We report production of nanostructured carbon foam by a high-repetition-rate,
high-power laser ablation of glassy carbon in Ar atmosphere. A combination of
characterization techniques revealed that the system contains both sp2 and sp3
bonded carbon atoms. The material is a novel form of carbon in which
graphite-like sheets fill space at very low density due to strong hyperbolic
curvature, as proposed for ?schwarzite?. The foam exhibits ferromagnetic-like
behaviour up to 90 K, with a narrow hysteresis curve and a high saturation
magnetization. Such magnetic properties are very unusual for a carbon
allotrope. Detailed analysis excludes impurities as the origin of the magnetic
signal. We postulate that localized unpaired spins occur because of topological
and bonding defects associated with the sheet curvature, and that these spins
are stabilized due to the steric protection offered by the convoluted sheets.Comment: 14 pages, including 2 tables and 7 figs. Submitted to Phys Rev B 10
September 200
