In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2

Abstract

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe₂As₂, 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 BaFe₂As₂ 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 BaFe₂As₂ actually rotates the easy axis of the collinear AF order near T_N/T_s, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.

BaFe₂As₂ is a parent compound for iron based superconductors, and this has motivated extensive study of its magnetic and electronics properties. Here, using neutron polarisation analysis, the authors demonstrate the critical importance of spin-orbit coupling to properties of BaFe₂As₂.