Identification of a new source of magnon relaxation in interface between epitaxial iron garnet ferrite films and GGG substrate

•Strong increase in the relaxation rate was investigated at temperatures below 100 K in epitaxial iron garnet films.•Compound with Gd3+ ions is formed in the transition layer, which has a considerable magnetic moment.•Unlike other relaxation mechanisms, this one cannot be compensated for.•For further improvement we suggest to use substrates without strong paramagnetic ions like a Y3Ga3Sc2O12 substrate.