Avoided Ferromagnetic Quantum Critical Point: Unusual Short-Range Ordered State in CeFePO

Abstract

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac-susceptibility, specific heat and muon-spin relaxation (µSR). Short-range static magnetism occurs below the freezing temperature Tg ≈ 0.7 K, which prevents the system from accessing the putative ferromagnetic quantum critical point. In the µSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.PACS numbers: 71.27.+a, 64.70.Tg, 76.75.+i, 75.50.Lk A long-standing question in the field of quantum criticality is whether a ferromagnetic (FM) quantum critical point (QCP) generally exists and, if not, which are the possible ground states of matter that replace it. Quantum critical points occur when a material is continuously tuned with an external parameter (pressure, magnetic field, etc.) between competing ground states at zero temperature [1, 2]. An FM-QCP then exists when it is possible to shift the Curie transition temperature T C of a ferromagnet continuously to zero where a second order quantum phase transition takes place. Quantum phase transitions occur at zero entropy and are driven by quantum rather than thermal fluctuations. These fluctuations diverge at the QCP modifying the excitation spectrum of a metal and leading to a fundamental instability of Landau's Fermi liquid (FL) [3]. Typical signatures of such a behavior are observed in magnetic, thermal and transport properties and are referred to as non-Fermi-liquid (NFL) phenomena [4].Although there is clear evidence for the existence of antiferromagnetic (AFM) QCPs, the FM-QCP case is controversial. In recent years, substantial experimental and theoretical efforts were made to further investigate this problem. However, a wide range of possibilities exists. On theoretical grounds, a 3-dimensional (3D) FM-QCP is believed to be inherently unstable, either towards a first order phase transition or towards an inhomogeneous magnetic phase (modulated/textured structures) [5][6][7]. Similar results have been obtained in 2D [5,8,9] CeRuPO [21] where the FM transition temperature is suppressed to T = 0 by hydrostatic pressure, exhibit a change into AFM order before reaching the QCP. There are Ce-based alloys (CePd 1−x Rh x [22]) and also d-electron metals (Ni 1−x V x [23]) where it seems that local disorder-driven mechanisms such as Kondo disorder or the quantum Griffiths phase (QGP) scenario are responsible for the NFL properties [24][25][26]. Broad and strongly T dependent NMR and µSR linewidths are indicative for such disorder-driven mechanisms. As a consequence spin-glass-like behavior is often found, e.g., in CePd 1−x Rh x , and power-law corrections to the thermodynamic and transport properties as well as in the local spi...

Quantum Order-by-Disorder in Strongly Correlated Metals

“…[66] that the fluctuation-induced helimagnet might be rather susceptible to the effects of disorder, possibly explaining the apparent spin glass behaviour in the region of the phase diagram of CeFePO(Ref. [67]) where helimagnetism might otherwise be predicted. More recently, two materials have emerged as compelling candidates for the emergence of fluctuation-induced helimagnetic order, PrPtAl [68] and LaCrGe 3 [69,70].…”

Pressure–Temperature–Magnetic Field Phase Diagram of Ferromagnetic Kondo Lattice CeRuPO

“…CeRuPO at 2.80 GPa and CeFePO are both located close to the magnetic instability. 20,26 In CeRuPO at 2.80 GPa, ρ(T ) starts to decrease below ∼ 100 K and shows a sharp decrease again below ∼ 10 − 15 K. The anomaly at high temperature is considered to originate from the combination of the Kondo effect and the CEF splitting, 23 and the shoulder at low temperature is attributed to the development of a coherent Kondo state. The characteristic coherent temperature T * was estimated from a kink in…”

Quantum critical fluctuations in layered YFe 2 Al 10

“…High pressure experiments evade the disorder that necessarily accompanies doping, but thus far only resistivity and susceptibility measurements have been reported. Complete experimental access is possible when doping is used to suppress T C → 0, but even small amounts of compositional inhomogeneity can obscure any intrinsic critical fluctuations of T C = 0 ferromagnetic transitions, resulting in interesting complications such as the Griffiths phase (12,13,14), as well as short ranged order, including spin glasses (15). Alternative ordered states that range from unconventional superconductivity in UGe 2 (16) and UCoGe (17), antiferromagnetic order in CeRu 2 Ge 2 (18), hidden order in URu 1−x Re x Si 2 (19), and spiral order in MnSi (20) may also emerge when ferromagnetism becomes sufficiently weak, potentially masking quantum critical fluctuations associated with the underlying T C = 0 ferromagnetic transition.…”