Fig. 1: Summary of the effect of uniaxial pressure on crystalline lattice and magnetic structures of Ba₂Fe₂As₂.

a Crystal and AF structure of BaFe₂As₂. The red arrows indicate the a-axis direction of magnetic moments. b Magnetic order parameters measured at Q₁ = (1, 0, 3) and Q = (0, 1, 3) under uniaxial pressure, revealing T_N = 143 K. The blue dashed curve denotes the magnetic order parameter measured on a strain-free sample with T_N = 136 K. c Scattering geometry of polarized neutron scattering experiment in the [H, 0, L] plane. d The reciprocal space, where the fluctuating moments along the a-, b-, and c-axis directions are marked as M_a, M_b, and M_c, respectively. e Spin arrangements of BaFe₂As₂ in the paramagnetic (left), near T_N (middle), and low-temperature AF state. f Temperature dependence of the static ordered magnetic moment M_a and M_c as determined from \({\sigma }_{x,z}^{\mathrm{SF}\,}\) at (1, 0, 1) and (1, 0, 3). The vertical error bars are estimated errors from fits-to-order parameters. g Comparison of temperature dependence of the strain-induced lattice distortion from ref. ²⁷ and our estimated M_c at  ~20 (blue solid line) and  ~45 (pink solid line) MPa.