Formation of magnetic nanoparticles by low energy dual implantation of Ni and Fe into SiO2

• First study of dual Ni and Fe low-energy ion implantation into SiO2.
• Small superparamagnetic nanoparticles are observed after implantation.
• These nanoparticles do not follow Bloch T3/2 law.
• Electron beam annealing leads to a bimodal size distribution and magnetic ordering.
• A spin-glass from uncompensated moments at the nanoparticle surface was observed.

Magnetic nanoparticles have been made by Ni and Fe implantation into a SiO2 film with a Ni:Fe ratio of 82:18 both before and after electron beam annealing (EBA). Superparamagnetic nanoparticles with diameters ∼4 nm were observed after implantation. The moment per implanted ion at high magnetic fields was significantly lower than that reported for bulk Ni1−xFex with a similar x, which may be due to some implanted ions not magnetically ordering and the appearance of antiferromagnetic phases. The high field moment did not follow Bloch's T3/2 law where T is the temperature. This behaviour is likely to be due to spin-waves propagating in the nanoparticle/NiyFe1−ySizOn matrix as well as the effect of disordered spins on the surfaces of the nanoparticles. After EBA, a bimodal size distribution was observed with large isolated particles closer to the surface and smaller nanoparticles further into the film. Such a distribution has not been previously reported for similar Fe or Ni implantation. All of the Ni and Fe moments have magnetically ordered and the high field moment can be modelled using Bloch's law and a small contribution from disordered moments in the shell with an average thickness of ∼0.3 nm.