Magnetic carbon: A new application for ion microbeams

In this study ferromagnetic microstructures in highly oriented pyrolytic graphite (HOPG) were created by 2.25 MeV proton microbeam irradiation and characterized using AFM/MFM and SQUID measurements. For this purpose, spots of 1.8 × 1.8 µm² size as well as cross-like patterns of 15 µm × 15 µm size were irradiated with different ion doses. Furthermore, arrays of 600 µm × 600 µm size containing 100 × 100 micro-spots were produced with ion doses of ~0.3 nC/µm². As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 × 1018 cm³ and 1 × 1021 cm³ for the range of ion doses used. The irradiated crosses showed strong magnetic signals and a complex domain structure in the MFM images. SQUID measurements on a HOPG sample successively irradiated with micro-spots arrays showed that the irradiation increased the magnetic moment of the sample up to ~0.6 × 10-5 emu and the remanent magnetization Mr(B = 0) up to 4.5 × 10-4 emu/g. However, annealing effects during irradiation have to be carefully taken into account as they can result in a complete loss of the magnetic ordering in the irradiated areas.