Tuneable Magnetic Properties of 3D Transition Metals Using Carbon Allotropes

Abstract

This research describes changes in the magnetic properties of 3d transition metals with hybrid nanocarbon and molecular carbon interfaces. Sputtered amorphous carbon (a-C) offers a simple and cheap pathway to tune the magnetic properties of transition metal thin films for magnetic memories and different spintronic applications. Here, amorphous nanocarbon films were RF-sputtered. Annealing the films changed the structure from amorphous to nanocrystalline as determined by Raman spectroscopy. The RF-sputtered a-C films were then coupled with pure iron (Fe) and iron nitride (FexN(1−x)) to study the magnetic properties for both hybrid interfaces at different annealing temperatures. Before annealing, hybridisation at the Fe/a-C interface leads to magnetic softening, with a reduction in coercivity (Hc) up to a factor of 5 for a Fe/a-C/Fe trilayer and a 10-30% lower saturation magnetisation as a function of the metal film thickness. On the other hand, after annealing, inter-diffusion and graphitisation of the carbon layer results in increased coercivity, also by a factor of 5, with the highest energy product being of the order of 53 kJ per m3 of iron in a 10 nm film annealed at 500◦C. Kerrmicroscopy images of the films show a reduction in domain size and increased pinning points after annealing. Similar results were obtained when Fe is doped with nitrogen, with a BH(max) of 9 kJ per m3 in a 9 nm FexN(1−x)/with RF-sputtered a-C was annealed at 500 ◦C. RF-sputtered carbon overlayers and post-processing can therefore be used to tune the anisotropy, domain configuration and magnetic properties of metallic thin films in a synthesis methodology that, for some applications in thin film technologies, could be simpler and cheaper than the use of heavy noble metals and/or rare earths. C60 was also coupled with Fe and Co and the resulting Fe/C60 bilayer samples show changes in Hc after annealing when the metallic layer is > 4 nm. Low temperature experiments at Co/C60 exhibited massive increases in coercivity (Hc). The Hc also rose as a result of annealing the film. Low temperature MR measurements point to an exchange spring effect. The research findings emphasise the importance of the hybridisation effects between the nanocarbon and molecular π orbitals and 3d bands of metals in controlling the magnetic properties at the interfaces before and after annealing.