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

RIKEN Accel. Prog. Rep. 53

Magnetic ordering and spin dynamics of Nd2 Ru2 O7 studied by µSR
U. Widyaiswari,∗1,∗2 H. Sakai,∗3 K. Inoue,∗3 N. Hanasaki,∗3 D. P. Sari,∗4 B. Kurniawan,∗2 and I. Watanabe∗1,∗2Pyrochlore ruthenate (R2 Ru2 O7 ) is one of the systems that have a pyrochlore structure with the generalformula A2 B2 O6 O’, where A is a trivalent rare-earth
ion and B is a tetravalent transition metal ion. Thisstructure is composed of two corner-sharing tetrahedral lattices, which induce the magnetic frustration ofspins on each corner of the tetrahedral lattices. Novelphysical properties such as the spin-glass,1) spin-ice,2)and metal-insulator transition accompanied by a magnetic transition,3–5) are induced by this magneticallyfrustrated state. Those exotic properties originate
from the competition and/or cooperation among theCoulomb interaction (U ), spin-orbit coupling (SOC)eﬀect, and exchange interaction between the rare-earth
and transition metal ions.
The temperature dependence of the speciﬁc heatof R2 Ru2 O7 shows a λ-type peak accompanied by asmall cusp in the temperature dependence of magnetic
susceptibility at the magnetic transition temperature,TN .6–8) TN shifts to the higher temperature side fromapproximately 76 K to 160 K with increase in the ionicradius of R.6) Ku et al. showed that three anomalies
occur in Nd2 Ru2 O7 at 146 K, 21 K, and 1.8 K.8) At
present, the origin of those anomalies is unclear. Accordingly, we investigated the magnetic properties ofNd2 Ru2 O7 by the muon spin relaxation (µSR) methodand compared the results with our previous results for
Nd2 Ir2 O7 .4,5)
The time evolution of the muon-spin polarization(µSR time spectrum) in zero ﬁeld (ZF) was measured
at several temperatures and analyzed by the following
two-component relaxation function:
A(t) = A1 exp(−λ1 t) + A2 exp(−λ2 t).

(1)

The ﬁrst and second components describe the fast andslow relaxation components, respectively. A1 and A2
are the asymmetry parameters while λ1 and λ2 are themuon relaxation rates of the two components.
We conﬁrmed from the current study an orderedstate of Ru spins below 146 K, showing a decrease in
the initial asymmetry parameter accompanied by theappearance of a critical slowing down behavior of theRu spin ﬂuctuations.9) Another anomaly of the muon
relaxation rate was detected around 30 K as a broad
dip in its temperature dependence.9) This anomaly
is unlikely to be due to the appearance of the magnetic ordering of Nd spins because no critical slowing
down behavior was observed at lower temperatures in
∗1
∗2
∗3
∗4

RIKEN Nishina Center
Department of Physics, Universitas Indonesia
Department of Physics, Osaka University
Graduate School of Engineering and Science, Shibaura Institute of Technology- 152 -Fig. 1. (a) ZF- and LF-µSR time spectra of Nd2 Ru2 O7
measured at various temperatures. An LF of 3850 G
was applied for LF-µSR measurement. (b) Temperature dependences of the slow relaxation rate in both
the ZF and LF condition.

Nd2 Ir2 O7 .4,5)
In order to clarify the anomaly around 30 K, we
measured µSR in the longitudinal-ﬁeld (LF) condition under an applied ﬁeld of 3850 G. Both ZF- andLF-µSR time spectra are shown in Fig. 1(a). We
conﬁrmed that the relaxation rate still showed theanomaly around 30 K in LF-µSR,9) but this anomaly
was slightly smaller compared to the case of ZF-µSR.
To obtain more information about the origin of theanomaly around 30 K, we are now planning to conduct
further µSR measurements of the LF scan at several
temperatures to investigate the change in spin dynamics.
References

1) J. G. Park et al., Physica B 328, 90 (2003).
2) M. J. Harris et al., Phys. Rev. Lett. 79, 2554 (1997).
3) K. Matsuhira et al., J. Phys. Soc. Jpn. 80, 094701(2011).
4) R. Asih et al., J. Phys. Soc. Jpn. 86, 024705 (2017).
5) H. Guo et al., Phys. Rev. B 88, 060411(R) (2013).
6) M. Ito et al., J. Phys. Chem. Solids 62, 337 (2001).
7) N. Taira et al., J. Solid State Chem. 152, 441 (2000).
8) S. T. Ku et al., J. Phys. Condens. Matt. 30, 155601(2018).
9) U. Widyaiswari et al., to be published.