  In the metallic magnet Nb$_{1-y}$Fe$_{2+y}$, the low temperature threshold of
ferromagnetism can be investigated by varying the Fe excess $y$ within a narrow
homogeneity range. We use elastic neutron scattering to track the evolution of
magnetic order from Fe-rich, ferromagnetic Nb$_{0.981}$Fe$_{2.019}$ to
approximately stoichiometric NbFe$_2$, in which we can, for the first time,
characterise a long-wavelength spin density wave state burying a ferromagnetic
quantum critical point. The associated ordering wavevector $\mathbf{q}_{\rm
SDW}=$(0,0,$l_{\rm SDW}$) is found to depend significantly on $y$ and $T$,
staying finite but decreasing as the ferromagnetic state is approached. The
phase diagram follows a two order-parameter Landau theory, for which all the
coefficients can now be determined. Our findings suggest that the emergence of
SDW order cannot be attributed to band structure effects alone. They indicate a
common microscopic origin of both types of magnetic order and provide strong
constraints on related theoretical scenarios based on, e.g., quantum order by
disorder.
