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

RIKEN Accel. Prog. Rep. 47 (2014)

Magnetic order in pyrochlore iridate Nd2 Ir2 O7 probed by employing
muon spin relaxation†
H. Guo,∗1,∗2 K. Matsuhira,∗3 I. Kawasaki,∗2 M. Wakeshima,∗4 Y. Hinatsu,∗4 I. Watanabe,∗2 and Z. Xu∗1
Pyrochlore iridates are highly suitable to investigate novel topological phases based on the networkof corner-sharing tetrahedra structures and the relatively large spin-orbit coupling (SOC) inherent in Ir 5delectrons.1) The interplay between SOC and electronelectron correlations (U ) produces characteristic electronic states. A series of R 2 Ir2 O7 (R-227, R = Nd-Ho)
compounds exhibit metallic or semi-metallic behavior
and undergo metal-insulator transitions (MITs) at atemperature TMI 2) while Pr-227 shows metallic behavior down to 0.3 K.3)
In this study, we focus on Nd-227, which shows
metallic behavior at high temperatures and undergoes
a MIT at T MI of about 30 K, and the magnetic susceptibility shows the bifurcation below T MI in zerofield-cooling (ZFC) and field-cooling (FC) conditions.2)
Muon spin precession is observed below TMI , and thespectra were fitted using the following function:
A(t) = A1 e−λ1 t + A2 cos(γµ Hint t + φ)e−λ2 t(1)where Hint is the internal field at the muon site, λ1 isthe muon spin-lattice relaxation rate, and λ2 and φ are
the damping rate and initial phase of the muon spin
precession, respectively.
The temperature dependence of the extracted parameters is shown in Fig. 1. Hint begins to increase
below TMI , following the Brillouin-type ordering andtends to saturate below about 20 K to a value of about
350 G. Below about 10 K, Hint increases again. λ2 continues to decrease below TMI and increases again fromthe same temperature at which Hint exhibits an increase. ¿From the inset of Fig. 1(a), it can be seen that
λ1 increases monotonically with decreasing temperature, reflecting the slowing down of the magnetic moments. A small increase is observed around TMI . However, no critical slowing down behavior is observed, asshown in Fig. 1(b). λ1 continues to increase below TMI
and shows a broad peak at around 10 K.
The increase in Hint below about 10 K is consistent
with the results of the neutron scattering experimentthat shows the ordering of Nd3+ moments, so it is attributed to the ordering of Nd3+ moments from ourmuon spin relaxation (µSR) experiment. The decreasein λ1 below about 10 K is then accounted for by thefreezing out of the magnetic fluctuations, and the increase in λ2 suggests that the distribution of the internal field becomes larger at the vicinity of the magnetic†∗1∗2∗3∗4ordering. The ordering below TMI is then attributed tothe ordering of the Ir4+ moments, suggesting its close
relationship with the MIT.

Condensed from the article in Phys. Rev. B 88, 060411(2013).
Department of Physics, Zhejiang University
RIKEN Nishina Center
Faculty of Engineering, Kyushu Institute of Technology
Division of Chemistry, Hokkaido University- 251 -Fig. 1. Temperature dependence of the extracted parameters from our fits to Eq. (1). (a) The internal field at
the muon site. (b) The damping rate of muon spin precession and the muon spin-lattice relaxation rate. The
inset in (a) shows the whole temperature range of λ1 .

According to a local spin-density approximation calculation including U and SOC, the magnetic structureof the Ir sublattice is the all-in/all-out type, which does
not break the lattice periodicity; therefore, the Slatertransition is ruled out to account for the relationshipbetween the MIT and the magnetic transition. On
the other hand, the Lifshitz-like transition in which
the hole band and electron band are moved downward
and upward, respectively, due to the specific magnetic
structure and the large SOC of Ir 5d electrons may
explain the mechanism of MIT.
The saturated internal field from the ordered Ir4+
moments is found to be much smaller than that in thecase of the other pyrochlore iridates with a magnetic
insulating ground state. This implies a stronger hybridization between the Ir 5d and the O 2p electronic
orbitals in Nd-227.
References
1) X. Wan et al., Phys. Rev. B 20, 205101 (2011).
2) K. Matsuhira et al., J. Phys. Soc. Jpn. 80, 094701(2011).
3) S. Nakatsuji et al., Phys. Rev. Lett. 96, 087204 (2006).