  We investigate the structural and electronic origin of antiferromagnetic
(AFM) coupling in the Yttrium iron garnet (YIG) and permalloy (Py) bilayer
system at the atomic level. Ferromagnetic Resonance (FMR) reveal unique hybrid
modes in samples prepared with surface ion milling, indicative of
antiferromagnetic exchange coupling at the YIG/Py interface. Using atomic
resolution scanning transmission electron microscopy (STEM), we found that AFM
coupling appears at the YIG/Py interface of the tetrahedral YIG surface formed
with ion milling. The STEM measurements suggest that the interfacial AFM
coupling is predominantly driven by an oxygen-mediated super-exchange coupling
mechanism, which is confirmed by the density functional theory (DFT)
calculations to be energetically favorable. Thus, the combined experimental and
theoretical results reveal the critical role of interfacial atomic structure in
determining the type magnetic coupling in a YIG/ferromagnet heterostructure,
and prove that the interfacial structure can be experimentally tuned by surface
ion-milling.
