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

RIKEN Accel. Prog. Rep. 49 (2016)

μSRinvestigation
investigationofof novelnovel magnetism
magnetism inμSR
in thethe4d
4dHeisenberg-Kitaev
Heisenberg-Kitaev
honeycomb
α-RuCl3 3compound α-RuCl
honeycomb compound
S. Yoon,*1,*2 S.-H. Do,*3 Y. S. Kwon,*4 K.-Y. Choi,*3 I. Watanabe,*1,*2 and B. J. Suh*2
The Kitaev honeycomb spin model has been considered
as an approach to study the spin liquid state.1)-3) This modelis based on the two-dimensional honeycomb network ofmagnetic ions with strong spin orbit coupling. This
environment can suppress the Heisenberg AF exchange,4),5)
and it can result in anisotropic exchange depending on thebond direction.5) This exchange leads to a highly frustrated
spin state with spatial and spin degrees of freedom.1)-3) This
fractional spin excitation that reflects the frustrated spinstate can be translated by an emergence of Majorana
fermions with their own antiparticles.1)-3),5)
Honeycomb ruthenate, α-RuCl3, was suggested as acandidate to realize the Kitaev spin model.6) It provides
proper environments for the Kitaev spin model, a cubic
environment composed of Cl- ions for revealing strong spin
orbit coupling, and honeycomb layers connected by weakvan der Waals forces for realizing the ideal two-dimensional
environment for Kitaev exchanges.6) It exhibits two distinct
magnetic anomalies, at 6 K, and 14 K in the DC
susceptibility. Also, significant exchange anisotropy isfound by observing the different Weiss temperatures with
opposite signs in the different external field orientations.7)-9)221.67 K
6.01 K6.43 K
6.63 K

Asymmetry (%)

20

1816140123456Time OµsP

150

Average local field (G)7.01 K
8.99 K
11.50 K
68.02 K

120
90

References

60
30002468101214

Temperature (K)Fig. 1. (upper panel) Time differential spectra of the muonpolarization at several temperatures in the zero field
condition, and (lower panel) temperature dependence of theinternal field extracted by fitting muon time spectra at thezero field with an assumption of the magnetic ordered state*1RIKEN Nishina Center
Department of Physics, The Catholic University of Korea*3
Department of Physics, Chung-Ang University*4
Department of Emerging Materials Science, DGIST*2

Our synthesized honeycomb ruthenate single crystal
shows three distinct anomalies at 6 K, 9.5 K, and 12.5 K in
the result of the heat capacity measurement, and themagnetic susceptibility exhibits the result similar to those in
the previous reports.7)-9) Besides, this material is found to be
similar to the ideal two-dimensional environment as arhombohedral lattice was revealed in the crystal structure
analysis. In this environment, the effects induced bystacking faults in the honeycomb layers are expected todecrease. For instance, different magnetic transitions
emerge from different stacking orders in the neutron
diffraction results.10) Additionally, a single anomaly at 14 K
is revealed in honeycomb ruthenate with a monoclinic
lattice.11)
Fig. 1 shows the muon time spectra at several
temperatures in the zero field condition (upper panel), andthe temperature dependence of internal fields extracted byfitting muon time spectra with an assumption of themagnetic ordered state (lower panel). The muon time
spectra obtained at the zero field exhibit the clear oscillation
below 6 K, and changes of the initial asymmetry, whichindicate the magnetic ordered state. In addition, single
exponential relaxation is exhibited over 13 K. With these
phenomena, we analyzed the muon time spectra based on
the magnetic ordered state below 13 K as shown in Fig. 1(lower panel). Furthermore, we observed similar relaxations
between 4 μs and 6 μs at 1.7 K, and 6 K. These relaxations
can provide evidence of the frustrated spin state.
In summary, we performed a microscopic investigation ofhoneycomb ruthenate with a rhombohedral lattice using
spin polarized muons. We analyzed the muon polarizations
below 13 K in the zero field condition with an assumptionof the magnetic ordered state revealed by the clear
oscillation below 6 K, and single exponential relaxation
over 13 K.
1) W. Witczak-Krempa et al., Annu. Rev. Condens. Matter Phys.
5, 57 (2014).
2) J. G. Rau et al., arXiv:1507.06323.
3) R. Schaffer et al., arXiv:1512.02224.
4) A. Kitaev, Ann. Phys. 321, 2 (2006).
5) G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 102, 017205(2009).
6) K. W. Plumb et al., Phys. Rev. B 90, 041112 (2014).
7) Y. Kubota et al., Phys. Rev. B 91, 094422 (2015).
8) J. A. Sears et al., Phys. Rev. B 91, 144420 (2015).
9) M. Majumder et al., Phys. Rev. B 91, 180401 (2015).
10) A. Banerjee et al., arXiv:1504.08037.
11) R. D. Johnson et al., Phys. Rev. B 92, 235119 (2015).

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