Direct ESR evidence for magnetic behavior of graphite oxide

Recently, there have appeared theoretical works on the magnetic properties of graphene and graphene nanoribbons envisaging possible spin-based applications along with fundamental scientific insight. The theoretical efforts, however, appear not paralleled by experimental investigation to test magnetic properties. Yet, room temperature ferromagnetism (RTFM) has recently been experimentally reported in few layer graphene oxide (G-600) Wang et al. (Nano Lett., 9, 2009, 220), the origin of which remains still unexplored. In an attempt to trace the origin of RTFM, we here report on low temperature K-band electron spin resonance (ESR) observations on G-600. Two distinct C-related paramagnetic signals are revealed, both of a Lorentzian shape: (a) a broad signal at g=2.00278, which can be attributed to graphitic-like (GL) carbon; (b) a narrower signal at g=2.00288, which is associated with free radical like (FL) carbon. No other signals could be detected. We speculate that the GL ESR signal may come from the conductive π-carriers propagating in the interior of graphite oxide sheets, while the FL ESR signal may stem from the edges of the sheets due to non-bonding localized electronic states. It is suggested that the long range direct/indirect exchange interaction between GL and FL C-related magnetic spin centers may lead to the reported RTFM, thus pointing to the C origin of the RTFM.