Both experimental and theoretical studies of the magnetic properties of
micrographite and nanographite indicate a crucial role of the partial oxidation
of graphitic zigzag edges in ferromagnetism. In contrast to total and partial
hydrogenation, the oxidation of half of the carbon atoms on the graphite edges
transforms the antiferromagnetic exchange interaction between graphite planes
and over graphite ribbons to the ferromagnetic interaction. The stability of
the ferromagnetism is discussed.Comment: 14 pages, 6 figure

Boukhvalov, D. W.

da Silva, R. R.

Kopelevich, Y.

Moehlecke, S.

English

arXiv

Both experimental and theoretical studies of the magnetic properties of micrographite and nanographite indicate a crucial role of the partial oxidation of graphitic zigzag edges in ferromagnetism. In contrast to total and partial hydrogenation, the oxidation of half of the carbon atoms on the graphite edges transforms the antiferromagnetic exchange interaction between graphite planes and over graphite ribbons to the ferromagnetic interaction. The stability of the ferromagnetism is discussed. © 2011 American Physical Society.8323Makarova, T.L., Magnetic properties of carbon structures (2004) Semiconductors, 38 (6), pp. 615-638. , DOI 10.1134/1.1766362Miller, J.S., (1992) Adv. Mater., 4, p. 298. , ADVMEW 0935-9648 10.1002/adma.19920040414Miller, J.S., (1992) Adv. Mater., 4, p. 435. , ADVMEW 0935-9648 10.1002/adma.19920040614Murakami, Y., Suematsu, H., (1996) Pure Appl. Chem., 68, p. 1463. , PACHAS 0033-4545 10.1351/pac199668071463Makarova, T., Han, K.H., Esquinazi, P., Da Silva, R.R., Kopelevich, Y., Zakharova, I.B., Sundqvist, B., (2003) Carbon, 41, p. 1575. , CRBNAH 0008-6223 10.1016/S0008-6223(03)00082-4Kopelevich, Y., Esquinazi, P., Torres, J.H.S., Moehlecke, S., (2000) J. Low Temp. Phys., 119, p. 691. , JLTPAC 0022-2291 10.1023/A:1004637814008Esquinazi, P., Setzer, A., Höhne, R., Semmelhack, C., Kopelevich, Y., Spemann, D., Butz, T., Lösche, M., (2002) Phys. Rev. B, 66, p. 024429. , PRBMDO 1098-0121 10.1103/PhysRevB.66.024429Coey, J.M.D., Venkatesan, M., Fitzgerald, C.B., Douvalis, A.P., Sanders, I.S., Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite (2002) Nature, 420 (6912), pp. 156-159. , DOI 10.1038/nature01100Cervenka, J., Katsnelson, M.I., Flipse, C.F.J., (2009) Nature Phys., 5, p. 840. , 1745-2473 10.1038/nphys1399Fujita, M., Wakabayashi, K., Nakada, K., Kushakabe, K., (1996) J. Phys. Soc. Jpn., 65, p. 1920. , JUPSAU 0031-9015 10.1143/JPSJ.65.1920Shibayama, Y., Sato, H., Enoki, T., Endo, M., (2000) Phys. Rev. Lett., 84, p. 1744. , PRLTAO 0031-9007 10.1103/PhysRevLett.84.1744Son, Y.-W., Cohen, M.L., Louie, S.G., Half-metallic graphene nanoribbons (2006) Nature, 444 (7117), pp. 347-349. , DOI 10.1038/nature05180, PII NATURE05180Son, Y.