Reply to Comment by D. Spemann et al [EPL 98 (2012) 57006, arXiv:1204.2992]

This article is a reply to the Comment by D. Spemann et al (arXiv:1204.2992) in response to our paper 'Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite' (EPL, 97 (2012) 47001).Comment: Reply to arXiv:1204.2992 Comment by D. Spemann et al re arXiv:1201.6374 by Sepioni et a

Revealing common artifacts due to ferromagnetic inclusions in highly-oriented pyrolytic graphite

We report on an extensive investigation to figure out the origin of room-temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray microanalysis revealed the presence of micron-size magnetic clusters (predominantly Fe) that are rare and would be difficult to detect without careful search in a scanning electron microscope in the backscattering mode. The clusters pin to crystal boundaries and their quantities match the amplitude of typical ferromagnetic signals. No ferromagnetic response is detected in samples where we could not find such magnetic inclusions. Our experiments show that the frequently reported ferromagnetism in pristine HOPG is most likely to originate from contamination with Fe-rich inclusions introduced presumably during crystal growth.Comment: 8 pages, 7 figure

Limits on intrinsic magnetism in graphene

We have studied magnetization of graphene nanocrystals obtained by sonic exfoliation of graphite. No ferromagnetism is detected at any temperature down to 2 K. Neither do we find strong paramagnetism expected due to the massive amount of edge defects. Rather, graphene is strongly diamagnetic, similar to graphite. Our nanocrystals exhibit only a weak paramagnetic contribution noticeable below 50K. The measurements yield a single species of defects responsible for the paramagnetism, with approximately one magnetic moment per typical graphene crystallite.Comment: 2nd version, modified in response to comment

Coherent, mechanical control of a single electronic spin

The ability to control and manipulate spins via electrical, magnetic and optical means has generated numerous applications in metrology and quantum information science in recent years. A promising alternative method for spin manipulation is the use of mechanical motion, where the oscillation of a mechanical resonator can be magnetically coupled to a spins magnetic dipole, which could enable scalable quantum information architectures9 and sensitive nanoscale magnetometry. To date, however, only population control of spins has been realized via classical motion of a mechanical resonator. Here, we demonstrate coherent mechanical control of an individual spin under ambient conditions using the driven motion of a mechanical resonator that is magnetically coupled to the electronic spin of a single nitrogen-vacancy (NV) color center in diamond. Coherent control of this hybrid mechanical/spin system is achieved by synchronizing pulsed spin-addressing protocols (involving optical and radiofrequency fields) to the motion of the driven oscillator, which allows coherent mechanical manipulation of both the population and phase of the spin via motion-induced Zeeman shifts of the NV spins energy. We demonstrate applications of this coherent mechanical spin-control technique to sensitive nanoscale scanning magnetometry.Comment: 6 pages, 4 figure

Spin-half paramagnetism in graphene induced by point defects

Using magnetization measurements, we show that point defects in graphene - fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene's structural stability.Comment: 14 pages, 14 figure

Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons

Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While the low temperature ferromagnet-like feature is observed in both types of ribbons, such room temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a 'positive exchange bias'. Electron spin resonance measurements infer that the carbon related defects may responsible for the observed magnetic behaviour in both types of ribbons. Furthermore, proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance provides no indications for the presence of potassium (cluster) related signals, emphasizing the intrinsic magnetic nature of the ribbons. Our combined experimental results may infer the coexistence of ferromagnetic clusters with anti-ferromagnetic regions leading to disordered magnetic phase. We discuss the origin of the observed contrast in the magnetic behaviours of these two types of ribbons

sp-Electron Magnetic Clusters with a Large Spin in Graphene

Motivated by recent experimental data (Sepioni, M. et al. Phys. Rev. Lett. 2010, 105, 207205), we have studied the possibility of forming magnetic clusters with spin S> 1/2 on graphene by adsorption of hydrogen atoms or hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface of graphene during the delamination of HOPG led to the formation of seven-atom or seven-OH-group clusters with S=5/2 that were of a special interest. The coincidence of symmetry of the clusters with the graphene lattice strengthens the stability of the cluster. For (OH)7 clusters that were situated greater than 3 nm from one another, the reconstruction barrier to a nonmagnetic configuration was approximately 0.4 eV, whereas for H7 clusters, there was no barrier and the high-spin state was unstable. Stability of the high-spin clusters increased if they were formed on top of ripples. Exchange interactions between the clusters were studied and we have shown that the ferromagnetic state is improbable. The role of the chemical composition of the solvent used for the delamination of graphite is discussed.Comment: 22 pages, 1 table, 4 figures. Minor changes, few refs added. Accepted to ACS Nan

Structure and magnetism of disordered carbon

Contains fulltext : 111434.pdf (preprint version ) (Open Access

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping

Contains fulltext : 123194.pdf (preprint version ) (Open Access