Incommensurate Magnetism in FeAs Strips: Neutron Scattering from CaFe$_4$As$_3$

TL;DR

This study reveals incommensurate and complex magnetic order in CaFe$_4$As$_3$, with phase transitions at 89.6 K and 25.6 K, linked to Fermi surface nesting in FeAs strips, using neutron diffraction.

Contribution

It provides detailed neutron diffraction analysis of magnetic phases and transitions in CaFe$_4$As$_3$, highlighting the role of Fermi surface nesting in its magnetic behavior.

Findings

Incommensurate magnetic order develops below 89.6 K.

A first-order transition at 25.6 K introduces a transverse component.

Fermi surface nesting correlates with magnetic ordering.

Abstract

Magnetism in the orthorhombic metal CaFe$_4$As$_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (${\bm q}_{\rm m}\approx (0.37-0.39)\times{\bm b}^{\ast}$) and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_{\rm N}=89.63(6)$ K with a 3D Heisenberg-like critical exponent $\beta=0.365(6)$. A 1st order transition at $T_2=25.6(9)$ K is associated with the development of a transverse component, locking ${\bm q}_{\rm m}$ to $0.375(2){\bm b}^{\ast}$, and increasing the moments from 2.1(1) to 2.2(3) $\mu_{\rm B}$ for Fe$^{2+}$ and from 1.3(3) to 2.4(4) $\mu_{\rm B}$ for Fe$^+$. The {\it ab-initio} Fermi surface is consistent with a nesting instability in cross-linked FeAs strips.