Multistep Development of the Hyperfine Fields in ?-(BEDT-STF)^sub 2^FeCl^sub 4^ Studied by Mössbauer Spectroscopy

We have investigated the magnetic properties of λ-(BEDT-STF)2FeCl4 by Mössbauer measurement to reveal the origin of the paramagnetic-like behavior of the 3d spin system observed in λ-(BETS)2FeCl4 using the donor substitution effect. We found that the temperature dependence of the hyperfine fields shows multistep development below TN of 16 K. From comparison of the Mössbauer results and a theoretical model, we demonstrated that the multistep development is caused by coupling between the 3d spins and the π spins with strong spin correlation. We propose that the multistep development of the hyperfine fields is an intrinsic behavior induced by the π–d interaction.