-W., Cohen, M.L., Louie, S.G., Energy gaps in graphene nanoribbons (2006) Physical Review Letters, 97 (21), p. 216803. , http://oai.aps.org/oai?verb=GetRecord&Identifier=oai:aps.org: PhysRevLett.97.216803&metadataPrefix=oai_apsmeta_2, DOI 10.1103/PhysRevLett.97.216803Barone, V., Hod, O., Scuseria, G.E., Electronic structure and stability of semiconducting graphene nanoribbons (2006) Nano Letters, 6 (12), pp. 2748-2754. , DOI 10.1021/nl0617033Yazyev, O.V., Katsnelson, M.I., Magnetic correlations at graphene edges: Basis for novel spintronics devices (2008) Physical Review Letters, 100 (4), p. 047209. , http://oai.aps.org/oai?verb=GetRecord&Identifier=oai:aps.org: PhysRevLett.100.047209&metadataPrefix=oai_apsmeta_2, DOI 10.1103/PhysRevLett.100.047209Koskinen, P., Malola, S., Hakkinen, H., (2008) Phys. Rev. Lett., 101, p. 115502. , PRLTAO 0031-9007 10.1103/PhysRevLett.101.115502Girit, C.O., Meyer, J.C., Erni, R., Rossell, M.D., Kisielovski, C., Yang, L., Park, C.H., Zettl, A., (2009) Science, 323, p. 1705. , SCIEAS 0036-8075 10.1126/science.1166999Takahara, K., Takai, K., Enoki, T., Sugihara, K., (2007) Phys. Rev. B, 76, p. 035442. , PRBMDO 1098-0121 10.1103/PhysRevB.76.035442Kusakabe, K., Maruyama, M., (2003) Phys. Rev. B, 67, p. 092406. , PRBMDO 1098-0121 10.1103/PhysRevB.67.092406Boukhvalov, D.W., Katsnelson, M.I., (2008) Nano Lett., 8, p. 4378. , NALEFD 1530-6984 10.1021/nl802234nCampos-Delgado, J., Romo-Herrera, J.M., Jia, X., Cullen, D.A., Muramatsu, H., Kim, Y.A., Hayashi, T., Terrones, M., (2008) Nano Lett., 8, p. 2773. , NALEFD 1530-6984 10.1021/nl801316dKosynkin, D.V., Higginbotham, A.L., Sinitskii, A., Lomeda, J.R., Dimiev, A., Price, B.K., Tour, J.M., (2009) Nature (London), 458, p. 872. , NATUAS 0028-0836 10.1038/nature07872Soler, J.M., Artacho, E., Gale, J.D., Garcia, A., Junquera, J., Ordejon, P., Sanchez-Portal, D., The SIESTA method for ab initio order-N materials simulation (2002) Journal of Physics Condensed Matter, 14 (11), pp. 2745-2779. , DOI 10.1088/0953-8984/14/11/302, PII S0953898402307379Perdew, J.P., Zunger, A., (1981) Phys. Rev. B, 23, p. 5048. , PRBMDO 1098-0121 10.1103/PhysRevB.23.5048Janotti, A., Wei, S.-H., Singh, D.J., (2001) Phys. Rev. B, 64, p. 174107. , PRBMDO 1098-0121 10.1103/PhysRevB.64.174107Boukhvalov, D.W., Katsnelson, M.I., (2010) J. Phys. D: Appl. Phys., 43, p. 175302. , JPAPBE 0022-3727 10.1088/0022-3727/43/17/175302Monkhorst, H.J., Park, J.D., (1976) Phys. Rev. B, 13, p. 5188. , PRBMDO 1098-0121 10.1103/PhysRevB.13.5188Gonzalez, J.C., Munoz, M., Garcia, N., Barzola-Quiquia, J., Spoddig, D., Schindler, K., Esquinazi, P., Sample-size effects in the magnetoresistance of graphite (2007) Physical Review Letters, 99 (21), p. 216601. , http://oai.aps.org/oai?verb=GetRecord&Identifier=oai:aps.org: PhysRevLett.99.216601&metadataPrefix=oai_apsmeta_2, DOI 10.1103/PhysRevLett.99.216601Esquinazi, P., Speeman, D., Höhne, R., Setzer, A., Han, K.-H., Butz, T., (2003) Phys. Rev. Lett., 91, p. 227201. , PRLTAO 0031-9007 10.1103/PhysRevLett.91.227201Ohldag, H., Esquinazi, P., Arenholz, E., Spemann, D., Rothermel, M., Setzer, A., Butz, T., e-print arXiv: 0905.4315 (unpublished

Boukhvalov D.W.

