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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Successive Transitions in Spin-dimer Compound Cs3 V2 Cl9H. Kikuchi,∗1,∗3 Y. Fujii,∗2 and I. Watanabe∗3ZF
19.7K
10.1K
5.7K
3.2K
3.7K
1.6K

Asymmetry

0.10024time (µsec)

68Fig. 1. Temperature dependence of the ZF-µSR spectra ofCs3 V2 Cl9 .
∗1∗2∗3

Department of Applied Physics, University of Fukui
FIR Center, University of Fukui
RIKEN Nishina Center0.15
Zero Field0.10-1

λs (µs )A3 M2 X9 (A = Cs, Rb : M = transition metal elements: X = Cl, Br) compounds with trigonal spacegroup P63 /mmc are composed of isolated di-nuclear
complexes [M2 X9 ]3− and their magnetic properties are
explained within an isolated or weakly coupled spin
dimer model. Because these spin dimers are arranged
in a triangular form, the spin frustration eﬀect is expected to appear when magnetic phase transition occurs via ﬁnite interdimer interactions. Cs3 V2 Cl9 , one
member of the A3 M2 X9 family with magnetic ion V3+(S = 1), was previously studied via magnetic susceptibility and inelastic neutron scattering measurement
using powder sample1) and no magnetic ordering wasobserved above 1.5 K. We recently synthesized a single
crystal of Cs3 V2 Cl9 and measured susceptibility χ(T )and speciﬁc heat, and we found successive phase transition at TN ≈ 4 K and Tn ≈ 15 K.2)
These successive phase transitions are not explained
within the framework of the isolated dimer model, andthey show the presence of non-negligible interdimer interaction. χ(T ) shows no anomaly at Tn although TN
is accompanied by the cusp-like anomaly of χ(T ). Thelower temperature transition is suggested to be an antiferromagnetic transition although the spin structure
is not clear. The higher temperature transition is nota mere crystal structure transition because the transition temperature depends on the applied magneticﬁeld.
We measured the µSR of Cs3 V2 Cl9 to clarify the nature of the successive phase transitions. Figure 1 shows
the zero-ﬁeld muon spin relaxation (ZF-µSR) spectra

0.050.00
01020Temperature (K)30Fig. 2. Temperature dependence of the relaxation rate λs .

measured down to 1.6 K. At relatively high temperatures, the spectra follow Gaussian curves. As thetemperature decreases, the spectra changes from Gaussian to an exponential curve following as exp(−λs t) +af exp(−λf t), where λs and λf denote slow and fast
relaxation rates, and as and af represent amplitudesof the asymmetry of slow and fast components, respectively. Solid lines in Fig. 1 are ﬁtted results. Figure 2
shows the temperature dependence of λs . A distinct
peak is observed at around TN , which conﬁrms that amagnetic long range order occurs at this temperature.
No anomaly of lambda is observed at Tn , which indicates that an internal ﬁeld does not appear. The phasetransition at Tn is not accompanied by the internalﬁeld, whereas the value of Tn depends on the appliedmagnetic ﬁeld. One candidate for the transition at Tn
is a spin nematic order wherein quadrupole moments,
not magnetic moments, play the role of an order parameter. Because the nematic state does not break thetime-reversal symmetry, usual magnetic probes including muon do not detect this transition.3) Several theoretical studies indicate the occurrence of the nematicorder in an S = 1 triangular lattice antiferromagnetor spin dimer magnets. The ﬁndings obtained by thisµSR experiments make it more likely that Tn is thenematic transition.
References

1) B. Leuenberger et al., Inorg. Chem. 25, 2930 (1986).
2) H. Kikuchi, T. Tanaka, Y. Fujii, A. Matsuo, K. Kindo,
Abstract of the 19 International Conference on Magnetism. July 8–13, 2012 Busan, Korea, p. 167.
3) A. Smerald et al., Phys. Rev. B 88, 184430 (2013).

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