Clusterization versus interaction

1. Experimental study of defective graphite and defective diamonds: a possible way to bulk magnetic carbon? Clusterization versus interaction

2. Oxygen eroded graphite it would require a ferromagnetic cluster of about 2600 ppm of Fe. As previously reported, the total content of iron in our samples was determined to be around 40–60 ppm range. Other ferromagnetic metals such as nickel and cobalt were detected at concentrations around 1 ppm 265 ppm was measured in this sample

3. Oxygen-eroded graphite weight • AM1 Graphite + 1% B: it is ferromagnetic (special pressure conditions) • AM108 Graphite + 5% B: diamagnetic • AM312 Graphite + 1% B: diamagnetic (the same idea as 10-8) • AM322 crystalline graphite (non-magnetic), obtained from the simple graphite - CuO reaction, where the ferromagnetic graphite is obtained No boron in it. • AM331 Graphite, similar as 32-2, but obtained with BaO2 (barium peroxide) instead of CuO

4. Temperature dependence for all samples Paramagnetic upturn has no correlation with the ferro properties! AM1 Graphite + 1% B: Note: 312 and 1 have the same Curie tail, but 312 is not ferro! AM312 Graphite + 1% B: AM322 crystalline graphite, No boron in it. AM331 Graphite, similar to 32-2, but obtained with BaO2 AM108 Graphite + 5% B: No boronno tail The most diamagnetic is 108, but at the same time it has the greatest paramagnetic contrubution. More boron, more Curie tail

5. RT field dependencies

6. AM312 AM331 AM1 Nonlinearity at 1.9 K AM322 crystalline graphite 10 milli AM331 Graphite with BaO2 10 milli AM108 Graphite + 5% B: 20 milli AM312 Graphite + 1% B: 8 milli AM1 Graphite + 1% B: 160 milli Control sample of graphite with 0 ppm iron: 1 milli

9. AM312: nonlinearity is absent at 10 K. Red – 1.9 K

10. AM1: the same nonlinearity at RT and 1.9 The response of AM1 is 1.5 times higher that could be expected from all iron atoms taken together. This can be in the case if iron nanoclusters are tens atoms in size. However, these clusters will behave superparamagnetically which is not observed. From the field dependencies at 1.8 and 295 K we estimated that the cluster sized should exceed 1000000 atoms. Such clusters should be seen in transmission microscopy Clusters should be about 50 nm in size But why iron, not oxide or cabide? Formation of air stable carbon-skinned iron nanocrystals from FeC2 Kentaroh Kosugi, M. Junaid Bushiri, and Nobuyuki NishiAppl. Phys. Lett., Vol. 84, No. 10, 8 March 2004 The segregated carbons grow as graphitic sheets parallel to the growing Fe lattice plane. This direct bonding is due to an accidental matching of the Fe–Fe distance 2.866 Å with that of the C1–C4 distance ~2.842 Å of the hexagonal rings in graphite

11. We present a study of the changes in the magnetic and electronic properties of small, deposited Fe clusters upon exposure to the graphite surface. The clusters exhibit strong X-ray magnetic circular dichroism (XMCD) at the L3 edge while matrix isolated in a thin Ar film. XMCD and photoemission experiments show that the clusters are driven into a nonmagnetic state by the interaction to graphite. Our results support earlier calculations for adatoms and dimers and extend their validity to larger cluster sizes. They also provide a basis for an understanding of the magnetic properties of carbon encapsulated transition metal particles. Small Fe, Co and Ni clusters possess enhanced magnetizations compared to the respective bulk materials when formed of less than 500–700 atoms. Interestingly, recent magnetic studies on moderately size selected FeN (N 300) clusters deposited on graphite [11] have not fully reproduced these strong magnetization enhancements. Likewise, carbon encapsulation, while being an efficient way to protect transition metal particles from oxidation, generally leads to cluster ensembles displaying reduced magnetizations at small particle sizes

12. We have presented a detailed in situ analysis of the interaction of small Fe clusters with the graphite surface. While the clusters exhibit strong L2;3- XMCD when matrix isolated in an Ar thin film, the magnetic contrast in X-ray absorption is entirely lost, when the substrate interaction is switched on. Changes in spectral position and line shape of the resonant L3 absorption indicate the reorganization of the clusters’ electronic level system, in agreement with previous predictions for transition metal adatoms and dimers on graphite. 3s – photoelectron spectra show, that the ground state of the exposed Fe clusters is indeed nonmagnetic. Whether this observation is related to the formation of hcp iron clusters on HOPG deserves careful consideration in future experiments. On the basis of our present results and considering other experimentalfindings on magnetic nanoparticles we are led to believe that the presence of graphite-like carbon is detrimental to nanoscale magnetic materials formed from the late 3d transition metals

13. Indeed, transmission microscopy revealed the spherical nanoclusters about 20 nm in diameter (ca.30 000 atoms) which were suspected to be iron clusters. However, diffraction patterns of the spherical particles look similar to carbon diffraction patterns. This, the problem of distribution of iron atoms in ferromagnetic graphite matrix remains an open question.

