Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 219-220, 886 (2004)
Proceedings of the Sixteenth International Conference on Ion Beam Analysis

D. Spemann, K. -H. Han, P. Esquinazi, R. Höhne and T. Butz

Abstract
In this study ferromagnetic microstructures were created in highly oriented pyrolitic graphite (HOPG) by proton microbeam irradiation. For this purpose, spots of 1, 2 × 2 and 3.5 × 3.5 μm² were irradiated with ion fluences ranging from 3.1 × 10¹⁶ to 4.7 × 10¹⁹ cm⁻² using 2.25 MeV proton and 1.5 MeV helium ion microbeams. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 × 10¹⁹ and 4 × 10²² cm⁻³ for the proton irradiation. The irradiated spots, which were characterized with atomic force microscopy (AFM) and magnetic force microscopy (MFM), show a clear swelling of the HOPG crystal proportional to the ion fluence. Strong magnetic force gradients were found even for the lowest proton fluences. Contrary to the topography, the magnetic force gradient changes after the application of a magnetic field. This rules out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to much weaker magnetic signals only, which indicates that hydrogen plays a significant role in the formation of the magnetic moments and ordering. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements.