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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Observation of Cu spin fluctuations in over-doped regime La2-x Srx CuO4
nanoparticles
S. Winarsih,∗1 T. Adachi,∗2 T. Goto,∗2 B. Kurniawan,∗3 B. Soegijono,∗3 and I. Watanabe∗4
The ﬁrst observation of exchange anisotropy or exchange bias was reported for cobalt particles capped bycobalt oxide.1) The shifted hysteresis loop in ﬁeld-cooledsuggested the occurrence of exchange coupling betweenthe ferromagnetic spin of cobalt with the antiferromagnetic spin of cobalt oxide. For gold nanoparticles encapsulated by butanethiol, clear evidence of magnetism wasobserved when the size of the gold cluster was 2.2 nm, as
studied by muon-spin relaxation (µSR).2) The relaxationof muon spins decreased with increasing temperaturein the longitudinal ﬁeld, (LF-µSR), indicating the existence of a magnetic moment. Even though magnetism
was clearly observed, this study could not distinguishwhether the magnetic moment was located at the encapsulated molecules or inside the gold nanoparticles.
The existence of ferromagnetic spins at the surface wasalso suggested to lead to the observation of a hysteresis
loop at 10 K in CuO nanoparticles.3) An anomalously
enhanced Curie term at low temperatures in high-Tc superconductor cuprate (HTSC), La2-x Srx CuO4 , with x =0.10–0.30, when the particle size was reduced to 113 nm
was also linked to the role of the spins at the surface.4)
This suggests that the surface eﬀect plays an importantrole in both free-standing nanoparticles, like CuO andLa2-x Srx CuO4 , and nanoparticles capped by molecules,
like oxide-coated particles of cobalt and nanogold cappedby butanethiol. However, the mechanism of the role ofthe surface causing the emergence of ferromagnetism in
nanoparticles is still an open question.
We aimed to investigate nano-sizing eﬀects in
La2-x Srx CuO4 because the observation of possible ferromagnetism is a new interesting phenomenon in HTSCs.
In the bulk case, a ferromagnetic phase was predictedin the heavily overdoped regime where the superconductance was suppressed.5,6) This appearance of ferromagnetism in La2-x Srx CuO4 from the underdoped to theoverdoped regime is a new question in HTSCs because
superconductivity and magnetic ordering are believed to
be interconnected.
To synthesize free-standing nanoparticles, the chemical reaction method is believed as one of the best methods because it is a bottom-up method.7) A bottomup method implies spontaneous self-assembly from theatomic level to the nanoparticle level. The detailed synthesis route is reported in our former paper.8)µSR measurements on La2-x Srx CuO4 with x = 0.20
and particle size of 46 nm were performed at the RIKENRAL Muon Facility, Rutherford-Appleton Laboratory,
∗1
∗2
∗3
∗4

Department of Physics, Universitas Padjadjaran
Department of Engineering and Applied Sciences, Sophia University
Department of Physics, Universitas Indonesia
RIKEN Nishina CenterFig. 1. Temperature dependence of relaxation rate of muonspins, λ, of La2-x Srx CuO4 with x = 0.20 and particle sizeof 46 nm.

UK, using a pulsed positive surface muon beam. Figure 1 displays the temperature dependence of the relaxation rate of the muon spins. The relaxation rate starts
rapidly increasing below 10 K. This indicates the slowing
down of the Cu spin ﬂuctuations below 10 K, which, inturn, suggests that magnetic correlations are developed
at low temperatures. Compared to the bulk case, nomagnetic correlation was observed even at 0.3 K.9)
Our present result reveals that nano-sizing induces
weak magnetism owing to the development of Cu spin
ﬂuctuation. Moreover, our magnetization results showthat the superconductivity is strongly suppressed inLa2-x Srx CuO4 nanoparticles. These results suggest an
anticorrelation between the magnetic order and the superconductivity and show that the superconductivity inan HTSC is strongly inﬂuenced by the magnetic order,
which can be explained by the stripe theory. We provided a new insight of studying Cu spin dynamics byinvestigating the nano-sizing eﬀects in an HTSC, which
is typically examined based on the impurity eﬀects on
the Cu site.9–11)
References

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3) M. S. Seehra, A. Punnoose, Solid State Commun. 128,299 (2003).
4) Y. Yin et al., J. Phys. Chem. C 117, 3028 (2013).
5) A. Kopp et al., Proc. Natl. Acad. Sci. USA 104, 6123 (2007).
6) J. E. Sonier et al., Proc. Natl. Acad. Sci. USA 107, 17131(2010).
7) A. G. Kolhatkar et al., Int. J. Mol. Sci. 14, 15977 (2013).
8) S. Winarsih et al., Key. Eng. Mater. 966, 357 (2019).
9) T. Adachi et al., Phys. Rev. B 78, 134515 (2008).
10) T. Adachi, et al., Phys. Rev B 70, 060504(R) (2004).
11) Risdiana, et al., Phys. Rev. B 77, 054516 (2008).

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