Magnetic carbon: A new application for ion microbeams

In this study ferromagnetic microstructures in highly oriented pyrolytic graphite (HOPG) were created by 2.25MeV proton microbeam irradiation and characterized using AFM/MFM and SQUID measurements. For this purpose, spots of 1.8×1.8μm2 size as well as cross-like patterns of 15μm×15μm size were irradiated with different ion doses. Furthermore, arrays of 600μm×600μm size containing 100×100 micro-spots were produced with ion doses of ∼0.3nC/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 ∼0.6×10−5emu and the remanent magnetization Mr(B=0) up to 4.5×10−4emu/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.