  Chiral symmetry breaking of phonons plays an essential role in emergent
quantum phenomena owing to its strong coupling to spin degree of freedom.
However, direct experimental evidence of the chiral phonon-spin coupling is
lacking. In this study, we report a chiral phonon-mediated interlayer exchange
interaction in atomically controlled ferromagnetic metal (SrRuO3)-nonmagnetic
insulator (SrTiO3) heterostructures. Owing to the unconventional interlayer
exchange interaction, we have observed rotation of magnetic moments as a
function of nonmagnetic insulating spacer thickness, resulting in a spin spiral
state. The chiral phonon-spin coupling is further confirmed by phonon Zeeman
effects. The existence of the chiral phonons and their interplay with spins
along with our atomic-scale heterostructure approach open a window to unveil
the crucial roles of chiral phonons in magnetic materials.
