Inducing Ferromagnetic Behavior in Cu Nanoparticles and Thin Films through Nonstoichiometric Oxidation

The unconventional magnetism of Cu nano materials has been studied in highly pure and capping-ligand-free nanoparticles and thin films. Gas-phase synthesis of these materials allows for the independent study of size and surface effects and their capacity for inducing magnetism in Cu. Superconducting quantum interference device (SQUID) room-temperature (300 K) measurements displayed no size correlation to the ferromagnetic behavior observed in Cu nanoparticles ranging from 4.5 +/- 1.0 to 9.0 +/- 1.8 nm in diameter. Moreover, magnetic quartz crystal microbalance in situ tests of 4.5 +/- 1.0 nm nanoparticles under vacuum conditions showed magnetic behavior only after the onset of oxidation. SQUID analysis conducted on Cu thin films exposed to several heat treatments demonstrated minor oxidation inducing higher ferromagnetic responses compared to extended oxidation. Further analysis of nanomaterial samples exhibiting the highest magnetic responses indicated a Cu/O atomic ratio of similar to 3-5:1, suggesting nonstoichiometric oxidation as the source of the ferromagnetic signature.