Magnetic and lattice coupling in single-crystal SrFe$_2$As$_2$: A neutron scattering study

TL;DR

This neutron scattering study of SrFe$_2$As$_2$ reveals that its structural and magnetic phase transitions are first order, coinciding at around 201.5 K, and are strongly coupled with evidence suggesting an order-disorder nature.

Contribution

The paper provides detailed experimental evidence showing the coupled first-order structural and magnetic transitions in SrFe$_2$As$_2$, highlighting the transition's order-disorder character.

Findings

Structural and magnetic transitions coincide at 201.5 K.

Transitions are first order with phase coexistence.

Evidence of an order-disorder transition mechanism.

Abstract

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_\texttt{O}$ = $T_\texttt{N}$ = (201.5 $\pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than \emph{T}$_\texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.