Tuneable Magnetic Properties of 3D Transition Metals Using Carbon Allotropes
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.