Magnetic properties of graphene

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Abstract

Graphene – a monolayer of carbon atoms densely packed in a honeycomb lattice – was isolated for the first time in 2004 and, since then, has established itself as one of the most remarkable materials available to condensed matter scientists today. Theory predicts a whole spectrum of magnetic phenomena in graphene, including several mechanisms for intrinsic ferromagnetism and spin-ordering effects that arise due to its low-dimensionality and highly unusual electronic properties (e.g. Dirac-like spectrum). In this experimental work, SQUID (Superconducting Quantum Interference Device) magnetic measurements have been carried out in graphene laminates with masses up to ≈ mg obtained by ultrasonic exfoliation of highly oriented pyrolytic graphite (HOPG) in N-methyl-pyrrolidone (NMP). Scanning electron microscopy (SEM) and X-ray diffraction experiments revealed that the laminates are made of decoupled graphene crystallites with typical flake size below 50 nm. Atomic force microscopy (AFM) measurements carried out for graphene suspensions dispersed onto a SiO2 substrate allowed the recognition of thin crystallites associable to single and double-layer graphene. X-ray dispersive fluorescence (XRF) and electron dispersive diffraction (EDX) confirmed the laminates chemical purity with absence of metals and/or magnetic inclusions. Pristine laminates exhibit Curie paramagnetism noticeable below ≈ 50 K, which contributes to about one moment per crystallite at 2 K. The laminates are strongly diamagnetic, although a decrease of the diamagnetic susceptibility by about three times with respect to graphite was observed for fields applied perpendicularly to the ab plane. The same graphene laminates were employed as a reference system to study magnetism of point defects, such as fluorine adatoms and vacancies generated through ion irradiation. The unambiguous spin value J=1/2 found for both species of defects confirms theoretical expectations. In the case of fluorine atoms a magnetic moment of 1 µB per ≈ 1000 adatoms was obtained, associated to the tendency of fluorine to cluster in graphene. Vacancies produced a value of the magnetic moment much closer to the expected 1 µB for point defects. No sign of defect related ferromagnetism was observed. On the other hand, our study performed on NT-MTD HOPG crystals (the same adopted for the fabrication of our graphene laminates), revealed ferromagnetic signals up to 3∙10-3 emu/g. Backscattering electron microscopy (BSE), performed alongside EDX chemical analysis, confirmed that the observed magnetic behaviour is due to ferromagnetic inclusions, such as magnetite and titano-magnetite. Therefore, weak and poorly reproducible ferromagnetic-like signals in graphene laminates were attributed to the same contaminations present in the original material (i.e. HOPG).

Keyword(s)

graphene; magnetism; spin

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