Structure and magnetic properties of irradiated Fe-Fe oxide core-shell nanoclusters

A cluster deposition method was used to produce a film of loosely aggregated particles of Fe-Fe₃O₄ coreshell nanoclusters with an 8 nm iron core size and 2 nm oxide shell thickness. The film of particles on a silicon substrate was irradiated with 5.5 MeV Si²⁺ ions to a fluence of 10¹⁶ cm^-2 near room temperature, and computer simulations based on the SRIM (Stopping and Range of Ions in Matter) code show that the implanted Si species stops near the filmsubstrate interface. The ion irradiation creates a structural change in the film with corresponding chemical and magnetic changes. X-ray diffraction shows that the core size and chemistry stay the same but the shell becomes FeO that grows to a thickness of 17 nm. Helium ion microscopy shows that the previously separate particles have densified into a nearly continuous film. Major loop magnetic hysteresis measurements show a decrease in saturation magnetization that we attribute to the presence of the antiferromagnetic (AFM) FeO shell. First-order reversal curve measurements on the irradiated film performed with a vibrating sample magnetometer show that the AFM shell prevents the particles from interacting magnetically, leading to low coercivity from the iron core and little bias field from the core interactions. These results, and others reported previously on different compositions (Fe₃O₄ or FeO+Fe₃N nanoclusters), show that the ion irradiation behavior of nanocluster films such as these depends strongly on the initial nanostructure and chemistry.