The nature of the magnetic and structural phase transitions in BaFe$_{2}$As$_{2}$

TL;DR

This study investigates the magnetic and structural phase transitions in BaFe$_2$As$_2$, revealing they occur simultaneously as a second order transition with critical behavior consistent with a 2D Ising model.

Contribution

The paper provides detailed neutron diffraction analysis showing coupled magnetic and structural transitions in BaFe$_2$As$_2$, with evidence of critical scattering and biquadratic coupling, advancing understanding of phase transition nature in iron pnictides.

Findings

Transitions are simultaneous and continuous with no hysteresis.

Magnetic order follows a power law with an exponent near 1/8.

Evidence of three-dimensional critical scattering and coupling between order parameters.

Abstract

We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped, iron pnictide compound BaFe$_2$As$_2$. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power law dependence $\phi(T)^2\propto(1-\frac{T}{T_N})^{2\beta}$ with $\beta=0.103\pm0.018$; a value near the $\beta=0.125$ expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system.