Antiferromagnetic Insulating Phase of λ-(BETS)2FeCl4
Studied by Electron Spin ResonanceYugo Oshima1, Taehoon Lee1,2, Heng-bo Cui1, and Reizo Kato1
12Condensed Molecular Materials Laboratory, RIKEN,
Graduate School of Science, Department of Condensed Matter Physics, Hokkaido University

Molecular conductors with finite π-d interaction have attracted considerable interests for the past few
decades since interaction between the itinerant π-electron and localized d-electron yields some intriguing
physical phenomena. λ-(BETS)2FeCl4 is one of the such materials which shows interesting physicalproperties. Although it is well-known that this salt shows superconducting state above 17 T when themagnetic field is applied along the conducting plane [1], the ground state at low temperature is alsointeresting. The high-temperature phase of this system is paramagnetic and metallic (hereafter, the PMphase). Then, this salt shows a metal-insulator (MI) transition, and becomes simultaneously
antiferromagnetic at TMI = 8.3 K known as the antiferromagnetic insulating (AFI) phase. The origin of theantiferromagnetic state, whether the d-electrons or the π-electrons trigger the antiferromagnetic state, isdiscussed for a long time. However, recent specific heat measurements revealed that the broad excess
specific heat observed below TMI can be fit with the Schottky peak in which the six energy levels of Fe3+
(S=5/2) is split by an internal field of 4 T. Therefore, it is now considered that only π-electrons become
antiferromagnetic (inducing an internal field of 4 T), and the d-electrons of Fe3+ remain paramagnetic [2].
However, previous ESR studies did not observe electron paramagnetic resonance (EPR) in the AFI phase,and only antiferromagnetic resonance (AFMR) was reported. [3-7] Meanwhile, many dielectric anomalies
are reported nearby the phase boundary of the PM and AFI phases [3,6]. We think that the observation ofdielectric anomalies just below TMI are due to the non-localized π-electrons, which affect the magnetism of
the system, and are the origin of the excess specific heat. Hence, to resolve these issues, we are performing
ESR measurements on λ-(BETS)2FeCl4, and studying its angular and temperature dependences in detail.
Last fiscal year, we have studied the EPR of λ-(BETS)2FeCl4 in the PM phase, and reported that theangular dependence has some characteristic features owing to the strong π-d interaction [7]. Moreover, wehave proposed a method to find the antiferromagnetic
easy-axis from the angular dependence of EPR. Thisfiscal year, we have applied the magnetic field along
the easy-axis by using this method, and studied theAFMR in detail.
Figure 1 is the angular dependence of AFMR using
the X-band ESR system. In general, a bubble structure,
which consists of spin-flop resonance and theeasy-axis mode of AFMR, is observed for alow-frequency region such as the X-band when themagnetic field is applied nearby the antiferromagneticeasy-axis. The center of the bubble corresponds to theeasy-axis, and the top and the bottom part of thebubble corresponds to the spin-flop resonance and theFigure 1 Angular dependence of AFMR near the easy-axis.

easy-axis mode of AFMR, respectively.
In consistency with the previous report, the spin-flopresonance is observed around 1150 mT (top part of thebubble) [8]. Moreover, the easy-axis mode of AFMR
shifts to lower field as the temperature increases, whichis also consistent with the previous report [4-6].
However, we have observed for the first time that theeasy-axis (i.e. center of the bubble) tilts as a function oftemperature.
Anomalous AFMR signals are also observed in thehigh magnetic field region. Although the frequency
dependence of AFMR shows a conventional easy-axis
mode of AFMR, the AFMR signal splits above 6 T (seeopen and solid triangles in Fig. 2). The split of AFMR

Figure 2 Frequency-resonance field plots of AFMR for B //easy-axis at 2 K

suggests a transition to a different ground state, and it
seems to be an intrinsic behavior since the signal split isalso observed for B//a- and B//c-axes as shown in Fig. 3.
Moreover, the easy-axis mode seems to change to thehard-axis mode for the B//c-axis, and vice versa for theB//a-axis above 4 T as shown in Fig. 3.
In summary, the AFMR below 5 K shows the typicalbehavior of the antiferromagnet with uniaxial anisotropy.
However, interesting behavior, including the tilt of theeasy-axis and the AFMR signal spilt, has been observed
near the boundary between the AFI and PM phases. It is
supposed that the dielectric anomalies observed at thephase boundary are the origin of the observed anomalous

Figure 3 The frequency dependence of the resonance fieldfor B//c- and a-axes.

antiferromagnetic state.

References[1] S. Uji et al., Nature, 410, 908 (2001).

[2] H. Akiba et al., J. Phys. Soc. Jpn., 78, 033601 (2009).

[3] Y. Oshima etal., Magnetochemistry, 3, 10 (2017). [4] L. Brossard et al., Eur. Phys. B, 1, 439 (1998). [5] T. Suzuki et al., Phys. Rev.
B, 67, 020408(R) (2003). [6] I. Rutel et al., Phys. Rev. B, 68, 144435 (2003). [7] T.-H. Lee et al., J. Phys. Soc. Jpn., 87,114702 (2018). [8] T. Sakaki et al., Synth. Met., 120, 759 (2001).