The compound lambda-(BETS)2FeCl4 provides an effective demonstration of the interaction of pi-conduction electron and d-electron localized moment systems in molecular crystalline materials where antiferromagnetic insulating and magnetic field induced superconducting states can be realized. The metal-insulator transition has been thought to be cooperative, involving both the itinerant pi- electron and localized d-electron spins where antiferromagnetic order appears in both systems simultaneously. However, recent specific heat data has indicated otherwise [Akiba et al., J. Phys. Soc. Japan 78,033601(2009)]: although the pi-electron system orders antiferromagnetically and produces a metal-insulator transition, a "mysterious" paramagnetic d-electron state remains. We report 57Fe Mossbauer measurements that support the paramagnetic model, provided the d-electron spins remain in a fast relaxation state below the transition. From the measured hyperfine fields, we also determine the temperature dependence of the pi-d electron exchange field.
@article{arxiv.0909.1096,
title = {Mossbauer spectroscopy study of the "mysterious" magnetic transition in lambda-(BETS)2FeCl4},
author = {J. C. Waerenborgh and S. Rabaca and M. Almeida and A. Kobayashi and B. Zhou and J. S. Brooks},
journal= {arXiv preprint arXiv:0909.1096},
year = {2015}
}