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

RIKEN Accel. Prog. Rep. 52 (2019)

Reduction in Néel Temperature of La2 CuO4 Nanoparticles
S. Winarsih,∗1,∗2 F. Budiman,∗3,∗4 H. Tanaka,∗3 T. Goto,∗5 T. Adachi,∗5 B. Kurniawan,∗2 B. Soegijono,∗2 andI. Watanabe∗1,∗2Recent reports on antiferromagnetic nanoparticles focus on the reduction in Néel temperature (TN ), the emergence of superparamagnetic behavior, and the increasein the band gap value.1–3) However, there is no uniﬁed
research on their magnetically ordered state. The causeof TN reduction has also not been investigated. In order to overcome this situation, a study on the magnetic
properties of La2 CuO4 nanoparticles (LCO NPs) using muon spin relaxation (µSR) and NMR is suggested.
La2 CuO4 is a mother compound of high-Tc cuprate oxides that has been well established by experiments andtheoretical methods; thus, a probing nano-sized eﬀect inthis LCO system can be achieved.4)
LCO NPs used in the present study were successfully
obtained through the sol-gel method. A detailed explanation about sample preparation has been reported inour previous paper.5) Zero-ﬁeld (ZF) µSR on LCO NPs
was carried out at RIKEN-RAL Muon Facility in theUK, using a single pulse positive surface muon beam.
ZF-µSR time spectra of LCO NPs with a particle sizeof 96 nm are shown in Fig. 1. The time spectra wereanalyzed by using Eq. (1). The ﬁrst component represents muon spin precession and the second component
represents the slow relaxation behavior beyond 1 µs. It
is shown that muon spin precession does exist in LCONPs and it disappears at 100 K. Muon spin precession
indicates the formation of the long-range ordered (LRO)

Fig. 2. Temperature dependence of the internal field at themuon site in LCO NPs with a particle size of 96 nm.

state.
A(t) = A1 e−λ1 t cos(ωt + ϕ) + A2 e−λ2 t(1)Figure 2 depicts the temperature dependence of theinternal ﬁeld at the muon site, Hint , of LCO NPs witha particle size of 96 nm. It is shown that Hint starts toincrease below ∼ 60 K, indicating that the TN of thissample is ∼ 60 K; this is strongly suppressed comparedto the bulk LCO, which has a TN of ∼ 320 K.4) Hint
reaches a saturated value ≈ 420 G at low temperatures.
This means that LCO NPs have almost the same saturated internal ﬁeld value as that observed in bulk LCO.4)
It is concluded that the nano-sized eﬀect causes the reduction in TN but does not aﬀect the internal ﬁeld at
the muon site. These two phenomena are also observedin a hole- doped La2-x Srx CuO4 bulk system.6) However,
the resistivity of LCO NPs does not show indication ofsuperconducting behavior. The reduction in the TN ofLCO NPs may be owing to the phase separation into
fully antiferromagnetic and paramagnetic regions. Our139
La NMR results also show that phase separation does
exist in LCO NPs. Further analysis is required to describe this phase separation so that the cause of the reduction in TN of LCO NPs can be clearly explained.
ReferencesFig. 1. ZF-µSR time spectra of La2 CuO4 NPs with a particle
size of 96 nm.
∗1

∗2
∗3
∗4
∗5

RIKEN Nishina Center
Department of Physics, Universitas Indonesia
Department of Human Intelligence Systems, Kyushu Instituteof Technology
School of Electrical Engineering, Telkom University
Department of Physics, Sophia University

1) X. G. Zheng et al., Phys. Rev. B 72, 014464 (2005).
2) D. Caruntu et al., J. Phys. D, Appl. Phys. 40, 5801
(2007).
3) M. Enhessari et al., Mater. Sci. Semicond. Process 16,1517 (2013).
4) J. I. Budnick et al., Phys. Lett. A 124, 1 (1987).
5) S. Winarsih et al., Key. Eng. Mater (2019). [in press].
6) E. Stilp et al., Phys. Rev. B 88, 064419 (2013).

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