  The phase diagram of the iron arsenides is dominated by a magnetic and a
structural phase transition, which need to be suppressed in order for
superconductivity to appear. The proximity between the two transition
temperature lines indicates correlation between these two phases, whose nature
remains unsettled. Here, we find a scaling relation between nuclear magnetic
resonance (NMR) and shear modulus data in the tetragonal phase of
electron-doped $\mathrm{Ba(Fe_{1-x}Co_{x})_{2}As_{2}}$ compounds. Because the
former probes the strength of magnetic fluctuations while the latter is
sensitive to orthorhombic fluctuations, our results provide strong evidence for
a magnetically-driven structural transition.
