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

RIKEN Accel. Prog. Rep. 51 (2018)

Spin fragmentation in Nd2 Ir2 O7 and its carrier-doped dependence
R. Asih,∗1,∗2 J. Angel,∗1,∗3 K. Matsuhira,∗4 T. Nakano,∗2 Y. Nozue,∗2 and I. Watanabe∗1,∗2,∗3Pyrochlore iridates, R2 Ir2 O7 (R227, R is a rare-earthelement), provide an ideal platform of strongly frustrated systems to study the interplay between electronelectron correlation (U ) and spin-orbit interaction (SOI)
given from 5d electrons of Ir4+ .1) Among R227, Nd227
stands out as a fascinating system because of the additional interesting properties. Nd227 exhibits metallic behavior and undergoes metal-insulator transition(MIT) at TMI = 33 K.2) µSR and neutron-scatteringstudies on Nd227 showed the appearance of a long-range
magnetic ordering (LRO) of Ir4+ moments below TMI
followed by an additional LRO of Nd3+ moments below 10 K.3–5) With such progressive reports, however,the sizes of the magnetic ordered moments remain debatable. A reduction in the Nd3+ moments was foundin the recent neutron study6) compared with those estimated from the previous study3) and crystal electric ﬁeld(CEF) analysis,7) which is argued to be attributed to astrong quantum ﬂuctuation. This argument was also
indicated from µSR results on Nd227, which show anappreciable reduction on the internal ﬁeld at the muonsite (Hint ) compared with other R227 compounds.4,5)
These results signify a possible magnetic fragmentation
in Nd227, where ordered and ﬂuctuating phases occur simultaneously. The onset of magnetic ordering on Nd227
was also reported to be suppressed by hole doping via
Ca2+ substitution on the Nd3+ site, and TMI was foundto gradually decrease by increasing the Ca concentration.8) In this study, we investigated the existence ofmagnetic fragmentation in Nd227 and Ca-doped Nd227,(Nd1−x Cax )2 Ir2 O7 .
Longitudinal-ﬁeld (LF-µSR) measurements were performed to conﬁrm the emergence of ﬂuctuations in theordered phase of the compounds. Figure 1 displays
the temperature dependence of the relaxation rate λ of
(Nd1−x Cax )2 Ir2 O7 for x = 0.00, 0.05, 0.07 and 0.10 under an applied ﬁeld of 3.6 T. An appreciable peak in λwas observed at higher temperatures compared with ordered and meta-transition temperatures. For x = 0.07
and 0.10, a clear peak was observed at about 20 K despite the fact that neither muon-spin precession nor aslowing-down behavior was observed under the zero-ﬁeld(ZF) condition at this temperature. These results may
indicate that Nd and/or Ir have low-lying spin ﬂuctuations, which can be easily changed by temperatures andmagnetic ﬁelds. Figure 2 shows the ﬁeld dependence ofthe relaxation rate measured in Nd227 at 50 K (paramagnetic state), 15 K (ordered state of Ir moments), and1.5 K (ordered state of Ir and Nd moments). λ increases∗1∗2∗3∗4RIKEN Nishina Center
Department of Physics, Osaka University
Department of Condense Matter Physics, Hokkaido University
Graduate School of Engineering, Kyushu Institute of Technology

Fig. 1. Temperature dependence of the relaxation rate λ of(Nd1−x Cax )2 Ir2 O7 for x = 0.00, 0.05, 0.07, and 0.10 under the applied field of 3.6 T.

Fig. 2. Field dependence of the relaxation rate λ of Nd2 Ir2 O7
at 1.5 K, 15 K, and 50 K.

with an increase in the applied ﬁeld at 50 K and shows apeak around 2.5 T at 15 K, which signiﬁes that the spectrum density of the spin ﬂuctuations shifts down to thelower frequency side with decreasing temperature followed by the change in the dynamics of spins. At 1.5 K,
λ remains about 0.6 µs−1 in the ZF condition, and then,it decreases exponentially with an increasing ﬁeld showing Redﬁeld-like behavior, which indicates the maintenance of the spin ﬂuctuation even in the ordered state,i.e., a magnetic-fragmentation is realized in Nd227. To
further discuss the scheme of spin-ﬂuctuations in thesecompounds, it is necessary to collect more data points
at diﬀerent temperatures and applied ﬁelds.
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) K. Tomiyasu et al., J. Phys. Soc. Jpn. 81, 034709 (2012).
4) H. Guo et al., Phys. Rev. B 88, 060411(R) (2013).
5) R. Asih et al., J. Phys. Soc. Jpn. 86, 024705 (2017).
6) H. Guo et al., Phys. Rev. B 94, 161102(R) (2016).
7) M. Watahiki et al., J. Phys. Conf. Ser. 320, 012080(2011).
8) R. Asih et al., RIKEN Accel. Prog. Rep. 50, 23 (2017).

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