Persistent Photomagnetism in Superparamagnetic Iron Oxide Nanoparticles

Using light irradiation to manipulate magnetization over a prolonged period of time offers a wealth of opportunities for spin-based electronics and photonics. To date, persistent photomagnetism has been frequently reported in spin systems composed of molecular magnets; yet this phenomenon is rarely observed in nanoparticle-based systems comprised of transition metal oxides. Here, detailed studies of persistent photomagnetism in superparamagnetic iron oxide (Fe3O4) nanoparticles at temperatures below their blocking temperature are presented and it is demonstrated that the magnetization change does not occur through steady-state spin transitions or photothermal heating. Instead, it is found that exciton-spin exchange-coupling plays a critical role in modulating the magnetization by lowering the anisotropic energy barrier of Fe3O4 nanoparticles to facilitate their optically driven conversion from ferrimagnetic to superparamagnetic. Collectively, these insights establish a comprehensive understanding of the underlying photophysical processes that regulate photomagnetism in nanoparticle-based magnetic systems composed of transition metal oxides.