Structural and magnetic phase transitions in Na$_{1-\delta}$FeAs

TL;DR

This study investigates the structural and magnetic phase transitions in Na$_{1-eta}$FeAs using neutron scattering, revealing sequential phase changes and confirming the commonality of antiferromagnetic order in iron arsenide superconductors.

Contribution

It provides detailed insights into the phase transition sequence and magnetic properties of Na$_{1-eta}$FeAs, highlighting the separation of structural and magnetic transitions.

Findings

Structural transition from tetragonal to orthorhombic at 49 K

Antiferromagnetic order with small magnetic moment below 49 K

Superconductivity emerges below 23 K

Abstract

We use neutron scattering to study the spin and lattice structures of single crystal and powder samples of Na$_{1-\delta}$FeAs ($T_c = 23$ K). On cooling from room temperature, the system goes through a series of phase transitions: first changing the crystal symmetry from tetragonal to orthorhombic at 49 K, then ordering antiferromagnetically with a spin structure similar to that of LaFeAsO and a small moment (0.09$\pm$0.04 $\mu_B$), and finally becoming superconducting below about 23 K. These results confirm that antiferromagnetic order is ubiquitous for the parent compounds of the iron arsenide superconductors, and suggest that the separated structural and magnetic phase transition temperatures are due to the reduction in the c-axis exchange coupling of the system.