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We present a systematic study of transport and thermodynamic properties ofthe Laves phase system Nb$_{1-y}$Fe$_{2+y}$. Our measurements confirm thatFe-rich samples, as well as those rich in Nb (for $\mid y\mid\geq 0.02$), showbulk ferromagnetism at low temperature. For stoichiometric NbFe$_2$, on theother hand, magnetization, magnetic susceptibility and magnetoresistanceresults point towards spin-density wave (SDW) order, possibly helical, with asmall ordering wavevector $Q \sim 0.05$ \AA$^{-1}$. Our results suggest that onapproaching 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 thephase diagram, which is located near $y=+0.02$. Further reducing the Fe contentsuppresses the SDW transition temperature, which extrapolates to zero at$y\approx -0.015$. Around this Fe content magnetic fluctuations dominate thetemperature dependence of the resistivity and of the heat capacity whichdeviate from their conventional Fermi liquid forms, inferring the presence of aquantum critical point. Because the critical point is located between the SDWphase associated with stoichiometric NbFe$_2$ and the ferromagnetic order whichreemerges for very Nb-rich NbFe$_2$, the observed temperature dependences couldbe attributed both to proximity to SDW order or to ferromagnetism.

Magnetism in Nb(1-y)Fe(2+y) - composition and magnetic field dependence