Mossbauer spectroscopy study of the "mysterious" magnetic transition in lambda-(BETS)2FeCl4

TL;DR

This study uses Mossbauer spectroscopy to investigate the magnetic transition in lambda-(BETS)2FeCl4, revealing that the d-electron spins remain paramagnetic with fast relaxation below the transition, challenging previous cooperative models.

Contribution

The paper provides experimental evidence supporting a paramagnetic d-electron state in lambda-(BETS)2FeCl4 using Mossbauer measurements, clarifying the nature of its magnetic transition.

Findings

Supports paramagnetic d-electron model with fast relaxation

Determines temperature dependence of pi-d exchange field

Challenges previous cooperative transition theories

Abstract

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.