On the origin of the ferromagnetic signature in silver nanoparticles and thin films

Abstract

The unconventional ferromagnetism of Ag as been studied in highly pure nanomaterials. Gas-phase synthesis of nanoparticles and thin films allow for the independent study of size and ligand-free surface effects. No size correlation to magnetism in Ag nanoparticles ranging from 3.3 ± 0.9 to 7.8 ± 1.3 nm was determined by superconducting quantum interference device (SQUID) magnetometry. Additionally, magnetic quartz crystal microbalance (MQCM) under vacuum conditions of 3.3 ± 0.9 nm Ag NPs shows magnetic behaviour only after the onset of oxidation. SQUID magnetometry shows a higher magnetization for samples exposed to O₂ compared to inert atmospheres. This indicates non-stoichiometric oxidation at the surface as the reason for the observed magnetism as supported by an ∼8 : 1 Ag : O ratio from MQCM measurements. Additionally, heat-treated Ag thin films present a lower saturation magnetization compared to those kept in O₂ without heating. This latter observation could be the result of driving the oxidation to stoichiometric surface oxides AgO and Ag₂O which are known to be diamagnetic.