14. Nanodiamonds • Another carbon nanostructure which can help understanding the metal-dependent or metal-independent magnetic ordering in carbon structures is nanodiamond. Nanodiamond is a 5-nm sp3 hybridized particle surrounded by 2 – 3 layers of graphite. We have investigated pristine nanodiamonds, nanodiamonds intercalated with copper, nanodiamonds intercalated with copper and annealed in hydrogen at different temperatures. • All samples had linear field dependence of magnetization at room temperature, showing that the nanodiamonds are thoroughly purified from iron. • Annealing pristine nanodiamonds in hydrogen leads to the antiferromagnetic ordering, as it was previously described in [Osipov et al. Fuller, Nanot, Car. Nan. 14, 565 2006].

15. A disorder network of nanographites was investigated, where each nanographite has about 1 edge-inherited localized spin. The susceptibility for the samples situated around the metal-insulator threshold shows a cusp around 4–7 K in addition to the presence of a field-cooling effect. These behaviors are explained in terms of disordered magnetism caused by random strengths of inter-nano-graphite antiferromagnetic interactions mediated by p-conduction carriers.

16. For the acid-treated nanodiamond sample, the magnetization (M–H) curve measured at T = 1.9 K in magnetic fields H up to 5.5 T can be interpreted as a sum of the temperature-independent linear (M–H) curve with a negative slope and the Brillouin curve with a positive contribution of S = 1/2. This suggests that the magnetic phenomena are associated only with the localized defects with S = 1/2. The positive magnetization curve fitted to the Brillouin curve with S = 1/2 yields the concentration of localized spins of 4.2 · 1019 spins/g. No contribution of the transition metal ions (spin value S > 1/2) to the magnetization curve was observed. This supports the conclusion from the ESR estimations of the impurity concentrations that the residual paramagnetic impurities in the sample do not affect the intrinsic magnetic properties of the nanodiamond powder. The concentration of intrinsic paramagnetic centers estimated from the magnetization curve corresponds to several spins per nanoparticle The number of the surface carbon atoms in a diamond nanoparticle is 2400 Small number of dangling bonds, as compared with the total number of the surface carbon atoms in a particle, indicates that the surface bonds of a diamond core are almost totally terminated and the observable paramagnetic centers are due to the structural defects in the core or within the strained surface layer of the core.

18. Dependence of Hpp vs. Ns in the range of 3 · 1018– 6.5 · 1018 spins/g is roughly linear. This range of spin concentration corresponds to the well-graphitized nanographite particles, demonstrating a single ESR signal with a g-factor 2.0013–2.0014. Because in this range the number of edge-localized spins per particle is around 1 the ESR linewidth can be explained by the spin relaxation, in which the interaction between the localized spins is neglected. The spin relaxation takes place in the vicinity of the edges, where the edge-state spins interact with local phonon modes that are confined to the edges. Linewidth increases drastically for onions having ca. 2 spins per one particle. Therefore, the magnetic interaction between the localized spins in the individual particle plays an important role in the spin relaxation mechanism. The large linewidth indicates the magnetic interaction between the spins in the spin relaxation process, where the linewidth is determined by the competition between the dipole–dipole interaction and the exchange interaction.

19. In addition to characteristic structural defects, originating from dangling C–C bonds of sp3 sites and located mainly in the interior of the nanocrystals, the hydrogen-terminated ND crystals show a high concentration of excess free radicals (up to 1021 spin/g), which are due to structural defects (dangling C–C bonds) induced on the surface of diamond nanocrystals by hydrothermal treatment. Strong antiferromagnetic coupling is found between the spins localized on the surface. The specific features observed in the range below 70K for the samples treated 1, 6 and 24 hours can be related with the contributions from pairs and triads (dimers and trimers) of surface spins S ½ antiferromagnetically coupled to the total susceptibility. For the 24 hours treated sample there are ca. 7 dimers per particle and the exchange constant is approximately J/kB = 50 K, This conclusion is also well proved by the fact that the ensemble average number of surface spins (or dangling bonds) laying on one facet equals ca. 3 for the case the HyT treatment of 24 hours. Thus, the increase in the 2D density of surface spins due to long time HyT treatment leads to the decreasing in the average distance between the spins and as a result, the enhanced AFM coupling appears.