Moehlecke S.

Da Silva R.R.

Kopelevich Y.

Repositorio da Producao Cientifica e Intelectual da Unicamp

Effect Of Oxygen Adsorption On Magnetic Properties Of Graphite

D. W. Boukhvalov

S. Moehlecke

R. R. da Silva

Y. Kopelevich

Crossref

Physical Review B

Effect of oxygen adsorption on magnetic properties of graphite

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Both experimental and theoretical studies of the magnetic properties of micrographite and nanographite indicate a crucial role of the partial oxidation of graphitic zigzag edges in ferromagnetism. In contrast to total and partial hydrogenation, the oxidation of half of the carbon atoms on the graphite edges transforms the antiferromagnetic exchange interaction between graphite planes and over graphite ribbons to the ferromagnetic interaction. The stability of the ferromagnetism is discussed.8323Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Boukhvalov, DW

Moehlecke, S

da Silva, RR

Kopelevich, Y

PHYSICAL REVIEW B 83, 233408 (2011)
Effect of oxygen adsorption on magnetic properties of graphite
D. W. Boukhvalov,1,* S. Moehlecke,2 R. R. da Silva,2 and Y. Kopelevich2,†
1School of Computational Sciences, Korea Institute for Advanced Study (KIAS), Hoegiro 87, Dongdaemun-Gu,
Seoul, 130-722, Korean Republic
2Instituto de Fı́sica Gleb Wataghin, Universidade Estadual de Campinas, Unicamp 13083-859, Campinas, São Paulo, Brazil
(Received 12 December 2010; revised manuscript received 7 March 2011; published 27 June 2011)
Both experimental and theoretical studies of the magnetic properties of micrographite and nanographite indicate
a crucial role of the partial oxidation of graphitic zigzag edges in ferromagnetism. In contrast to total and partial
hydrogenation, the oxidation of half of the carbon atoms on the graphite edges transforms the antiferromagnetic
exchange interaction between graphite planes and over graphite ribbons to the ferromagnetic interaction. The
stability of the ferromagnetism is discussed.
DOI: 10.1103/PhysRevB.83.233408 PACS number(s): 81.05.uf
The occurrence of ferromagnetic-type (FM-type) behavior
above room temperature in metal-free carbon-based (CB)
structures has been previously reported by various research
groups.1 In most cases, the measured magnetic moment has
been rather small, so care must always be taken regarding the
effects of possible traces of ferromagnetic impurities, such
as Fe, Co, and Ni.2 Nevertheless, there exists convincing
experimental evidence supporting the intrinsic character of
ferromagnetism in CB materials.1
In particular, Murakami and Suematsu3 produced fer-
romagnetic order in C60 fullerene crystals by irradiating
them with light from a xenon lamp in the presence of
oxygen. They showed that the diamagnetism characteristic
of fullerenes had been overwhelmed by paramagnetic or
ferromagnetic responses after irradiation of the sample in
an oxygen atmosphere. The decrease of the ferromagnetic
moment after annealing and its increase after leaving the
sample in air for several months argue against the contributions
of Fe impurities. The authors estimate a magnetic moment of
0.1 μB per C60 molecule. The temperature dependence of
the saturation magnetization revealed an extraordinarily high
Curie temperature, TC = 800 K. Makarova et al.4 reported
similar effects for laser- and electron-beam-illuminated C60
films obtained either in air or in an oxygen-rich atmosphere.
Importantly, x-ray structural analysis performed on those
C60 samples revealed the coexistence of the C60 phase with
clusters of graphene layers.4 This fact is notable because of
the occurrence of ferromagnetism in bulk graphite.5–8
Bearing in mind that, in all cases, magnetism in CB
materials is associated with graphene layers, it is instructive
to explore the role of zigzag edges.9–14 In fact, the results of
recent experiments8,15 speak for the key role of graphite zigzag
edges in magnetism. In line with these experiments, zigzag
