Laves-phase ${\mathrm{Nb}}_{1+c}{\mathrm{Fe}}_{2-c}$ is a rare itinerant intermetallic compound exhibiting magnetic quantum criticality at $c_{\mathrm{cr}}\sim 1.5\%\mathrm{Nb}$ excess; its origin, and how alloying mediates it, remains an enigma. For ${\mathrm{NbFe}}_2$, we show that an unconventional band critical point above the Fermi level $E_{\mathrm{F}}$ explains most observations and that chemical alloying mediates access to this unconventional band critical point by an increase in $E_{\mathrm{F}}$ with decreasing electrons (increasing %Nb), counter to rigid-band concepts. We calculate that $E_{\mathrm{F}}$ enters the unconventional band critical point region for $c_{\mathrm{cr}}>1.5\%\mathrm{Nb}$ and by $1.74\%\mathrm{Nb}$ there is no Nb site-occupation preference between symmetry-distinct Fe sites, i.e., no electron-hopping disorder, making resistivity near constant as observed. At larger Nb (Fe) excess, the ferromagnetic Stoner criterion is satisfied.

• Received 9 February 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.206401