In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe(2)As(2)

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 effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.