Figure 4. Evolution of the character of the magnetic and nematic transitions in the spin-driven nematic theory.

For both panels the blue lines represent magnetic transitions, the red lines denote structural transitions and the violet lines represents the joint magneto-structural transition. The grey lines show the trends noted in the spin-driven nematic theory⁵ which differ from those found in experiments. Thick lines denote first-order transitions, whereas the thinner lines denote second-order transitions. For a, the control parameter is partial element substitution (doping) for the case of BaFe₂As₂. For b, the control parameter is pressure for the case of FeSe. T_s, T_s*,m and T_s,m denote the structural transition temperature, the magnetic ordering temperature associated with the discontinuous change in the orthorhombicity, and the joint structural and magnetic transition temperature, respectively.