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

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFeAs, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature T. 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 ( < T) and create in-plane resistivity anisotropy above T. Here we use neutron polarization analysis to show that such a strain on BaFeAs also induces a static or quasi-static out-of-plane (c-axis) AF order and its associated critical spin fluctuations near T/T.