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

Highlights

• 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 T^3/2 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.

Abstract

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 Ni_1−xFe_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_yFe_1−ySi_zO_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.

Introduction

Magnetic nanoparticles are being actively researched because they have a wide variety of potential applications that include magnetic resonance imaging contrast agents [1], [2], medical treatment and diagnostics [1], [2], waste water treatment [3], high density magnetic memory [4], [5], [6], and magnetic field sensors [7], [8], [9]. The nanoscale physics is different when compared with the bulk where the coercivity and magnetocrystalline anisotropy can depend on the nanoparticle size [10], [11], [12]. The magnon dispersion is known to change for small nanoparticles where the small nanoparticle size can lead to the opening of a magnon gap [13], [14], [15]. When the nanoparticle radius becomes small enough it is possible for the thermal energy to exceed the magnetocrystalline anisotropy energy and this leads to superparamagnetism with negligible hysteresis [16]. This can be advantageous for magnetic sensing applications where negligible hysteresis makes it easier to measure small magnetic fields. Nanostructured composites containing magnetic nanoparticles can display magnetotransport properties not seen in the bulk [7], [17], [18], [19]. For example, spin tunnelling between nanoparticles in a semiconducting matrix can lead to large magnetoresistances [17], [18], [19].

Different methods are being pursued to create magnetic nanoparticles. Most nanoparticles have been made in powder form using chemical solution methods [14], [15], [19], [20], [21], [22], [23], [24], [25], [26]. Nanoparticle composites have been made by mixing powders in polymers [27] and they have also been made by pulsed laser deposition [28], [29] and other methods [30]. Ion implantation is a relatively new method that can be used to create nanostructured magnetic materials [7], [11], [12], [31], [32], [33], [34], [35]. The advantages of ion implantation over other methods include the ability to control to high precision the depth and concentration at a nanoscale level. We have recently shown that low energy Fe ion implantation into SiO₂ followed by electron beam annealing (EBA) leads to superparamagnetic Fe nanoparticles in the surface region that have a large room temperature magnetoresistance that can be useful for magnetic sensing applications [7]. A range of other magnetic nanoparticles have been made by ion implantation into SiO₂ that includes Ni [11], [12], [33] and Co [12] nanoparticles. Dual implantation of Co and Pt has been reported that leads to an enhanced coercivity and magnetic remanence [12] but there have been no reports of dual Fe and Ni implantation. Implanting Fe and Ni might be expected to lead to Ni_1−xFe_x where the bulk compound is known to display a large anisotropic magnetoresistance, low coercivity, and high permeability [16].

In this paper, we report the results from Rutherford backscattered spectrometry (RBS), transmission electron microscopy (TEM) and magnetization measurements on SiO₂ films implanted with Ni and Fe ions at low energies before and after EBA. We show below that as-implanted films contain superparamagnetic nanoparticles and EBA at 1000 °C for 1 h leads to ferromagnetic order and a bimodal size distribution and a different nanoparticle depth distribution that is not seen after only Fe or Ni ion implantation and EBA.