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, resolve a long-wavelength spin density wave state. The associated ordering wavevector \$$\backslash$mathbf\{q\}_\{$\backslash$rm SDW\}=\$(0,0,\$l_\{$\backslash$rm SDW\}\$) is found to depend significantly on \$y\$ and \$T\$, staying finite but decreasing as the ferromagnetic state is approached. The results indicate that the interplay between FM and SDW order in NbFe\$_2\$ is consistent with the theoretically predicted scenario of a hidden ferromagnetic quantum critical point, which is masked by emerging modulated magnetic order.     «

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, resolve a long-wavelength spin density wave state. The associated ordering wavevector \$$\backsl...     »