Magnetic order and spin dynamics across a ferromagnetic quantum critical point: μ\muSR investigations of YbNi4_4(P1−x_{1-x}Asx_x)2_2

In the quasi-1D heavy-fermion system YbNi$_4$(P$_{1-x}$As$_x$)$_2$ the presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_c$ $\approx 0.1$ with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation ($\mu$SR) experiments on polycrystals of this series to study the magnetic order and the low energy 4$f$-electronic spin dynamics across the FM QCP. The zero field $\mu$SR measurements on pure YbNi$_4$(P$_{2}$ proved static long range magnetic order and suggested a strongly reduced ordered Yb moment of about 0.04$\mu_B$. With increasing As substitution the ordered moment is reduced by half at $x = 0.04$ and to less than 0.005 $\mu_B$ at $x=0.08$. The dynamic behavior in the $\mu$SR response show that magnetism remains homogeneous upon As substitution, without evidence for disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows 1/$T_{1}T\propto T^{-n}$ with $1.01 \pm 0.04 \leq n \leq 1.13 \pm 0.06$. The critical fluctuations are very slow and are even becoming slower when approaching the QCP.Comment: 6 pages, 4 figure

Ferromagnetism and superconductivity in P-doped CeFeAsO

We report on superconductivity in CeFeAs1-xPxO and the possible coexistence with Ce- ferromagnetism (FM) in a small homogeneity range around x = 30% with ordering temperatures of T_SC = T_C = 4K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to T^N_Fe ~ 40K and does not shift to lower temperatures with further increase of the P concentration. Therefore, a quantum-critical-point scenario with T^N_Fe -> 0K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and X-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short range AFM Fe-ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering.Comment: 5 pages, 4 figures, published in Phys. Rev. B (Rapid Communication, Editors suggestion

Field-induced coupled superconductivity and spin density wave order in the Heavy Fermion compound CeCoIn5

The high field superconducting state in CeCoIn5 has been studied by transverse field muon spin rotation measurements with an applied field parallel to the crystallographic c-axis close to the upper critical field Hc2 = 4.97 T. At magnetic fields >= 4.8 T the muon Knight shift is enhanced and the superconducting transition changes from second order towards first order as predicted for Pauli-limited superconductors. The field and temperature dependence of the transverse muon spin relaxation rate sigma reveal paramagnetic spin fluctuations in the field regime from 2 T < H < 4.8 T. In the normal state close to Hc2 correlated spin fluctuations as described by the self consistent renormalization theory are observed. The results support the formation of a mode-coupled superconducting and antiferromagnetically ordered phase in CeCoIn5 for H directed parallel to the c-axis.Comment: 5 paes, 4 figure

Avoided ferromagnetic quantum critical point: Unusual short-range ordered state in CeFePO

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 ($\mu$SR). 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 $\mu$SR, 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.Comment: 5 pages, 4 figures, accepted for publication in Physical Review Letters, Link: http://prl.aps.org/accepted/6207bYdaGef1483c419928305372ce2d4419eb96

Magnetic order and spin dynamics in the proximity of a ferromagnetic quantum critical point: A {\mu}SR study of YbNi4P2

The local 4f-electronic spin dynamics and magnetic order in YbNi4P2 were studied by means of muon-spin relaxation measurements. Zero-field muon-spin relaxation proves static magnetic order with a strongly reduced ordered Yb3+ moment of (2.5-4.6) \times 10-2{\mu}B, below TC = 140 mK. Above TC, the muon-spin polarization P(t,B) is dominated by quasihomogeneous spin fluctuations and exhibits a time-field scaling relation P(t,B) = P(t/B{\gamma}), indicating cooperative critical spin dynamics in the system. At T = 190 mK, slightly above TC, {\gamma} = 0.81(5), suggesting time-scale invariant power-law behavior for the dynamic electronic spin-spin autocorrelation function.Comment: 5 pages, 4 figure

Interplay of Superconductivity, Antiferromagnetism, and Pauli Depairing in CeCoIn5_5

Low temperature magnetization M measurements down to 70~mK have been performed through the superconducting upper critical field H_c2 of CeCoIn5 with the field H parallel to the c axis of the tetragonal crystal. The crossing through the upper critical field H_c2(0) is marked by a sign change in the temperature dependence of M(T). A Fermi liquid law of the magnetization is observed in the superconducting phase while in the paramagnetic state above H_c2 a quasi-linear temperature dependence is obeyed. The specific heat below H_c2 is therefor dominated by the increase of \gamma (H), the linear term of the specific heat, and is marked by a singularity at H_{c2} (0). No indication of the so called Q-phase was observed below H_c2 (in contrast to the case with H applied in the basal plane).Comment: 4 pages, 5 figure