In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe$_2$As$_2$

TL;DR

Applying in-plane uniaxial pressure to BaFe₂As₂ induces out-of-plane antiferromagnetic moments and critical fluctuations, revealing that strain affects magnetic order orientation and must be considered when studying intrinsic properties.

Contribution

This study demonstrates that uniaxial strain not only detwins BaFe₂As₂ but also induces out-of-plane AF order and fluctuations, highlighting the importance of strain effects in magnetic property analysis.

Findings

Uniaxial pressure induces out-of-plane AF order in BaFe₂As₂.

Strain causes rotation of the AF easy axis near T_N/T_s.

Critical spin fluctuations are enhanced by uniaxial strain.

Abstract

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe$_2$As$_2$, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature $T_s$. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature $T_N$ ($<T_s$) and create in-plane resistivity anisotropy above $T_s$. Here we use neutron polarization analysis to show that such a strain on BaFe$_2$As$_2$ also induces a static or quasi-static out-of-plane ($c$-axis) AF order and its associated critical spin fluctuations near $T_N/T_s$. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe$_2$As$_2$ actually rotates the easy axis of the collinear AF order near $T_N/T_s$, and such effect due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.