half-metallic antiferromagnetism,11,12 energy-gap opening,13
and spiral-like magnetism14 have been theoretically predicted.
It is worth noting that, in contrast to unstable graphene
zigzag edges,15,16 the same edges in graphite are stable
at room temperature.8,17 Theoretical studies also suggest
that graphene-edge passivation affects magnetism.18,19 Both
theoretical19 and experimental20,21 work indicates the enor-
mous oxygen affinity of zigzag graphene edges, and complete
oxidation of these edges is expected to lead to a nonmagnetic
state due to the saturation of dangling bonds.19 In contrast to
graphene, zigzag edges in graphite do not reconstruct under
a nonoxidative cut. The production of graphite powder and/or
nanographites in an oxygen environment is the method used
to explore the role of oxygen in graphite-edge magnetism.
In this work, we report the results of density functional
theory (DFT) calculations for the effect of oxygen on the
zigzag-edge magnetism of graphene multilayers. We also
describe our experimental studies on the effect of oxygen
and other adsorbed gases on the magnetic properties of a
graphite powder. Density functional theory (DFT) calculations
were carried out with the pseudopotential code SIESTA.22 All
calculations were performed using the local density approxi-
mation (LDA),23 which is more suitable for the description
of graphite24 and was used for our previous modeling of
graphite functionalization.25 All calculations were carried out
for an energy mesh cutoff of 360 Ry and a 4 × 8 × 4 k-point
mesh in the Mokhorst-Park scheme.26 During optimization,
the electronic ground state was found self-consistently using
norm-conserving pseudopotentials for cores and a double-ζ
plus polarization basis of localized orbitals for carbon and iron.
Optimization of the forces and of total energies was performed
with accuracies of 0.04 eV/Å and 1 meV, respectively. When
creating images of the density of states, a smearing of 0.2 eV
was used.
To model realistic graphite, the nanoribbon was multiplied
along the z axis, using periodic boundary conditions and
taking into account AB (Bernal) stacking [Fig. 1(a)]. The total
number of carbon atoms in the studied nanographite supercell
is 128. To explore the specific role of oxygen, we modeled
total [Fig. 1(c)] and different types of partial [Figs. 1(d)–1(g)]
hydrogenation and oxidation.
Exchange energy is defined by the standard formula J =
(EFM − EAFM)/N , where N is the number of zigzag-edge
atoms with magnetic moments in the studied supercell. The
Curie temperature is calculated using the equation TC =
JS(S + 1)/3kB , where J is the sum of exchange energies
in all three (x is across the graphene planes, y is between
nearest neighbors in the zigzag edge, and z is between the
two graphene layers in graphite) directions (see Fig. 1) of
each magnetic atom on the zigzag edge, and S is the spin per
zigzag-edge step.
An activated graphite powder was prepared by cutting
and grinding a graphite rod at T = 300 K in different
atmospheres: Ar, He, N2, H2, O2, and air. The graphite rod
was from Carbon of America Ultra Carbon, sold by Alfa Aesar
233408-11098-0121/2011/83(23)/233408(4) ©2011 American Physical Society
BRIEF REPORTS PHYSICAL REVIEW B 83, 233408 (2011)
FIG. 1. (Color online) Optimized geometry structures of a
nanographite sample with nonpassivated zigzag edges (a), with zigzag
edges with total oxidation (b), and with different levels [1/16 (c),
1/8 (d), and 1/2 (e), (f)] of oxidation. Carbon atoms from different
graphite layers are shown in black and yellow (black and light
gray) and oxygen atoms in cyan (medium gray). All differences in
energy between ferromagnetic and antiferromagnetic configurations
are reported in meV. In parentheses, the same values are given for the
case of hydrogenation of the edges.
(stock No. 40766), AGKSP grade, (ultra “F” 99.9995 % purity
or 5 ppm of total impurities and maximum of 1 ppm of
impurities per element). The powder was produced by cutting
