Temperature Dependence of Internal Field by Analysis of Specific Heat on an Organic Conductor λ-BETS₂FeCl₄

Abstract

-BETS2FeCl4 (BETS = bis (ethylenedithio) tetraselenafulvalene) system exhibits a mysterious paramagnetic metal (PM)-antiferromagnetic insulator (AFI) phase transition. Just after AF ordering, the large entropy of Fe 3d localized spin still remains. To investigate why the mysterious free 3d spin exists in the  spin AF ordering state, we measured the specific heat in the vicinity of the PM-AFI transition. The formation process of this AF ordering is discussed in the context of a low-dimensional spin network. It is proposed that the FeCl4 crystal field plays a crucial role in this magnetic ordering via a -d interaction.

Stabilization of Long-Range Order by Additional Anisotropic Spins in Two-Dimensional Isotropic Heisenberg Antiferromagnets —A Possible Model of an Organic Compound with Magnetic Anions—

“…The Schottky model exhibits the six-level Schottky-type specific heat, which agrees well with the experimental data in λ-(BETS) 2 FeCl 4 [18]. Some authors adopted this model [18,22,33], and reproduced not only the specific heat [18] but also the Mössbauer effect [33]. In the analysis of the specific heat by Akiba et al [17], an additional scale factor is introduced to change the magnitude of specific heat.…”

Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

“…They proposed that the difference of T N by the Fe content is due to the fluctuation of the π-electron that is suppressed by the magnetic anisotropy introduced by the Fe spins [23]. The paramagnetic Fe model was further extended by introducing 2D Ising-like transition and double Schottky model, to explain the heat capacity behavior nearby T N and the field dependence, respectively [24,25].…”

Scaling Theory for Critical Phenomena in Coupled Magnetic Systems —Critical Exponents of πd Antiferromagnets—

“…The phase transition and critical phenomena in coupled magnetic systems have been examined experimentally [1][2][3][4][5] and theoretically [6][7][8][9] in connection with the organic πd antiferromagnet λ-(BETS) 2 FeCl 4 , [10][11][12][13][14][15][16][17] where BETS stands for bis(ethylenedithio)tetraselenafulvalene. In this compound, the long-range order is principally sustained in a π-electron system 2) despite the large length of d spins.…”

“…The phenomena in the vicinity of T c are also intriguing; for example, the magnetic specific heat C(T ) exhibits an extremely sharp peak around T c . [1][2][3][4][5] This phenomenon can be understood 9) as a consequence of the mismatch between the energy scales of T c and J 1 , which is a unique feature of this compound as mentioned above. Furthermore, it was predicted 9) that the system undergoes a pseudo second-order phase transition with a small drop in the order parameter at T c (the transition finally becomes a first-order transition), which originates from a reduction of the effective interaction between the π electrons 7) due to a logarithmic divergence of the fluctuations at T c .…”

“…Because the leading terms are retained in these definitions, we obtain δ ℓ = max(δ (1) ℓ , δ (2) ℓ ). Hence, using Eq.…”