Incommensurate spin-density wave and magnetic lock-in transition in CaFe4As3

TL;DR

This study reveals the magnetic phase transitions in CaFe4As3, showing an incommensurate spin-density wave below 85K and a lock-in transition to a commensurate structure around 25K, with detailed magnetic modeling.

Contribution

The paper provides the first neutron diffraction analysis of CaFe4As3, identifying its magnetic structures and the nature of its phase transitions, including a lock-in transition to a commensurate magnetic order.

Findings

Long-range magnetic order appears below 85K.

Incommensurate modulation with δ~0.39 is observed.

A first-order transition locks δ into 3/8 at ~25K.

Abstract

The magnetic structure for the newly discovered iron-arsenide compound \CaFeAs has been studied by neutron powder diffraction. Long-range magnetic order is detected below 85K, with an incommensurate modulation described by the propagation vector k=(0,$\delta$,0), $\delta\sim$ 0.39. Below $\sim$ 25K, our measurements detect a first-order phase transition where $\delta$ locks into the commensurate value 3/8. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density-waves with magnetic moments directed along the \textit{b}-axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular the drop in resistivity at the lock-in transition.