Magnetic neutron diffraction study of Ba(Fe1-xCox)2As2 critical exponents through the tricritical doping

TL;DR

This study uses magnetic neutron diffraction to analyze the critical exponents of magnetic phase transitions in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ near the tricritical point, revealing different universality classes depending on doping levels.

Contribution

It provides detailed measurements of magnetic critical exponents across doping levels, identifying the transition from 3D Ising to tricritical behavior in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$.

Findings

Critical exponent $eta$ varies with doping, indicating different universality classes.

Near the tricritical doping, the system exhibits mean-field tricritical behavior.

The results align with theoretical models for magnetic phase transitions.

Abstract

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for $x$ = 0.039, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the ($x$,$T$) plane, $x_{tr} \approx $0.022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long range antiferromagnetic order that may be described near the transition by simple power laws, with $\beta$ =~0.306$\pm$0.060 for $x$ =~0.039, $\beta$ =~0.208$\pm$0.005 for $x$ =~0.022, and $\beta$ =~0.198$\pm$0.009 for $x$ =~0.021. For the $x$ =~0.039 sample, the data are reasonably well described by the order parameter exponent $\beta$ =~0.326 expected for a 3D Ising model while the $x$ =~0.022 and $x$ =~0.021 samples are near the $\beta$ =~0.25 value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.