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

Abstract

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 ͉y͉ Ն 0.02͒, show bulk ferromagnetism at low temperature. For stoichiometric NbFe 2 , on the other hand, magnetization, magnetic susceptibility, and magnetoresistance results point toward spin-density wave ͑SDW͒ order, possibly helical, with a small ordering wave vector Q ϳ 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 Ϸ −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.

Doping-driven magnetic instabilities and quantum criticality ofNbFe2

“…In this section we utilise doped supercell calculations to investigate the evolution of N (E F ) with both hole (Nb) and electron (Fe) doping. We note that doped NbFe 2 samples are disordered as is evidenced by the significant increases in the low temperature resistivities that occur upon doping 7 . For these calculations we have employed the lattice parameters derived from experiment 7 and relaxed all internal coordinates.…”

“…Also, we have made a simple approximation to the disorder by applying a gaussian broadening of width 41meV to our density of states to reflect the decreased lifetime of electrons in the states near the Fermi level due to disorder. This is calculated from the measured low temperature resistivity in a Drude approximation 7,26,27 . In Nb-rich Nb 1.065 Fe 1.935 we find that N (E F ) = 4.6 eV −1 FU −1 .…”

“…Recent experimental work has further probed the behavior of the temperature-doping phase diagram of high purity NbFe 2 via measurements of the temperature dependence of resistivity, specific heat and magnetic susceptibility 7,8 . The resulting phase diagram is shown in Fig.…”

Spectroscopic study of metallic magnetism in single-crystallineNb1−yFe2+y

“…In the Nb 1−y Fe 2+y composition series, the magnetic state depends sensitively on composition [8][9][10][11] . Nbrich (y < −0.02) and Fe-rich (y > 0.005) Nb 1−y Fe 2+y are weakly ferromagnetic (FM) at low temperature, which may be attributed to changes in the electronic structure [12][13][14][15] .…”

Field-tuned Fermi liquid in quantum critical YFe2Al10

“…By comparison, the description of QC behavior is much more limited in systems where magnetism is derived from the d-electrons. It is well established that pressure or field can drive magnetic ordering to T=0, via a first or second order transition [24][25][26][27][28] . Other compounds have singular temperature dependencies in specific heat and magnetic susceptibility [29][30][31][32][33][34][35] that signal their proximity to a QCP, although the use of doping to approach the QCP can replace the intrinsic critical fluctuations with mean field behavior 27,36,37 .…”