Magnetism of adsorbed oxygen on carbon nanohorns

Magnetic susceptibilities for adsorbed O2 on heat-treated single-wall carbon nanohorns (HT-SWNHs) showed anomalous features as compared with those on as-grown SWNHs. Magnetism for the latter case was similar to that for bulky solid O2, indicating the magnetic phase transitions associated with gaseous to liquid and in solid states (γ-β and β-α). On the other hand, magnetism associated with γ-β transition was not observed for O2 adsorbed on HT-SWNHs. For HT-SWNHs, O2 can get into the interior of SWNH from the nano-windows, which were opened by heat-treatment, and solidify. We therefore concluded that the γ-β transition was suppressed in the nano-meter sized space.