Phase transitions and iron-ordered moment form factor in LaFeAsO

TL;DR

This study investigates the structural and magnetic phase transitions in LaFeAsO single crystals using neutron scattering, revealing the coexistence of tetragonal and orthorhombic phases, and determining the iron magnetic moment at low temperature.

Contribution

It provides detailed insights into the structural transition temperatures, phase coexistence, and magnetic form factor of iron in LaFeAsO, advancing understanding of iron pnictide properties.

Findings

Structural transition at T_S ≈ 156 K and sharp transition at T_P ≈ 148 K.

Emergence of antiferromagnetic order at T_N ≈ 140 K with iron moments along the orthorhombic a axis.

Magnetic moment size of approximately 0.8 μ_B at 9.5 K.

Abstract

Elastic neutron scattering studies of an optimized LaFeAsO single crystal reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O) structural transition occurs at $T_\texttt{S} \approx 156$ K, and it exhibits a sharp transition at $T_\texttt{P} \approx 148$ K. We argue that in the temperature range $T_\texttt{S}$ to $T_\texttt{P}$, T and O structures may dynamically coexist possibly due to nematic spin correlations recently proposed for the iron pnictides, and we attribute $T_\texttt{P}$ to the formation of long-range O domains from the finite local precursors. The antiferromagnetic structure emerges at $T_\texttt{N} \approx 140$ K, with the iron moment direction along the O \emph{a} axis. We extract the iron magnetic form factor and use the tabulated $\langle j_0\rangle$ of Fe, Fe$^{2+}$ and Fe$^{3+}$ to obtain a magnetic moment size of $\sim$0.8 $\mu_\texttt{B}$ at 9.5 K.