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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Antiferromagnetic ordering of λ-(BEST)2 FeCl4 observed by µSR
measurement
T. Kobayashi,∗1 D. P. Sari,∗2,∗3 K. Koyanagawa,∗1 K. Satoh,∗1 H. Taniguchi,∗1 A. Kawamoto,∗4 and I. Watanabe∗3In the organic conductor λ-(BETS)2 FeCl4 (see Fig. 1for BETS), there is a signiﬁcant strong exchange interaction between the 3d electrons of Fe ions, and π electronsof BETS molecules, i.e., π-d interaction. This results inexotic properties such as a ﬁeld-induced superconductivity1) and a metal-insulator transition with antiferromagnetic (AF) ordering, where 3d electrons are paramagnetic
even below the AF transition temperature indicated by
the heat capacity measurement.2) To understand these
phenomena, a deep understanding of the π-d interaction mechanism is required. As a clue to this, the magnetism of λ-(STF)2 FeCl4 has been recently evaluated.
As shown in Fig. 1, the STF molecule is a molecule inwhich part of the selenium in the BETS molecule is replaced by sulphur. Magnetisation, NMR, and Mössbauer
measurements showed that λ-(STF)2 FeCl4 exhibits AF
ordering at 16 K, where both 3d and π electrons are ordered, whereas further development of the internal magnetic ﬁeld owing to the polarized magnetisation of the 3dspins was observed below 8 K.3–5) In addition, µSR measurements demonstrated that the slow precession signals
due to the AF order of π-electrons and the fast precession signals due to the polarization of 3d spins appear
from diﬀerent temperatures. The magnetic properties
that appeared when the BETS molecule was replaced by
the STF molecule are thought to be caused by changes
in the π-d and d-d interactions. Therefore, to evaluate
the relationship between the molecular substitution andthe change in the π-d interaction, we focused on the substitution from BETS to BEST molecules (see Fig. 1 forBEST). Because the electron density of the outer chalcogen atoms is small, the expansion of the orbitals by thesubstitution from S to Se is expected to cause steric hindrance and reduce the π-d interaction.
According to the reported magnetisation measurementof λ-(BEST)2 FeCl4 , it exhibits paramagnetic behaviour
down to 4 K.6) However, because the observed spin susceptibility is dominated by 3d spins, the magnetism of π
spins is almost concealed. To reveal the magnetic orderof the 3d and π electrons, a microscopic measurementthat can sensitively detect the internal ﬁeld is necessary.

∗2∗3∗4For this purpose, we performed µSR measurements onλ-(BEST)2 FeCl4 .
Figure 2 shows the µSR time spectra at several temperatures. We observed that the initial asymmetry
changes abruptly between 28.4 K and 25.6 K. The time
spectra A(t) are ﬁtted by the stretched exponential function, A(t) = A exp[−(λt)β ] + Abg , where A and Abg
are the asymmetries due to the muons stopped inside
the sample and at the sample holder, λ is the relaxation rate, and β is the stretched exponent, respectively.
From this analysis, we deduced that the initial asymmetry A indeed decreases below ∼30 K, as shown in
the inset of Fig. 2, although its transition temperature
is not clear. Because the AF order of π spin was alsoobserved in λ-(BEST)2 GaCl4 consisting of nonmagnetic
Ga ions, the change in the initial asymmetry observedin λ-(BEST)2 FeCl4 can be understood by the AF order.
In fact, the disappearance of the paramagnetic EPR signal was observed at 25 K in recent ESR measurements,suggesting an AF transition.
Compared to λ-(STF)2 FeCl4 , it is interesting to knowwhether the fast precession signal due to the polarization of 3d spins is observed or not. Within our experimental error, no signiﬁcant spectral change was observed from 25.6 K to 2.11 K, suggesting that only theπ spins are magnetically ordered. In λ-(BETS)2 FeCl4 ,unlike λ-(BEST)2 GaCl4 , the anisotropy of the magnetic susceptibility was observed with AF transition.
Therefore, we conﬁrmed that the magnetic state of λ(BEST)2 GaCl4 is diﬀerent from that of λ-(BETS)2 FeCl4
and λ-(STF)2 FeCl4 . These results provide important
clues for the mechanism of π-d interaction.
References

Fig. 1. Structures of donor molecules.
∗1Fig. 2. Zero-field µSR time spectra of λ-(BEST)2 FeCl4 . Inset: temperature dependence of initial asymmetry.

Graduate School of Science and Engineering, Saitama University
College of Engineering, Shibaura Institute of Technology
RIKEN Nishina Center
Graduate School of Science, Hokkaido University

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3) S. Fukuoka et al., J. Phys. Soc. Jpn. 87, 093705 (2018).
4) T. Minamidate et al., Phys. Rev. B 97, 104404 (2018).
5) S. Fukuoka et al., Phys. Rev. B 101, 184402 (2020).
6) H. B. Cui et al., Chem. Lett. 34, 254 (2005).

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