Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 53

Study on magnetism of defective reduced graphene oxides
R. Asih,∗1,∗2 D. P. Sari,∗3,∗2 M. A. Baqiya,∗1 Darminto,∗1 and I. Watanabe∗2The origin of magnetism in graphene-based materials has been determined to be related with the presence of defects in their sheets.1–3) A graphene-derived
material that has a signiﬁcant amount of defects is reduced graphene oxide (rGO). The magnetic state of rGO
has been reported to vary depending on the defect state
and, thus, on the preparation history. Paramagnetic,4)super-paramagnetic,5) and even room-temperature ferromagnetic states1,2) have been reported to coexist inrGO.6) A magnetic ordering is expected to appear in thepresence of a suﬃcient amount of defects in rGO.3) In
this study, we investigate the presence of possible roomtemperature magnetic ordering in rGO prepared using
the green-synthesized method7) by means of muon-spin
relaxation (µSR).
The investigation of the Raman spectra of the synthesized rGO revealed the presence of a signiﬁcant amountof defects and that the amount of defects can be enhanced by thermal reduction.7) In addition to sp2 carbon hybridization (C = C), the main functional group,
the presence of diﬀerent types of oxygen functionalities
was observed in the synthesized rGO.7,8) The magnetization at 300 K was found to increase with an increasing amount of defects, and the enhancement was
not attributed to magnetic impurities.7) Moreover, a divergence in χ(T ) was observed under ﬁeld cooling (FC)and zero-ﬁeld cooling (ZFC) conditions, which might indicate that the blocking temperature is noticeable at300 K. Thus, the µSR experiment was subsequently performed to microscopically investigate magnetism in rGO
at 300 K.
The inset in Fig. 1 shows the zero-ﬁeld (ZF) time spectrum of the synthesized rGO at 300 K. An appreciablemuon-spin precession on top of the decaying signal wasobserved in the spectrum, which is somehow similar tothat observed in as-prepared graphene.9) The spectrum
can be well ﬁtted by a single precession component anda Lorentzian decay. The ﬁtting results in a hyperﬁne
ﬁeld of about 6 G. Such coherent muon-spin precession
could indicate the presence of long-range magnetic ordering and/or the formation of a muonium (Mu)-H entangled state.9) Meanwhile, the Lorentzian decay signiﬁes muon diﬀusion through a carbon surface.10) Under
the assumption of the Mu-H dipolar interaction, a dipolar ﬁeld of approximately 5 G is obtained which is equal
to a Mu-H internuclear distance of 1.344 ± 0.005 . Furthermore, a longitudinal-ﬁeld (LF) was applied to decouple the muon spin from the electron spin. The LF
dependence of the muon-spin repolarization fraction is∗1
∗2
∗3

Department of Physics, Institut Teknologi Sepuluh Nopember,Indonesia
RIKEN Nishina Center
Department of Regional Environment Systems, Shibaura Institute of Technology

Fig. 1. Field dependence of the muon-spin repolarization
fraction of the synthesized rGO at 300 K fitted by a simple model for the muonium radical (Eq. (1)). Inset: zerofield (ZF) µSR spectrum (at 300 K) of the synthesizedrGO fitted by the summation of Lorentzian decay and asingle precession component, as described in the text.

displayed in Fig. 1, which can be well ﬁtted by a simple
model for isotropic muonium with the function describedin Eq. (1).10)
Pexp = Pdia + Arad [x2 /(x2 + 1)],

(1)where x = B/Bhyp . Bhyp is the hyperﬁne ﬁeld for freemuonium. Pdia and Arad are the diamagnetic fraction
and the amplitude of the radical fraction, respectively.
The obtained Bhyp is about 4.35 ± 0.24 G, which is almost as large as that procured from the ZF spectrum.
This Bhyp value corresponds to a hyperﬁne frequency of12(1) MHz, which is close to the instrument cut-oﬀ of
13 MHz. This can be the reason why ∼45% of the implanted muons were not observed in the ZF spectrum.
Furthermore, a more detailed analysis in conjunction
with ﬁrst-principles calculations is required to conﬁrm
that the observed muon-spin precession originates from
Mu-H interaction to form the CHMu group as suggested
for hydrogenated graphene.9)
References

1) S. Qin et al., Carbon 78, 559 (2014).
2) G. Khurana et al., Nanoscale 5, 3346 (2013).
3) J. Tǔcek et al., Chem. Soc. Rev. 47, 3899 (2018).
4) M. Sepioni et al., Phys. Rev. Lett. 105, 207205 (2010).
5) S. K. Sarkar et al., Physica E 64, 78 (2014).
6) P. Z. Sun et al., Nano Res. 7, 1507 (2014).
7) D. Darminto et al., IEEE Trans. Mag. 54, 1 (2018).
8) R. Asih et al., Mat. Sci. Forum 966, 290 (2019).
9) M. Riccò et al., Nano Lett. 11, 4919 (2011).
10) M. Gaboardi et al., J. Mater. Chem. A 2, 1039 (2014).

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