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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Spin dynamics in Pyrochlore Nd2 Mo2 O7
L. J. Chang,∗1 D. P . Sari,∗2 M. C. Hatnean,∗3 G. Blalkrishnan,∗3 Y. Yasui,∗4 and I. Watanabe∗5
Nd2 Mo2 O7 is metallic and exhibits a ferromagnetictransition at a Curie temperature Tc = 93 K. In contrast
with other Nd-pyrochlore systems, such as Nd2 Sn2 O7and Nd2 Zr2 O7 , where the magnetic moments are localized, Nd2 Mo2 O7 possesses delocalized electrons, which
may host novel phenomena associated with the interaction between the localized moments Nd3+ and the itinerant moment Mo4+ . Particularly, Nd2 Mo2 O7 exhibits
unusual spin chirality,1,2) and possesses a two-in-two-out(AIAO) spin ice spin conﬁguration on Nd3+ . Unlike theAIAO magnetic order of Ir4+ in Nd2 Ir2 O7 , the XY typeorder of Ru4+ in Nd2 Ru2 O7 , and Mo4+ in Nd2 Mo2 O7 orders along the 001 direction. In Nd2 Mo2 O7 , the anomalies appearing at 93 K in both the magnetic and heat capacity is attributed to the Mo4+ ordering temperature,
whereas the ordering of Nd3+ becomes signiﬁcant below
30 K.3) Therefore, it is interesting to study the spin dynamics in the delocalized electron system of Nd2 Mo2 O7
and compare them to those of other Nd-pyrochlores byµSR technique. Moreover, applied ﬁelds in Nd2 Mo2 O7
may change the ground state of Nd3+ .3) µSR experiments were carried out at ARGUS spectroscopy, RIKENRAL, between the temperature ranges of 5–300 K to obtain the information below and above the magnetic transition temperatures of 30 and 93 K. Figure 1(a) shows
the zero-ﬁeld mSR relaxation spectra at 7.5, 35, 95,
and 160 K and the ﬁtted curves using the function ofA1 exp(−λ1 t) + A2 exp(−λ2 t) + A3 . The plots of the ﬁtted parameters are shown in Fig. 1(b). The initial dropof the muon relaxation below 95 K indicates the magnetic order of the compound, which can also be conﬁrmedby the increasing of the relaxation rate λ1 in Fig. 1(b).

Fig. 1. (a) zero-ﬁeld µSR relaxation spectra at 7.5, 35,95, and 160 K and ﬁtted curves using the function ofA1 exp(−λ1 t) + A2 exp(−λ2 t) + A3 ; (b) plots of the ﬁttedparameters.
∗1
∗2
∗3
∗4
∗5

Department of Physics, National Cheng Kung University
College of Engineering, Shibaura Institute of Technology
Department of Physics, University of Warwick
Department of Physics, Meiji University
RIKEN Nishina CenterFig. 2. Temperature-dependence of the asymmetry spectraof muon relaxation signal between 5 and 300 K in theapplied ﬁeld of 4000 G. The spectra can be separated into
three diﬀerent temperature intervals of 30 K and 93 K,
the transition temperatures of Nd3+ and Mo4+ ordering
temperatures, respectively.

The initial drop made it impossible to determine themagnetic volume fraction of the AIAO spin structure.
However, the transition at 30 K cannot be picked up
clearly by muon relaxation. The asymmetry of the slow
relaxation parameter exhibits a magnetic anomaly. Longitudinal ﬁelds were applied up to 4000 G to evaluate
the decoupling of muons on the sites. An applied constant of 4000 G in the temperature-dependent scans wasalso utilized in the experiments. The transitions at 30
and 93 K can be clariﬁed in the measurements as shownin Fig. 2, in which the relaxations of the spectra areseparated to three groups in three diﬀerent temperature
ranges. Proper models have to be proposed to ﬁt thesecurves and higher ﬁelds are necessary to decouple themuon relaxations for further study on this compound.
At 5 K, the signiﬁcant diﬀerences of the asymmetry
and muon relaxation have been observed before and after an applied ﬁeld (not shown in the ﬁgure). Similar
behaviour does not appear at a higher temperature of
85 K. This hysteresis at 5 K indicates ferromagneticcomponents at low temperature. In our experiments,
this hysteresis even happens in a small applied ﬁeld of
100 G. The magnetic structure of Nd2 Mo2 O7 , had beenreported as a spin chirality “umbrella” structure,4) whichis consistent with the observation made in the µSR spectra at low temperatures. This phase occurs below theNd3+ ordering temperature only, and does not exist below the Mo4+ ordering temperature between 93 K and
30 K. However, muon precession was not observed in theexperiments. We are working on a proper model to ﬁt
the spectra below 30 K.
References

1) K. A. Ross et al., Phys. Rev. B 84, 174442 (2011).
2) A. Maisuradze et al., Phys. Rev. B 92, 094424 (2015).
3) Y. Yasui et al., J. Phys. Soc. Jpn. 72, 865 (2003).
4) Y. Taguchi et al., Science 291, 2573 (2001).
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