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Graphical abstract Highlights • Near-surface superparamagnetic Ni 1−x Fe x nanoparticles in SiO 2 with x = 0.47 and 0.20 but the same Ni fluence. • Higher fraction of ordered moment in the cores for x = 0.47. • Spin-stiffness highest for x = 0.20. • Negligible demagnetization and magnetic dipole interactions between nanoparticles. • Enhanced superparamagnetic dM/dH for x = 0.47 due to larger nanoparticle sizes. Abstract Structural and magnetic measurements were performed on SiO 2 films containing near-surface Ni 1− x Fe x superparamagnetic nanoparticles made by low energy ion implantation into SiO 2 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− x Fe 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. d M /d H 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 d M /d H is consistent with that expected from a derivative of the Langevin function with negligible demagnetization where larger nanoparticles are expected to have a higher d M /d H. [ABSTRACT FROM AUTHOR]

Fe 3 O 4 nanoparticles have been made by a co-precipitation method with an average size of 13 nm. Raman measurements show that there is also a small fraction of maghemite that is not seen in the XRD data. The saturation magnetization is high for this preparation method and the coercive field is low at 300 K. The high field magnetic moment cannot be fitted to a Bloch temperature dependence over the full temperature range, which is likely due to the effect of the Verwey transition at 120 K. Pressed powders shows a magnetoresistance of up to −6.5% at 8 T and 300 K. The magnetoresistance can be fitted to a model where there is a spin-dependent tunnelling between nanoparticles with a spin-disordered shell. The temperature dependence of the resistivity can be attributed to nanoparticle electrostatic charging effects. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles have been made by Ni and Fe implantation into a SiO 2 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 Ni 1− x Fe x 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 T 3/2 law where T is the temperature. This behaviour is likely to be due to spin-waves propagating in the nanoparticle/Ni y Fe 1− y Si z O n 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. [ABSTRACT FROM AUTHOR]

Highlights: [•] New method of arc discharge used to synthesise permalloy containing nanoparticles. [•] The highest quality powders were made using a 78% Ni permalloy rod in N2. [•] The Saturation moment was slightly less and the coercive field was low (3mT). [•] MR contributions from the spin-dependent tunneling between the particles. [Copyright &y& Elsevier]