Magnetism In Nb1-Yfe2+Y: Composition And Magnetic Field Dependence

We present a systematic study of transport and thermodynamic properties of the Laves phase system Nb1-yFe2+y. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for vertical bar y vertical bar >= 0.02), show bulk ferromagnetism at low temperature. For stoichiometric NbFe2, 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 similar to 0.05 A degrees(-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 approximate to-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 NbFe2 and the ferromagnetic order which reemerges for very Nb-rich NbFe2, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.