Temperature Dependence of Internal Field by Analysis of Specific Heat on an Organic Conductor λ-BETS 2 FeCl 4

-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 the Fe 3d localized spin still remains. To investigate why the mysterious free 3d spin exists in the  spin AF ordered 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.