Magnetism inNb1−yFe2+y: Composition and magnetic field dependence

We present a systematic study of transport and thermodynamic properties of the Laves phase system ${\text{Nb}}_{1\ensuremath{-}y}{\text{Fe}}_{2+y}$. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for $|y|\ensuremath{\ge}0.02$), show bulk ferromagnetism at low temperature. For stoichiometric ${\text{NbFe}}_{2}$, on the other hand, magnetization, magnetic susceptibility, and magnetoresistance results point toward spin-density wave (SDW) order, possibly helical, with a small ordering wave vector $Q\ensuremath{\sim}0.05\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$. Our results suggest that on approaching the stoichiometric composition from the iron-rich side, ferromagnetism changes into long-wavelength SDW order. In this scenario, $Q$ changes continuously from 0 to small, finite values at a Lifshitz point in the phase diagram, which is located near $y=+0.02$. Further reducing the Fe content suppresses the SDW transition temperature, which extrapolates to zero at $y\ensuremath{\approx}\ensuremath{-}0.015$. Around this Fe content magnetic fluctuations dominate the temperature dependence of the resistivity and of the heat capacity which deviate from their conventional Fermi-liquid forms, inferring the presence of a quantum critical point. Because the critical point is located between the SDW phase associated with stoichiometric ${\text{NbFe}}_{2}$ and the ferromagnetic order which reemerges for very Nb-rich ${\text{NbFe}}_{2}$, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.