We investigate the evolution of the electrical resistivity of BaFe(2)As(2) single crystals with pressure. The samples used were from the same batch, grown using a self-flux method, and showed properties that were highly reproducible. Samples were pressurized using three different pressure media: pentane-isopentane (in a piston-cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a Bridgman cell). Each pressure medium has its own intrinsic level of hydrostaticity, which dramatically affects the phase diagram. An increasing uniaxial pressure component in this system quickly reduces the spin density wave order and favours the appearance of superconductivity, which is similar to what is seen in SrFe(2)As(2).

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.

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.

We have investigated single crystals and polycrystals from the series Nb1−yFe2+y, −0.004 ≤ y ≤ 0.018 by electron spin resonance, muon spin relaxation and Mössbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Feexcess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric NbFe2 display spin density wave order with small magnetic moment amplitudes of the order ∼ 0.001 − 0.01µB /Fe. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in NbFe2.

Quantum critical points (QCPs) emerge when a 2nd order phase transition is suppressed to zero temperature. In metals the quantum fluctuations at such a QCP can give rise to new phases including unconventional superconductivity. Whereas antiferromagnetic QCPs have been studied in considerable detail ferromagnetic (FM) QCPs are much harder to access [1,2]. In almost all metals FM QCPs are avoided through either a change to 1st order transitions or through an intervening spin-density-wave (SDW) phase. Here, we study the prototype of the second case, NbFe 2 . We demonstrate that the phase diagram can be modelled using a two-order-parameter theory in which the putative FM QCP is buried within a SDW phase. We establish the presence of quantum tricritical points (QTCPs) at which both the uniform and finite q susceptibility diverge.The universal nature of our model suggests that such QTCPs arise naturally from the interplay between SDW and FM order and exist generally near a buried FM QCP of this type. Our results promote NbFe 2 as the first example of a QTCP, which has been proposed as a key concept in a range of narrow-band metals, including the prominent heavy-fermion compound YbRh 2 Si 2 [3].