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

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

μSR study of the stabilization mechanism of antiferromagnetic state
in molecular π-d system λ-(BEDT-STF)2 Fex Ga1−x Cl4
S. Fukuoka,∗1 Y. Ito,∗1 A. Kawamoto,∗1 D. P. Sari,∗2 and I. Watanabe∗3
In some molecular conductors, the coexistence ofstrongly correlated π electrons and localized 3d spinsis realized by introducing magnetic molecules, such as−FeCl−
4 and FeBr4 , as anion molecules. Such coexistentsystems are known as π-d systems. In π-d systems,
the magnetic interactions between strongly correlated
π electrons and localized 3d spins (π-d interaction) giverise to interesting magnetic and conducting properties.
λ-(BEDT-STF)2 FeCl4 , where BEDT-STF denotes
bis(ethylenedithio)dithiadiselenafulvalene, exhibits anantiferromagnetic ordering at 16 K.1,2) In the alloy
compound of λ-(BEDT-STF)2 Fe0.2 Ga0.8 Cl4 , an antiferromagnetic ordering is observed at 8 K. In contrast, λ-(BEDT-STF)2 GaCl4 shows no magnetic ordering down to 300 mK. These results indicate thatthe introduction of π-d interaction stabilizes the antiferromagnetic ground state. However, the stabilization
mechanism has not yet been demonstrated. This study
aims to clarify the stabilization mechanism of the antiferromagnetic ground state induced by π-d interaction.
In this study, we performed zero ﬁeld (ZF) μSR
measurements below 10 K and longitudinal ﬁeld (LF)
μSR measurements at base temperature and 10 K forλ-(BEDT-STF)2 Fe0.1 Ga0.9 Cl4 (Fe-0.1) and λ-(BEDTSTF)2 Fe0.05 Ga0.95 Cl4 (Fe-0.05), respectively.
Figure 1 shows the temperature dependence of theZF time spectra of Fe-0.1 and Fe-0.05. In this analysis,
we ﬁtted the time spectra by the following functions.
A(t) = A0 exp(−λ0 t) + GKT (t) + Abg ,(1)A(t) = A1 exp(−λ1 t) + A2 cos(ωt + φ)exp(−λ2 t)(2)+ Abg ,A(t) = A3 exp(−λ3 t) + A4 exp(−λ4 t) + Abg . (3)Here, Ai and λi denote the initial asymmetries andrelaxation rates, respectively. Abg is the backgroundcontribution derived from the muons stopped in thesample holder. G KT (t) is the Kubo-Toyabe function.
ω and φ are the precession frequency and phase ofmuon spin precession, respectively. The time spectra measured at high temperatures were ﬁtted using
Eq. (1), and those of Fe-0.1 and Fe-0.05 measured below 7 K were ﬁtted using Eqs. (2) and (3), respectively.
We conﬁrmed that the shape of the ZF time spectraof Fe-0.1 changes below 7 K, suggesting that an antiferromagnetic transition occurs around 7 K. In contrast,no clear change was observed in the shape of the ZF
time spectra down to 1.5 K for Fe-0.05. As complemen∗1∗2∗3Fig. 1. Temperature dependence of the ZF time spectra of(a) Fe-0.1 and (b) Fe-0.05.

tary experiments, we performed 13 C NMR measurements for λ-(BEDT-STF)2 Fe0.05 Ga0.95 Cl4 and conﬁrmed that no peak was observed in the temperaturedependence of the spin-lattice relaxation rate (1/T 1 )down to 4 K. These suggest that T N of Fe-0.05 is lower
than 4 K or does not exist and antiferromagnetic ordering is drastically stabilized in the small Fe content
region below x = 0.1. These results are consistent
with theoretical studies suggesting that the antiferromagnetic ground state is stabilized even for small
π-d interactions.3) However, our results indicate that
there exists a phase boundary between the no- andantiferromagnetic-ordered states in the low Fe content
region between x = 0 and 0.1, which is diﬀerent fromthe theoretical prediction. Detailed analysis regarding
the development of internal ﬁelds from the results of
μSR time spectra and comparison with the results oftheoretical studies are in progress.
References
1) S. Fukuoka et al., Phys. Rev. B 101, 184402 (2020).
2) S. Fukuoka et al., J. Phys. Soc. Jpn. 87, 093705 (2018).
3) H. Shimahara, K. Ito, J. Phys. Soc. Jpn. 83, 114702(2014).

Graduate School of Science, Hokkaido University
College of Engineering, Shibaura Institute of Technology
Meson Science Laboratory, RIKEN- 115 -1