Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

Investigation of magnetic ordered states in the pyrochlore iridates(Nd,Ca)2 Ir2 O7 probed by μSR
R. Asih,∗1,∗2 D. P. Sari,∗1,∗2 J. Angel,∗1,∗3 K. Matsuhira,∗4 T. Nakano,∗2 Y. Nozue,∗2 andI. Watanabe∗1,∗2,∗3
Pyrochlore iridates R2 Ir2 O7 (R227, R is a lanthanide element), have attracted growing interest because of their potential for realizing new topologicalstates in the presence of strong spin-orbit coupling(SOC ) and electron correlation (U ), such as the Mott
insulator, Weyl semimital, and axion insulator.1) Interestingly, the electron correlation (U ) in these compounds can be systematically tuned by changing theionic radius of the R-ion (r ). R227 shows systematic
metal-insulator transition (MIT) at TMI , which gradually decreases by increasing the ionic radius of the R3+ion, and its boundary lies between R= Nd and Pr2) .
Abundant emergent quantum states have been theoretically predicted to occur on the boundary of MIT.1) In
order to unravel those states, it is necessary to ﬁnely
tune U in this MIT-critical region. One way to do
this is to substitute a nonmagnetic ion such as Ca forNd, (Nd1−x Cax )2 Ir2 O7 , which leads to the doping ofholes in the Ir 5d band, and hence drives the transition
from insulator to metal at the ground state and simultaneously suppresses magnetic orders. In this study,
we systematically investigated changes in magnetic ordered states of Nd2 Ir2 O7 due to hole doping by meansof μSR measurements.
Pure Nd2 Ir2 O7 exhibits metallic behavior and undergoes MIT at TMI = 33 K.2) Our μSR study on
Nd2 Ir2 O7 showed the appearance of a long-range magnetic order of Ir4+ moments below TMI followed by anadditional magnetic order of Nd3+ moments below 10K.3)4) In the dilute hole-doped system x = 0.01, this Ir
ordering appears at a lower temperature of around 26K, as displayed in Fig. 1, indicating the suppressionof the onset of the magnetic ordering. The zero-ﬁeld(ZF) time spectra showed spontaneous muon-spin precession below 26 K, which was then well analyzed by
the following function.
A(t) = Ar e−λr t + Aω cos(γμ Hint t + ϕ)e−λω t

The temperature dependences of the parameters obtained from the analysis of the ZF-μSR data are shownin Fig. 2. The dilute hole-doping gradually suppressed
the onset of magnetic ordering and the internal ﬁeld
coming from the Ir4+ ordering, while the internal ﬁeld
coming from the Nd3+ ordered moments tended to increase below 5 K. The critical slowing down in the relaxation rate (Fig. 2b) indicates that Nd3+ moments
form a static ordering below about 10 K that does notrely on Ca concentration. Further measurements will
be conducted on the intermediate and heavy Ca-doped
systems to complete the magnetic phase diagram of(Nd1−x Cax )2 Ir2 O7 .

Fig. 1. Zero ﬁeld time spectra of (Nd1−x Cax )2 Ir2 O7 x =0.01 at the early time region. Solid lines show ﬁts tothe data described in the text.

(1)

The ﬁrst component expresses the relaxing behavior
with initial asymmetry Ar and relaxation rate λr ,and the second one expresses the muon-spin precession with initial asymmetry Aω , damping rate λω andphase of the precession ϕ. Here γμ and Hint are the gyromagnetic ratio of the muon spin (2π × 13.55 kHz/G)
and the internal ﬁeld at the muon site, respectively.
∗1∗2∗3∗4Fig. 2. Parameters derived from ﬁtting Eq. 1 to the zeroﬁeld μSR data of (Nd1−x Cax )2 Ir2 O7 . (a) Internal ﬁeld
at muon sites Hint and (b) relaxation rate λr . Solidlines are guides for the eye.

RIKEN Nishina Center
Department of Physics, Osaka University
Department of Condense Matter Physics, Hokkaido University
Graduate School of Engineering, Kyushu Institute of TechnologyReferences
1) X. Wan et al., Phys. Rev. B 83, 205101 (2011).
2) K. Matsuhira et al., J. Phys. Soc. Jpn. 80, 094701 (2011).
3) H. Guo et al., Phys. Rev. B 88, 060411(R) (2013).
4) R. Asih et al., J. Phys. Soc. Jpn. 86, 024705 (2017).

- 23 -