Tuneable Magnetic Properties of 3D Transition Metals Using Carbon Allotropes

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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.