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

RIKEN Accel. Prog. Rep. 50 (2017)

Study of the spin dynamics of a honeycomb ruthenate using spin polarizedmuons
㻖3 䠆3
㻖3 䠆3
㻖2
㻖1,㻖2 䠆1,䠆2
䠆2
S. Yoon,
Do,Do
K.-Y.
Choi,B.
Suh,andWatanabe
SungwonYoon㻖1,,䠆㻖21,S.-H.
S.-H.
, K.-Y.
Choi, J.B.
J. Suh,䠆2 I.andI. Watanabe

Honeycomb ruthenate α-RuCl3 was recently suggested asa candidate to realize the Kitaev-Heisenberg spin model forthe following reasons.1) Ru3+ ions can be in the low spinstate because of the competition between the spin orbit
coupling and the electronic correlation on the octahedral
environment surrounded by Cl- ions. Also, they form anearly two-dimensional honeycomb network for the weakcoupling by a van der Waals force among honeycomb layers.
These environments are expected to serve thebond-dependent anisotropic exchange coupling between
Ru3+ ions by an orthogonally bridged Cl- ion in thehoneycomb network.
We prepared a single crystal sample of α-RuCl3 with arhombohedral lattice observed by the X-ray diffraction. In
this lattice, the anisotropic exchange can be revealed for theclose cubic environment in the two-dimensional honeycomb
layer. In addition, we carried out the measurements for DC
susceptibility and specific heat. The DC susceptibility resultshows a magnetic anomaly around 6 K. The specific heat
result also exhibits a sharp peak around 6 K, as well as two
broad humps around 10 K and 13 K. These anomalies arelower than previously presented results.2-4)
In order to obtain details of the magnetic ordered states,
we performed a local probe investigation using spinpolarized muons at the RIKEN-RAL Muon Facility because
muons are highly sensitive to a small magnetic field. In themuon polarization at zero field, we confirmed results
similar to the previous experiment,5) two magnetictransitions around 6 K and 13 K. It was also found that bothtransitions are close to the two dimensional Ising spin stateby fitting the phenomenological equation of the magnetic
ordered state..

Fig. 1. (Left panel) Muon polarization spectra in several
longitudinal fields at 0.5 K (up), and 5.5 K (down),respectively. (Right panel) Longitudinal field dependencesof the slow relaxation rates with their fits at 0.5 K and 5.5 K,
respectively.
䠆1

RIKEN Nishina Center
Department of Physics, The Catholic University of Korea
䠆3
Department of Physics, Chung-Ang University
䠆2In addition, we obtained information about the electronic
spin fluctuations from the muon polarization in the externallongitudinal field by fitting with the superposition of two
exponential functions. We then extracted the relaxation rateof the slow-relaxing component on the latter time region. As
shown in Fig. 1, the slow relaxation rates of the whole
spectra are strongly dependent on the external longitudinal
field. The spin diffusive model is used to describe thebehavior of the electronic spin fluctuation.6) It was foundthat the result of the relaxation rates at 5.5 K is close to thetwo-dimensional diffusive model rather than the result at
0.5 K which is almost inversely proportional to thelongitudinal field.

Fig. 2. (Left panel) Several time differential spectra of themuon polarization at some temperatures in the longitudinal
fields of 10 G and 200 G, respectively. (Right panel)
Temperature dependence of the slow relaxation rates fromthe muon polarization at longitudinal fields of 10 G and 200
G, respectively.
Moreover, we also extracted the relaxation rate on thelatter time region by fitting with the superposition of two
exponential functions from the muon polarization at several
temperatures in the longitudinal fields of 10 G and 200 G, asshown in Fig. 2. Both relaxation rates on latter times in thelongitudinal fields of 10 G and 200 G are dependent on thetemperature and exhibit a sharp peak around 6 K. Then, as
the temperature cools, both relaxation rates increase, andbecome almost saturated below 2 K.

References
1) K. W. Plumb et al., Phys. Rev. B 90, 041112 (2014).
2) Y. Kubota et al., Phys. Rev. B 91, 094422 (2015).
3) J. A. Sears et al., Phys. Rev. B 91, 144420 (2015).
4) M. Majumder et al., Phys. Rev. B 91, 180401 (2015).
5) R491 in the 2015 RIKEN Accelerator Program Report.
6) F. L. Pratt et al., Hyperfine Interact. 106, 33 (1997).

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