Ni1−xFex nanoparticles made by low energy dual ion implantation into SiO2

Abstract

Ni1−xFex nanoparticles were made by implanting Ni and Fe at 10 keV into a SiO2 film and electron beam annealing (EBA) with a Ni:Fe fluence ratio of 44:56. The as-implanted nanoparticles were superparamagnetic with average diameters of ∼4 nm. The high field moment followed Bloch’s T3/2 law where T is the temperature and there was a spin-glass component from disordered surface spins that was not previously reported for an implanted film with a Ni:Fe fluence ratio of 82:18 made under similar conditions. EBA led to non-spherical near surface Ni1−xFex nanoparticles with widths ranging from ∼60 nm to ∼220 nm where some of the nanoparticles were surrounded by a Fe1-z1Siz1Oz2 shell. Unlike the previous report on a 82:18 film, we do not find a bimodal particle size distribution with a significant fraction of smaller nanoparticles. The saturation moment is also less than that found in the bulk, which may be due to the Fe1-z1Siz1Oz2 shell that is antiferromagnetic or does not magnetically order. The saturation moment can be fitted to Bloch’s T3/2 law and a thin spin-disordered shell. Our results show that the Fe fraction in the ferromagnetic nanoparticles is less than the expected x = 0.56 and dual ion beam implantation and EBA with a Fe fluence fraction of between 0.56 and 0.18 could result in pure Ni1−xFex nanoparticles without excess Fe or a significant number of smaller nanoparticles.