Understanding Optomagnetic Interactions in Fe Nanowire–Au Nanoring Hybrid Structures Synthesized through Coaxial Lithography

Ferromagnetic Fe nanowires surrounded by two plasmonic Au nanorings (Fe_AuNRsNWs) were synthesized by coaxial lithography (COAL). They were characterized under irradiation (three different wavelengths) and an external magnetic field applied either parallel or perpendicular to the long axes of the wires using magnetic force microscopy (MFM) and Kelvin probe force microscopy (KPFM). The field directs the magnetic spin alignment of the nanowires. Through these studies, two important properties of Fe_AuNRsNWs, which derive from plasmon–spin interactions, have been discovered. First, with both parallel and perpendicular spin alignments (relative to the wire axis), hot-electron-enhanced magnetization is observed, as evidenced by plasmon-induced changes in both the surface potential and MFM phase values. Second, the observed enhancement when the spin alignment is perpendicular to the wire axis is smaller than that observed for parallel alignment. Both of these observations are a consequence of hot electron transfer from the Au nanorings to the Fe nanowire and the interaction between the plasmon-induced internal magnetic field and the external magnetic field in the Fe_AuNRsNW. This conclusion is further supported by finite-difference time domain (FDTD) and finite element method (FEM) simulations. In addition to being an excellent testbed for examining plasmon–spin interactions as a way to control magnetic spins with light, these Fe_AuNRsNWs, and their ring structure-dependent properties point toward a pathway for tailoring plasmon–spin interactions.