Magnetic exchange interaction at the heterostructure of Tris(8-hydroxyquinoline)iron with different coordination symmetry and valence states on Ni ferromagnetic surface

We investigated the interfacial magnetic exchange interaction between the organic semiconductor, tri(8-hydroxuquinoline)iron(III) (Feq₃) on nickel film which displays perpendicular magnetization near the critical thickness of spin reorientation transition. By varying the adlayer thickness, we observed a redox process at interface that induced the formation of an interfacial hybrid state which involves the change of valence state and local crystal geometry in central iron of Feq₃. The redox process indicates the electron transfer from Ni surface to adsorbed Feq₃, resulting in trivalent Fe(III) to divalent Fe(II) and octahedral to tetrahedral coordination. Besides, our results demonstrate a strong ferromagnetic coupling at the interface between adsorbed Feq₃ and Ni surface. Hysteresis loops exhibit a second hysteretic transition in 1.75 Å-thick Feq₃ on Ni when subjected to vary magnetic fields, indicating the presence of pinning spins within the interface of Feq₃/Ni magnetic moment. These pinning spins also enhanced the coercivity field. The mechanistic study via DFT calculations further illustrate the strong adsorption energy of divalent Fe(II)q₃ with tetrahedral coordinate and imbalanced spin polarized electrons transfer from Ni surface to adsorbed Feq₃ via N and O ligands that plays the key roles for the strong exchange coupling and significant magnetic hardening in Ni/Feq₃ heterojunctions.