Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Magnetic moments and ordered states in pyrochlore iridates
Nd2 Ir2 O7 and Sm2 Ir2 O7 studied by muon-spin relaxation†
R. Asih,∗1,∗2 N. Adam,∗1,∗3 S. S. Mohd-Tajudin,∗1,∗3 D. P. Sari,∗1,∗2 K. Matsuhira,∗4 H. Guo,∗5
M. Wakeshima,∗6 Y. Hinatsu,∗6 T. Nakano,∗2 Y. Nozue,∗2 S. Sulaiman,∗3 M. I. Mohammed-Ibrahim,∗3P. K. Biswas,∗7 and I. Watanabe∗1,∗2,∗3,∗8Pyrochlore iridates, R2 Ir2 O7 (R227, R = Nd, Sm,Eu, Gd, Tb, Dy, or Ho), which have the relatively
large spin-orbit coupling (SOC) inherent in Ir 5d electrons and a d-f exchange interaction, have been suggested to show peculiar electronic properties.1) R227
also shows metal-insulator transitions (MITs) at TMI ,which seems to be accompanied by magnetic transitions. TMI gradually decreases from 141 K for R = Hoto 117 K for R = Sm with the increasing ionic radiusof the trivalent R ion. TMI suddenly drops to 33 K for
Nd227, and no MIT is observed in Pr227, which showsmetallic behavior.2) Owing to diﬃculties in observing
Ir magnetic ordering by means of neutron studies inthese compounds, muon-spin relaxation (µSR) studies
have been proven to directly conﬁrm the appearanceof long-range magnetic ordering (LRO) in some of theR227 compounds.3–5)
In this study, we investigate the magnetic orderingsand structures of Nd227 and Sm227, which are particularly important as they lie in the boundary of MIT.
In the case of Nd227, we observed the additional LROof Nd moments below 10 K and found a saturated internal ﬁeld at the muon site (Hint ) of approximately
530 G at 1.5 K, which conﬁrmed indications suggested
by previous neutron scattering6) and µSR studies.5) In
the case of Sm227, spontaneous muon-spin precession
was observed below TMI = 117 K, which indicated theappearance of LRO of Ir moments below TMI . The
parameters obtained from the ﬁtting to the zero-ﬁeldµSR data on Sm227 are shown in Fig. 1. The solid line
in Fig. 1(a) indicates the temperature dependence ofthe resistivity, showing a clear transition at TMI . Theinternal ﬁeld at the muon site, Hint , rapidly increases
just below TMI and saturates at the temperature region
between 60 K and 20 K. With further decrease in temperature, Hint decreases below approximately 10 K. As
shown in Fig. 1(b), the relaxation rate λ1 is relatively
constant in the paramagnetic region at T > TMI andthen increases below TMI , forming a peak at approxi†
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Fig. 1. Temperature dependences of (a) internal field atthe muon sites Hint , (b) slow relaxation rate λ1 , and(c) damping rate of the muon-spin precession λ2 . The
broken line indicates TMI = 117 K. The area T > TMIindicates the paramagnetic region. The temperaturedependence of the resistivity of Sm2 Ir2 O7 is displayedin (a) by the solid line as a reference MIT.

mately 10 K, which indicates a slowing-down behavior
toward an LRO below this temperature. This, therefore, suggests the appearance of the additional LRO ofSm moments below 10 K. Increases in the damping rateof the muon-spin precession are also observed below
10 K and near the TMI , as indicated in Fig. 1(c), whichfurther indicates a broadening of the distribution ofHint . Dipole-ﬁeld calculations at the possible muon
stopping site show that the all-in all-out spin structure most convincingly explained the present µSR results with the lower limits of the magnetic-ordered moments determined as 0.12 µB /Ir4+ and 0.2 µB /Nd3+
in Nd227 and 0.3 µB /Ir4+ and 0.1 µB /Sm3+ in Sm227.
Further analysis indicated that the spin coupling between Ir and Nd/Sm moments was ferromagnetic for
Nd227 and antiferromagnetic for Sm227.

Condensed from the article in J. Phys. Soc. Jpn. 86, 024705(2017)RIKEN Nishina Center
Department of Physics, Osaka University
School of Distance Education, Universiti Sains Malaysia
Graduate School of Engineering, Kyushu Institute of Technology
Department of Physics, Zhejiang University
Department of Chemistry, Hokkaido University
Bulk µSR Group, Paul Scherrer Institut
Department of Condense Matter Physics, Hokkaido University

References
1) X. Wan et al., Phys. Rev. B 83, 205101 (2011).
2) K. Matsuhira et al., J. Phys. Soc. Jpn. 80, 094701 (2011).
3) S. Zhao et al., Phys. Rev. B 84, 180402(R) (2011).
4) S. M. Disseler et al., Phys. Rev. B 84 174441 (2012).
5) H. Guo et al., Phys. Rev. B 88, 060411(R) (2013).
6) K. Tomiyasu et al., J. Phys. Soc. Jpn. 81, 034709 (2012).

- 22 -