Ferromagnetic Quantum Critical Point in the Heavy-Fermion Metal YbNi ₄ (P _{1− x} As _x ) ₂

Abstract

Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi(4)(P(0.92)As(0.08))(2) diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.

Fluctuation-Driven Magnetic Hard-Axis Ordering in Metallic Ferromagnets

“…Quantum fluctuations close to a quantum critical point are potentially strong enough to drive a moment reorientation even in the absence of frustration. This is supported by recent experiments on YbNi 4 P 2 [12], which show quantum critical fluctuations with a uniform magnetic susceptibility that is an order of magnitude larger along the easy direction. Remarkably, at very low temperatures -well below the Kondo temperature -this system displays a switch of magnetic response anisotropy similar to YbRh 2 Si 2 and develops FM order along the hard direction.…”

“…Similar effects are possible in homogenous itinerant FMs. This is suggested by recent experiments that show unusual ordering of magnetic moments along hard magnetic directions [11,12].…”

“…Similar effects are possible in homogenous itinerant FMs. This is suggested by recent experiments that show unusual ordering of magnetic moments along hard magnetic directions [11,12].The first example is YbRh 2 Si 2 , which is a prototypical system for studying antiferromagnetic quantum criticality, but exhibits strong FM fluctuations [24]. Interestingly, isoelectronic cobalt substitution for rhodium or hydrostatic pressure stabilize FM order along the hard axis at low temperatures [11,25].…”

“…In many systems, quantum phase transitions are preempted by the formation of superconducting [5][6][7], modulated magnetic [8], or unusual spin-glass phases [9,10]. Other metallic FMs show an unexpected ordering along the magnetic hard axis [11,12].It is well understood that the first-order behavior arrises from the coupling of the magnetic order parameter to soft electronic particle-hole fluctuations, giving rise to non-analytic terms in the free energy [13][14][15][16]. Because of this interplay between low-energy quantum fluctuations, metallic FMs are very susceptible towards the formation of incommensurate magnetic [17], spin nematic [14] or modulated superconducting states [18].…”

“…In many systems, quantum phase transitions are preempted by the formation of superconducting [5][6][7], modulated magnetic [8], or unusual spin-glass phases [9,10]. Other metallic FMs show an unexpected ordering along the magnetic hard axis [11,12].…”