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

RIKEN Accel. Prog. Rep. 51 (2018)

Nano-size eﬀect on Néel temperature and magnetic ordering of
La2 CuO4S. Winarsih,∗1,∗2 F. Budiman,∗3 H. Tanaka,∗3 and I. Watanabe∗1,∗2,∗4
Finite-size eﬀects of antiferromagnetic materials,
which refer to the reduction in particle size of antiferromagnetic materials until they become nanoscale sized,
have received much attention in recent years.1,2) As predicted by Néel, there will be uncompensated spin fromthe surface due to the reduction in particle size of antiferromagnetic materials until they become nanoscale sized.
Because of this, antiferromagnetic nanoparticles exhibit
superparamagnetic properties, and the physical properties of antiferromagnetic nanoparticles will change.3)
La2 CuO4 (LCO) is chosen because it is one of the typical Mott insulating systems and is the parental compound of the superconducting high-Tc cuprate; therefore, this work can provide the basic knowledge forstudying the size eﬀect on Mott insulating systems.4)
Prof. Tanaka’s group as our collaborator the grain sizecan be controlled to be in the nanoscale range. As far as
we know, there has been no similar research on high-Tc
cuprates so far.
A previous measurement, which investigated othernanoparticle systems, proved that dc magnetic susceptibility cannot measure the TN value of an antiferromagnetic material.5) The transition temperature obtained bythis measurement is the blocking temperature or freezing temperature of the antiferromagnetic material, andnot TN .5) In this case, muon spin relaxation (µSR) wasused to investigate the magnetic ordering of this material. Prof. Tanaka’s group has already performed the dc
susceptibility measurement of LCO 96 nm.6) Therefore,
we can compare and analyze the results from susceptibility measurement and µSR measurement.
Zero-Field (ZF) µSR measurement at RAL, UK, using single pulsed muon beam, was carried out in orderto detect the muon spin precession of the material. TheZF-µSR time spectrum at various temperatures is shownin Fig. 1. The muon spin precession can be clearly seen
to start from 35 K, indicating that antiferromagnetic(AF) ordering starts at this temperature; however, theprecession disappears at 100 K. This suggests that theNéel temperature TN of this material is between 35 Kand 100 K. The value of TN is signiﬁcantly reduced compared to bulk LCO, which has TN = 240 ± 10 K.4)
Figure 2(a) depicts the temperature dependence of theinternal ﬁeld, Hint . Hint is zero at 100 K, since no muon
precession is observed at this temperature; it increases
with a decrease in temperature and reaches a saturated
internal ﬁeld of 400 G. The same tendency of Hint dependence on temperature and the same saturated internal
∗1
∗2
∗3
∗4

Department of Physics, Universitas Indonesia
RIKEN Nishina Center
Department of Human Intelligence Systems, Kyushu Instituteof Technology
Department of Physics, Hokkaido Universityﬁeld value is observed for LCO bulk material. However,in the bulk material, Hint is zero at 250 K. It meansthat although TN decreases with reduction in particle
size, the average magnetic moment and magnetic interaction are still the same.
The damping rate of muon spin precession, which iscaused by the static and dynamic ﬁeld dependence of
this material on temperature, is demonstrated in Fig. 2.
The damping rate seems to increase for temperatures
below 100 K, achieve a maximum value at 35 K, andthen decrease for temperatures below 35 K. The muon
spin depolarization dependence of this material on temperature, which is shown in Fig. 2(c), also has the sametendency. It is indicated that below 35 K, the spin ﬂuctuation slows down and there is an alignment of themagnetic moments of Cu2+ ; this means that long rangeordering starts at temperatures below 35 K.
We plan to complete ZF data between 35 K and 100 K
in order to evaluate the TN , and we are going to analyze
the internal ﬁeld of this material to ﬁnd out whether it
is coming from the static or dynamic spin ﬂuctuation bymeasuring the Longitudinal Field (LF)-µSR. Besides, weplan to measure the other LCO nanoparticle whose particle size is diﬀerent from that of this material. Hence,
the analysis on the nano-size eﬀect on Néel temperatureand magnetic properties can be conﬁrmed.

Fig. 1. ZF-µSR time spectrum of La2 CuO4 withparticle size of 96 nm.

Fig. 2. Temperature dependence of (a) the internal field atthe muon site; (b) the damping
rate of muon spin precession;and (c) muon spin depolarization rate of La2CuO4 96 nm.

References
1) M. M. Ibrahim et al., Phys. Rev. B 51, 2955 (1995).
2) S. Gider et al., Science 268, 77 (1955).
3) L. Néel, in Low Temperature Physics, (New York, 1962).
4) Budnick et al., Phys. Lett. A 124, 1, 2 (1987).
5) Tang et al., Phys. Rev. B 67, 054408 (2003).
6) H. Tanaka et al., (in preparation).

- 196 -