Exchange bias and large room temperature magnetoresistance in ion beam-synthesized Co nanoparticles in SiO2

Magnetic nanogranular materials have shown promising magnetic and electron transport properties, offering key advantages for a range of applications from spintronics components to magnetic field sensors. In this paper, the properties of Co nanoparticles synthesized using low-energy ion implantation and electron beam annealing (EBA) on SiO2 were investigated. EBA leads to the growth of crystalline face-centred cubic Co nanoparticles from small nanoparticles within a Co-rich region in the near surface. The as-implanted and EBA samples are ferromagnetic with Curie temperatures above 300 K. The saturated magnetic moment per implanted Co atom was measured to be as high as 4.25 +/- 0.5 mu(B). The moment per Co atom decreases and approaches that of bulk Co with increased EBA time. This suggests that there may be a ferromagnetic Co1-xSixOy phase that has not been previously reported. An exchange bias is observed and proposed to arise from a thin antiferromagnetic CoO layer surrounding the Co nanoparticles. We find a room temperature magnetoresistance as high as 22.8% at 8 T with linear behaviour above similar to 3 T. The linear magnetoresistance is likely to be due to a geometric magnetoresistance that is observed in inhomogeneous nanomaterials containing metallic nanoparticles in a semiconducting matrix. Thus, EBA leads to Co nanoparticles that are expected to be electronically spin polarized but there is no evidence for spin-dependent tunnelling. These unique characteristics could provide the base for novel devices.