and grinding the graphite rod on the edge- and side-area
of a new, clean circular diamond saw blade. The cutting
and grinding system was inside a plastic bag filled with
the gas. A continuous gas stream was also forced to blow
through a gas hose into the grinding area. From the x-ray
diffraction spectrum, we estimated the graphite crystallite
size to be La ≈ 1000 Å and Lc ≈ 400 Å. The powder
particle size was found to be 150 ± 70 μm by using sieves.
Magnetization M(T ,H ) measurements were performed using
a commercial superconducting quantum interference device
(SQUID, Quantum Design, MPMS5) magnetometer.
Figures 2 and 3 present our principal experimental ob-
servations. Figure 2(a) illustrates that, while the virgin bulk
sample is diamagnetic with a negligible magnetic hysteresis,
the sample prepared under oxygen exposure possesses a
pronounced ferromagnetic response at room temperature.
Figure 2(b) demonstrates the intrinsic (i.e., related to the
sample) ferromagnetism. This intrinsic ferromagnetism can
also be seen from Fig. 3, which shows that no ferromagnetic
response was detected for graphite powders prepared under
He, N2, or Ar gas environments, in contrast to the samples
prepared in the air or in an oxygen atmosphere.
Figure 4 illustrates another important experimental fact,
namely that the oxygen effect is reversible: the ferromagnetism
vanishes with time after the sample is removed from the
oxygen atmosphere. This observation provides unambiguous
evidence that the ferromagnetism of graphite is essentially
related to the presence of oxygen. Figure 5 emphasizes
that hydrogen does not induce ferromagnetism in graphite.
This observation should be compared to the pronounced
FIG. 2. (a) Magnetization M(H ) measured at T = 300 K for
the graphite samples: bulk (virgin) sample and powdered samples
prepared under oxygen (O2) atmosphere; (b) m(H ) measured for
the oxidized sample at T = 10 K, shown in comparison with the
negligible contribution from the sample holder.
ferromagnetism observed after proton irradiation.27,28 We
attribute the difference in the results to the much weaker effect
of adsorbed hydrogen on the electronic structure of zigzag
graphitic edges (see below) than that produced by high-energy
protons.29
To explore the role of oxygen adsorption, cases of total
[Fig. 1(b)], slight [Figs. 1(c) and 1(d)] and half oxidation
[Figs. 1(e) and 1(f)] of graphite zigzag edges were an-
alyzed. Optimized atomic structure and calculated energy
differences per edge unit between ferromagnetic and antifer-
romagnetic configurations are reported in Fig. 1. In nonox-
idized nanographite, much as in graphene,11,12,14 magnetic
interactions between the nearest neighbors belonging to the
same edge are ferromagnetic (Jy), but interactions across
the ribbon (Jx) are weakly antiferromagnetic. The magnetic
interactions between neighbors in different graphite layers are
also antiferromagnetic. The electronic structure of the studied
nanographite (Fig. 6) is similar to the half-metallic structure
reported for the case of graphene nanoribbons.11 The total
oxidation of graphite edges [Fig. 1(b)], much as in the case
233408-2
BRIEF REPORTS PHYSICAL REVIEW B 83, 233408 (2011)
FIG. 3. (Color online) Magnetization M(H ) measured at T =
10 K for the powdered graphite samples prepared in various gas
environments (see text for details).
of graphene,19 leads to the absence of magnetism due to the
saturation of dangling bonds on the zigzag edges.
Partial oxidation of graphite edges combines the passivation
of several dangling bonds with an increase in the number of
the states on the Fermi level (metallization) of the samples,
as noted before for the graphene case.20,21 As can be seen
in Figs. 1(c) and 1(d), a few oxygen atoms chemisorbed on
the edges significantly reduce antiferromagnetic interactions
over the ribbon and between graphite plates. Further oxidation
of graphene edges enhances the metallicity of nanographites
