  Interfaces between materials with differently ordered phases present unique
opportunities for exotic physical properties, especially the interplay between
ferromagnetism and superconductivity in the ferromagnet/superconductor
heterostructures. The investigation of zero- and pi-junctions has been of
particular interest for both fundamental physical science and emerging
technologies. Here, we report the experimental observation of giant oscillatory
Gilbert damping in the superconducting Nb/NiFe/Nb junctions with respect to the
NiFe thickness. This observation suggests an unconventional spin pumping and
relaxation via zero-energy Andreev bound states that exist only in the
Nb/NiFe/Nb pi-junctions, but not in the Nb/NiFe/Nb zero-junctions. Our findings
could be important for further exploring the exotic physical properties of
ferromagnet/superconductor heterostructures, and potential applications of
ferromagnet pi-junctions in quantum computing, such as half-quantum flux
qubits.
