Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 231, 433 (2005)
Nuclear Microprobe Technology and Applications

D. Spemann, P. Esquinazi, R. Höhne, A. Setzer, M. Diaconu, H. Schmidt and T. Butz

Abstract
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 M_r(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.