R. Feyerherm

μ+‐site in heavy‐fermion compounds

We report on the μ+‐site determination in different heavy‐fermion systems, resulting from the analysis of the angular dependence of parameters of the μSR spectra. After an introduction on the basics of the analysis of the μ+‐Knight shift, we briefly discuss the specific information accessible once the knowledge of the crystallographic site occupied by the implanted muon is gained.

μSR studies of borocarbides

The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of the borocarbide compounds RNi2B2C ( R=\,Tm, Er, Ho and Dy), which exhibit coexistence of superconductivity and magnetism. Some general features of their magnetic ground‐state and unusual results obtained in the Ho, Tm and Dy systems are discussed.

On the competition between superconductivity and magnetism in CeCu2Si2

Abstract Muon spin relaxation measurements in zero magnetic field on several polycrystalline samples provide evidence that superconductivity and magnetic order do not coexist on a microscopic scale but develop inhomogeneously in Ce 1+ x Cu 2+ y Si 2 . The distribution of magnetic and superconducting domains depends on the temperature and sensitively on variations of the stoichiometry. Superconductivity sets in first in the paramagnetic domains and, on further cooling, reduces the magnetically ordered volume fraction. However, in all samples down to T = 60 mK considerable portions remain magnetic.

Muon spin rotation spectroscopy on a UNiGa single crystal

Abstract UNiGa orders antiferromagnetically (AF) below 39 K with four different AF phases appearing with decreasing temperature. These phases are reflected in μSR spectra by different sets of frequencies. In paramagnetic range three different Knight shift signals were detected. The results can be explained assuming three different stopping sites.

μSR studies of low charge carrier density heavy‐fermion systems

We report on μSR studies in low charge carrier density heavy‐fermion systems with cubic symmetry. Whereas for Ce3Au3Sb_4 and for Sm3Te4 the μSR data point to the presence of some kind of spin‐glass‐like magnetic states at low temperature, for Yb4As3 preliminary μSR studies just show a slight increase of the depolarization rate which indicates a slight slowing down of the Yb‐moment fluctuations. Consequently for this latter system, the large linear term of the specific heat seems indeed to be connected to electronic excitations.

Dynamics of μ+ in Sc and ScHx

Abstract Muon spin rotation (μSR) experiments on single-cystal samples of ScHx, with x = 0, 0.05, 0.10 and 0.25, were performed in order to investigate the muon location and possible systematic variations in muon dynamics in these systems. The angular and magnetic field dependance of the μSR relaxation rates shows that at temperatures below 50 K the muon is localized at the interstitial tetrahedral (T) sites. The results for pure Sc, covering a wide range of high and low fields, are coherent with the picture that the muon performs fast tunnelling between two adjacent T sites, separated by 2z = 0.18c, above and below the a-b plane. This value for the muon position along the c axis is remarkably near the 2z = 0.194c found for D in Sc by neutron diffraction. The anisotropy of the relaxation rate at low magnetic fields varies sensitively with the H concentration, indicating a change in the ratio of the radial, muon-induced and the partly inherent, uniaxial electric field gradients at the position of the Sc nuclei.

μSR studies of Kondo disorder in UCu5−xPdx and CeCu5.9Au0.1

Abstract Muon spin rotation (μSR) spectra in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu5−xPdx (x = 1.0 and 1.5) and CeCu5.9Au0.1 yield estimates of the rms width (δχ)rms of the inhomogeneous susceptibility distribution in these systems. In UCu5−xPdx, the temperature dependence of (δχ)rms indicates a wide distribution of Kondo temperatures TK, resulting in a significant number of uncompensated U spins with low TK which cause NFL behavior. In contrast, μSR lines in CeCu5.9Au0.1 are too narrow for such “Kondo disorder” to account for NFL properties unless, as seems unlikely, the correlation length for the susceptibility disorder is long in this alloy.

Search for weak magnetic order in CeCu5-xAlx

Abstract The particular sensitivity of μ + SR on small magnetic moments is used to monitor the gradual loss in magnetic order from long range antiferromagnetism (CeCu 5 ) to almost pure Kondo behaviour (CeCu 3.5 Al 1.5 ). While in CeCu 5 a complex oscillating signal was observed, in the substituted compounds no evidence for any oscillations was found in the μ + SR spectra. On the other hand, distinct transitions to a state quasistatic electronic moments were detected, even for the compound with x = 1.5, where almost complete Kondo screening of the magnetic moments was anticipated. The values of the electronic moments involved were presumed to be substantially reduced with increasing Al concentration.

μ+SR in antiferromagnetic UNi4B

Abstract Positive-muon spin rotation (μ + SR) has been carried out in a single crystal of the hexagonal antiferromagnet UNi 4 B, for which the magnetic structure has recently been resolved by Mentink et al. No spontaneous precession was observed in zero field with the μ + spin directed along the rhombohedral b -axis; instead, rapidly and slowly relaxing components (comprising 70% and 30% volume fraction, respectively) were found. On the other hand, a frequency of 5 MHz was detected when the μ + spin was directed along the c -axis. Transverse-field measurements indicate that the μ + is not stopped at a highly symmetric site. The experimental results are compared with calculations based on the magnetic structure.

μ+ SR study of the magnetic order in CeCu5

Abstract It is the aim of our current work to study the magnetic ordering phenomena in Ce(Cu 1− x M x ) 5 compounds (M = Al, Ga). A variety of previous experiments showed that the substitution of Cu by Al (Ga) reduces the magnetic order present in CeCu 5 , until at around x = 0.8 the compounds exhibit heavy electron behaviour. In order to proceed systematically, the experiments were started by carefully studying the magnetic order in the binary compound CeCu 5 . For T > T N the isotropic Knight shift is found to scale with the susceptibility; the anisotropic Knight shift, however, can only be interpreted by assuming a strongly anisotropic susceptibility. The simultaneous presence of a non-oscillating signal and an oscillating one below T N = 4.1 K can be attributed to the formation of inhomogeneous magnetic ordering or to a complex antiferromagnetic structure.