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 cm−3 and 1 × 1021 cm−3 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.