Ga5O12.