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

We present a systematic study of transport and thermodynamic properties of the Laves phase system Nb$_{1-y}$Fe$_{2+y}$. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for $\mid y\mid\geq 0.02$), show bulk ferromagnetism at low temperature. For stoichiometric NbFe$_2$, on the other hand, magnetization, magnetic susceptibility and magnetoresistance results point towards spin-density wave (SDW) order, possibly helical, with a small ordering wavevector $Q \sim 0.05$ Å$^{-1}$. 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 $y\approx -0.015$. 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 NbFe$_2$ and the ferromagnetic order which reemerges for very Nb-rich NbFe$_2$, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.