Uniaxial pressure dependence of the magnetic ordered moment and transition temperatures in BaFe2-xNixAs2

We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic phase in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and superconducting BaFe$_{\mathrm{1.915}}$Ni$_{\mathrm{0.085}}$As$_{\mathrm{2}}$. We find that the ordering temperature (TN) increases in both compounds, consistent with calculations of the static spin susceptibility within the random-phase approximation. At low temperature, where uniaxial pressure mechanically detwins the crystals, we find that the magnetic ordered moment in BaFe$_{\mathrm{1.915}}$Ni$_{\mathrm{0.085}}$As$_{\mathrm{2}}$ increases by as much as 20{\%} under 40 MPa of pressure, whereas no change was observed in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. The doping and uniaxial pressure dependence of the magnetic ordered moment is captured by DFT$+$DMFT calculations which suggests the pressure-induced increase near superconductivity is closely related to the diverging nematic susceptibility. We will also present new spin wave data on fully detwinned BaFe$_{\mathrm{2}}$As$_{\mathrm{2.}}$