Structural and magnetic properties of near surface superparamagnetic Ni1-Fe nanoparticles in SiO2 formed by low energy dual ion implantation with different fluences

Abstract Low energy implantation of Ni and Fe into SiO2 films resulted in the formation of superparamagnetic Ni1-xFex nanoparticles for Ni fluences of 2 × 1016 at./cm2, 4 × 1016 at./cm2, and 6 × 1016 at./cm2 where the Ni:Fe fluence ratios were 47:56, 53:47, and 63:37, respectively. Small ∼5 nm Ni1-xFex nanoparticles were dispersed in the implantation region for the lowest Ni fluence. Increasing the Ni fluence resulted in a different nanoparticle morphology where larger nanoparticles appeared at the surface and small Ni1-xFex segregated regions to a depth of ∼20 nm. The average nanoparticle size in the surface region was ∼8 nm for Ni fluences of 4 × 1016 at./cm2 and 6 × 1016 at./cm2. The highest Ni fluence film also had smaller Ni1-xFex nanoparticles at a depth of ∼11 nm. The largest high field moment per implanted ion was found for the intermediate Ni fluence. The spin-stiffness was similar for all fluences and smaller than that expected for bulk Ni1-xFex. A small spin-disordered region was evident with the same low spin-freezing temperatures that may be due to a similar spin-disordered shells. dM/dH at 300 K was estimated and found to be highest for a Ni fluence of 4 × 1016 at./cm2 where it reached 62.