The evolution of magnetism for graphene oxide (GO) before and after chemical reduction was investigated by means of static magnetization and electron spin resonance (ESR) spectroscopy. Strong paramagnetism with a saturation magnetization of ∼1.2 emu/g and weak antiferromagnetic interactions were identified in pristine GO. Apart from spin-half defect centers, ESR spectroscopy indicated the excitation of high spin states, consistently with the high spin (S = 2) magnetic moments derived from the magnetization analysis, corroborating the formation of spatially “isolated” magnetic clusters in GO. A marked reduction of GO's magnetization (∼0.17 emu/g) along with an appreciable rise of diamagnetism (−2.4 × 10−6 emu/g Oe) was detected after chemical reduction by sodium borohydride, reflecting the drastic removal of paramagnetic defects and the concomitant growth of sp2 domains in reduced graphene oxide (rGO). ESR revealed a large drop of the spin susceptibility for rGO, which, in addition to the main paramagnetic Curie component, showed an appreciable Pauli contribution. The latter together with the g-factor shift and the broadening of the ESR line indicated the coupling of localized spins with conduction electrons. The rGO ESR signal presented a metallic line shape, which could be analyzed in terms of two separate spectral components, a broad one that may be related to defect states strongly coupled with itinerant spins within the sp2 clusters and a narrow one due to edge/vacancy defect spins, indicative of rGO's persistent structural inhomogeneity.
Ferromagnetic resonance (FMR) and ac conductivity have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3O4) and cementite (Fe3C) nanoparticles (30–50nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron. Ac conductivity measurements showed thermally activated behavior involving a range of activation energies and power law frequency dependence at high frequencies similar to conducting polymer composites randomly filled with metal particles. Ferromagnetic resonance measurements revealed a relatively narrow FMR line at high temperatures indicating the presence of ferromagnetic nanoparticles, where thermal fluctuations and interparticle interactions determine the FMR temperature variation. An abrupt change of the FMR spectra was observed at T<81K (ΔT⩽1K) coinciding with a sharp anomaly resolved in the temperature derivative of the ac conductivity. This behavior is attributed to the Verwey transition of Fe3O4 nanoparticles, where the concurrent skin depth variation unveils the FMR of large magnetite conglomerates and thus allows discriminating their contribution from relatively isolated nanoparticles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.