20. Samples: detonation sythesis nanodiamonds. Chamber is made from titanium. Postsynthesis treatment: NNO3 at 230 , HCl at 83 C 5 cycles 45 minutes each Intercalation with Cu, Au, and also with the transition metals Method: water solution of nitrates or acetates of these metals. Drying in hydrogen flow at room temperature, 550 or 900 C Granules 0.011mkm

22. Copper changes pi-electron structure • An interesting effect is found with copper intercalation. Copper is a diamagnetic metal; however, intercalation enhances paramagnetism of the samples. Simultaneously we observe the increase of the G peaks in the Raman spectra. One may speculate that this enhancement is due to the increase of graphitic contribution, but this is an unrealistic assumption. We conclude that copper resonantly enhances the pi-electron system, and from joint magnetic and Raman measurements we conclude that Cu does not form clusters but resides atom-by-atom on the nanodiamond surfaces.

23. Magnetic Force Microscopy measurements + Electric Force Microscopy: can one separate the magnetic signal from the signal due to the workfunction differences? The glove box is purged continuously by the weak nitrogen flow. The concentration of water vapors is controlled by the hygrometer. We pay so much attention to the quality of the atmosphere after the publication of Proksch [R. Proksch, Appl. Phys. Lett. 89 (2006) 113121 ] who points out that the presence of adsorbed water can influence the results of the experiments. In our MFM-EFM measurements we also observed light or dark regions in the images on the surface of freshly cleaved HOPG. We have found that water vapors condensate within ten minutes on the freshly cleaved HOPG surface, forming islands, mainly in the vicinity of the defects. Measurements of the thickness of the water layer give values about several nanometers. This gives strong response in the EFM measurements; 1 nm water layer increases twice the capacity between the tip and the sample at 80 nm height of the tip (water has the permittivity about 80 whereas the permittivity of air is close to unity).

24. Another effect that overlaps with the magnetic interactions in the system is the difference between the electron work functions W of the materials of the tip and the sample. This difference provides an electric bias between the tip and the sample. This effect is usually neglected in MFM studies of the metallic surfaces when the work function of the tip material is close to the work function of the material of the sample. However in the case of the carbon samples it is necessary to take this into account. For the Co/Cr tips W= 4.9 eV, for Pt/Ir W=5.5 eV. The work function of graphite is estimated in the range 4.5 - 4.9 eV. Surface contaminations can locally change the work function of the surface and provide undesirable contrast of the scans. Cleaving of the HOPG sample in nitrogen atmosphere allow reducing the effect of termination of the graphite plane edges by oxygen. . Parallel domain-like stripes that are frequently seen on the pictures are the results on the reflection of the laser light from the non-ideally flat surfaces and the interference of the reflected light. The main result of our work is that a large amount of magnetic features on carbon surfaces can be explained by the artefacts in the interpretation of the MFM data. However, in some cases there is evidence that the observed signal comes from the magnetic sources.

25. The most frequent defects are various linear features which do not contribute to the MFM images. These are folded strands and the majority of the step edges Magnetically dead intrinsic carbon defects.

26. Topology-related defects which always appear as a result of the proton bombardment. The characteristic feature is that they always look bright which corresponds to the repulsive signal, whereas the formation of paramagnetic defects must result in a dark (attractive) signals. AFM and MFM images of proton-bombarded graphite (14 x 14 m).

27. In order to explain the repulsive nature of the signal, we made the measurements of the phase shift at different voltages between the tip and sample. We obtained two portions of linear curves with a minimum at about -0.8 V which can be reasonably explained by the workfunction differences. The dependence of the MFM phase shift on the applied voltage. However, the fact that the applied voltage can compensate the magnetic signal, does not mean that the signal is not present et al. These two effects can mask each other.

28. In some cases we found the defects that are difficult to interprete without invoking magnetic interactions. These are the alternative repulsive-attractive signals and the defects which change attractive response to repulsive when the tip magnetization is reversed AFM MFM MFM

29. MFM conclusions Only sharp cleavage edges, and only some of them, show magnetic activity. The absence or presence of the magnetic contrast at step edges is not related to the numbers of stacking layers, It is not possible to ascribe the presence or absence of magnetic charges to either zigzag shaped edges or armchair shaped edges, respectively. The reason for this is that the zigzag direction is not a cutting direction for graphite: even if a linear defect is extended in the zigzag direction, the edge represents the series of zigzag-armchair patterns. Factors that may affect: relative amount on the zigzag patterns in the edge, different stacking sequences and/or the dangling bonds at the step edges either passivated with athmosphere atoms or free.