  We have clarified the origin of magnetically dead interface layer formed in
yttrium iron garnet (YIG) films grown at above 700{\deg}C onto gadolinium
gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The
diffusion-assisted formation of a Ga-rich region at the YIG / GGG interface is
demonstrated by means of composition depth profiling performed by X-ray
photoelectron spectroscopy, secondary ion mass spectroscopy and X-ray and
neutron reflectometry. Our finding is in sharp contrast to the earlier
expressed assumption that Gd acts as a migrant element in the YIG/GGG system.
We further correlate the presence of Ga-rich transition layer with considerable
quenching of ferromagnetic resonance and spin wave propagation in thin YIG
films. Finally, we clarify the origin of the enigmatic low-density overlayer
that is often observed in neutron and X-ray reflectometry studies of the YIG /
GGG epitaxial system.