(Fig. 6) that correspond to the delocalization of the unpaired
electrons localized on the zigzag edges11–14 and turns all
antiferromagnetic exchanges to ferromagnetic.
Thus, due to partial oxidation of nanographite edges,
one-dimensional ferromagnetism becomes two dimensional
[Fig. 1(e)] and three dimensional [Fig. 1(f)]. The Curie
temperature estimated for the case reported in Fig. 1(f) is
333 K, and the Curie temperature estimated for the case
with two-dimensional ferromagnetism [Fig. 1(e)] is 310 K.
Thus, these theoretical results predict the room-temperature
FIG. 4. M(H ) for the graphite powder prepared in an oxygen
(O2) environment and measured at T = 10 K immediately after
preparation and 11 days after the sample has been removed from
the oxygen atmosphere.
FIG. 5. (Color online) Magnetization M(H ) measured at T =
300 K for the powdered graphite samples prepared in hydrogen (H2)
and oxygen (O2) gas environments.
ferromagnetism in partially oxidized graphite powders, as
observed experimentally.
To explore aging (i.e., time-dependent effects), calculations
of the total energy of chemisorbed oxygen atoms for the
structures given in Figs. 1(c)–1(f) were performed using the
formula EO = (Enr − Epure)/N , where Enr is the energy of
the partially oxidized nanoribbon, Epure is the energy of
nonoxidized nanoribbon [see Fig. 1(a)], and N is the number
of oxygen atoms in the supercell. The lowest total energy of
oxygen corresponds to the lowest oxygen content on the edges
[Fig. 1(c)]. Adsorption of the next oxygen atom [Fig. 1(d)]
increases the total energy by about 59 meV/oxygen. The
cases of half passivation [Figs. 1(e) and 1(f)] are signifi-
cantly energetically unfavorable (475 and 387 meV/oxygen,
respectively). Hence, placing the samples in an oxygen-free
environment results in redistribution of adsorbed oxygen atoms
from areas with a higher concentration to nonoxidized edges
FIG. 6. (Color online) Spin-polarized densities of states for
graphite with nonoxidized zigzag edges (solid red line) and with
partially oxidized zigzag edges. Dashed green line corresponds to
the structure in Fig. 1(e) and the dotted blue line to the structure in
Fig. 1(f).
233408-3
BRIEF REPORTS PHYSICAL REVIEW B 83, 233408 (2011)
or in desorption of oxygen atoms, leading to the weakening or
even vanishing of the ferromagnetism.
The studies reported here of ferromagnetism in micro-
graphite and nanographite samples provide unambiguous
evidence for the crucial role of oxygen adsorption in room-
temperature ferromagnetism in graphite. We demonstrated
experimentally that hydrogen, nitrogen, helium, and argon
gases do not induce ferromagnetism in graphite. DFT calcu-
lations revealed significant changes in the electronic structure
of graphite ribbons caused by partial oxidation, transforming
antiferromagnetic interactions to ferromagnetic interactions in
two- and three-dimensional graphites. At the same time, ac-
cording to our calculations, partial hydrogenation of graphitic
edges does not lead to significant changes in the electronic
and magnetic structures of the nanographites. Experimental
results revealed the aging phenomenon, that is, the vanishing
of the ferromagnetism over time when the sample is removed
from the oxygen atmosphere. Theoretical modeling explains
this effect as a result of the reorganization of the oxygen atoms
into more energetically favorable antiferromagnetic config-
urations. The reported results as a whole demonstrate that
the electronic and magnetic properties of carbon nanosystems
can be strongly affected by graphitic-edge passivation, which
may be useful for the production and theoretical modeling of
magnetic graphitic nanoribbons.
This work was supported by CNPq, FAPESP, and CAPES.
We thank the Korea Institute for Advanced Study for providing
computing resources (KIAS Center for Advanced Computa-
tion Linux Cluster System) for this work.
*danil@kias.re.kr
†kopel@ifi.unicamp.br
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233408-4
