Defects in graphite may be magnetic and magnetostrictive as revealed by scanning tunneling microscopy

摘要

We use scanning tunneling microscopy to measure magnetic field induced strains in highly oriented pyrolytic graphite. This is done by using a scanning tunneling microscope with some magnetic components, which however do not produce an observable response within our resolution in the case of pure (99.999%) paramagnetic or diamagnetic metals (at the low field strengths applied). We study also ferromagnetic metals with this method for comparison. We find a relatively large (similar to that of permalloy) magnetostrictive response of graphite for the low applied field. The data shows saturation of the strain and also that the strain observed is localized and is not the cumulative strain from the mounted edge of the sample to the position of measurement, implying that volume is not conserved with the strains. We believe that the observed strains correspond to a signal of a ferromagnetic material and in this case may be due to the defects observed on the graphite planes.