The effect of different Fe concentrations on the structural and magnetic properties of near surface superparamagnetic Ni1-xFex nanoparticles in SiO2 made by dual low energy ion implantation

Structural and magnetic measurements were performed on SiO₂ films containing near-surface Ni_1-xFe_x superparamagnetic nanoparticles made by low energy ion implantation into SiO₂ with the same Ni fluences of 2 × 10¹⁶ at./cm² and two different Fe fluences so that x = 0.47 and x = 0.20. The x = 0.47 films had larger nanoparticles, a higher fraction of ordered moments in the nanoparticle cores, and thinner spin-disordered shells. The nanoparticle core spin-stiffnesses were slightly higher for x = 0.20 and smaller than that expected for bulk Ni_1-xFe_x for both Fe concentrations. There was no evidence for magnetic dipole interactions between the nanoparticles that have been reported in compressed superparamagnetic ferromagnetic nanoparticles. dM/dH was estimated and found to be highest for x = 0.47 where it reached 15 at 300 K and 56 at 100 K. The measured dM/dH is consistent with that expected from a derivative of the Langevin function with negligible demagnetization where larger nanoparticles are expected to have a higher dM/dH.