Mössbauer spectroscopy study of the magnetostructural and spin-state transitions in the breathing pyrochlore LiFeCr${}_4$O${}_8$

Abstract

We report on investigations of the complex magnetostructural and spin-state transitions in the breathing pyrochlore LiFeCr${}_4$O${}_8$ by means of magnetization, Mössbauer spectroscopy, and density functional theory (DFT) calculations. Three transitions corresponding to the ferrimagnetic transition at $T_N\sim 94$ K, the spin-gap transition at $T_{SG}\sim 50$ K, and the magnetostructural transition at $T_{MS}\sim 19$ K were observed from the $\ddot{I}‡$(T) curve, whereas only $T_N$ and $T_{MS}$ were evidenced for the Fe site from our Mössbauer measurements, suggesting that the spin-gap transition is absent at the Fe site. This indicates that the spin-gap transition is an effect of the breathing Cr${}_4$ lattice, in agreement with our DFT calculations from which we see nearly decoupled electronic states for the FeO${}_4$ and CrO${}_6$ units. From the temperature dependence of the hyperfine magnetic field we also observed a spin-state transition for the Fe spins at $T_{MS}$ consistent with earlier neutron diffraction measurements. These local characteristics are believed to be important for a complete understanding of the complex magnetostructural coupling effects observed in similar systems.

9 pages, 8 figures