NMR Detection of Temperature-Dependent Magnetic Inhomogeneities in URu2Si2

We present 29Si-NMR relaxation and spectral data in URu2Si2. Our echo-decay experiments detect slowly fluctuating magnetic field gradients. In addition, we find that the echo-decay shape (time dependence) varies with temperature T and its rate behaves critically near the Neel temperature TN, indicating a correlation between the gradient fluctuations and the transition to small-moment order. T-dependent broadening contributions become visible below 100 Kelvin and saturate somewhat above TN, remaining saturated at lower temperatures. Together, the line width and shift suggest partial lattice distortions below TN. We propose an intrinsic minority phase below $T_{\rm N}$ and compare our results with one of the current theoretical models.Comment: 2 pages RevTeX, 1 figure, SCES 99-Japan, to appear in Physica

Direct evidence for critical exchange coupling via Allayers in the heavy fermion compound CePd2Al3

The long-range planar-type antiferromagnetic ordering in the heavy-fermion compound CePd2Al3 is shown to critically depend on the exact occupation and distribution of aluminum atoms over their available lattice sites. A poly-crystalline sample (TN=2.7 K) is systematically compared with a nonordering single crystal, by means of susceptibility, specific heat, and site-specific Al-NQR/NMR experiments. Implications for other Al-containing heavy-fermion systems are discussed

Susceptibility inhomogeneity and non-Fermi liquid behavior in UCu_{5-x}Pt_x

Transverse-field muSR shifts and relaxation rates have been measured in the non-Fermi liquid (NFL) alloy system UCu_{5-x}Pt_x, x = 1.0, 1.5, and 2.5. At low temperatures the fractional spread in Knight shifts delta K/K approx deltachi/chi is gtrsim 2 for x = 1, but is only half this value for x = 1.5 and 2.5. In a disorder-driven scenario where the NFL behavior is due to a broadly distributed (Kondo or Griffiths-phase cluster) characteristic energy E, our results indicate that delta E/E_{rm av} approx (delta K/K)_{T=0} is similar for UCu_{5-x}Pd_x (x = 1 and 1.5) and UCu_4Pt, but is reduced for UCu_{5-x}Pt_x, x = 1.5 and 2.5. This reduction is due to a marked increase of E with increasing x; the spread delta E is found to be roughly independent of x. Our results correlate with the observed suppression of other NFL anomalies for x > 1 in UCu_{5-x}Pt_x but not in UCu_{5-x}Pd_x, and are further evidence for the importance of disorder in the NFL behavior of both these alloy systems.Comment: 4 pages, 2 figures, submitted to 10th International Conference on Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200

Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2

Muon spin relaxation (muSR) experiments have been carried out at low temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The longitudinal-field muSR relaxation function is exponential, indicative that the dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of the form \chi''(\omega) \propto \omega^{-y} f(\omega/T), \lim_{x\to0} f(x) = x for the dynamic spin susceptibility.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002 (Physica B

Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35 \pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield \gamma = 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.Comment: 4 pages, 4 figures, to be published in Physical Review Letter

Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials

We report on zero-field muon spin relaxation studies of cerium based heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1 compositions muon spin relaxation functions were found to be temperature independent across T_c; no evidence for the presence of electronic magnetic moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K and superconducting below T_c=0.8 K. Muon spin realxation spectra show the gradual onset of damped coherent oscillations characteristic of magnetic order below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85% of the sample volume. No change in muon precession frequency or amplitude is detected on cooling below T_c, indicating the microscopic coexistence of magnetism and superconductivity in this material.Comment: 6 pages with 3 figures. Revision with corrected axis label (mK) in Fig.