S. Barth

μ+SR-spectroscopy in intermetallic compounds containing heavy-electrons

Abstract μ + SR-investigations of various heavy-electron systems with different electronic ground states are presented. These experiments have revealed the presence of hitherto unknown weak magnetic correlations in some of these materials. The correlations partly coexist with superconductivity and can even develop without a distinct phase transition. They are quasistatic on the muon timescale in CeAl 3 , CeCu 2.1 Si 2 , U 2 Zn 17 , UCu 5 , URu 2 Si 2 , UPt 3 and U 0.977 Th 0.033 Be 13 , but not in the case of UCd 11 . The magnitude of the local magnetic fields at the muon sites points to small effective electronic f-moments ( B ) and/or small coherence lengths. This weak magnetism seems to be an important feature of the electronic ground state of heavy-electron materials and deserves further investigations both from the experimental and theoretical point of view.

Muon knight shift between 75 and 300 K in the high temperature superconductor YBa2Cu3O7

Abstract First μ+ Knight shift results (Kμ) in the system Ba2Cu3O7 between 75 and 300 K are reported. Above Tc ≈ 90 K Kμ is composed of a temperature independent part of 24±7 ppm and a Curie-like component. The absolute value of the relative field shift at 75 K is found to be smaller than expected by ∼ 8 3 π∥M∥ , where M is the macroscopic magnetization of the sample.

A comparative study of the magnetic heavy-electron materials U2Zn17 and UCu5 by μ+SR

A systematic μ+SR study of the magnetic heavy-electron systems U2Zn17 and UCu5 in the paramagnetic and in the magnetically ordered state is presented. In both systems the antiferromagnetic nature of the low-temperature phase could be at least partially confirmed, but the muon reveals significant differences with regard to the phase transition itself. UCu5 behaves like a model-antiferromagnet showing a drastic increase of the relaxation rate both below and above TN, two spontaneous frequencies in the ordered phase, and a Knight shift above TN which scales with the bulk susceptibility. In contrast U2Zn17 shows a loss of μ+ asymmetry by 20% below TN, which is independent of the external field but can be quenched in sufficiently strong longitudinal fields. No scaling of the Knight shift and the susceptibility was observed and no critical increase of the relaxation rate λ. Most astonishing is the strong and nonlinear field dependence of λ above and below TN in both compounds. The absence of longitudinal relaxation demonstrates the static origin of λ.

Positive muons in bismuth: determination of their sites, detection of a large local lattice contraction and giant associated electric field gradients, evidence for short and long range diffusion

Using a high purity Bi single crystal the temperature and orientation dependence of the zero and transverse field muon spin relaxation rate has been studied in detail. The results imply that the μ+ occupies one of the two possible interstitial sites in the distorted rhombohedral crystal structure of Bi below 10 K and the other site above 80 K. At both sites the nearest neighbor Bi atoms are found to be shifted towards the μ+ by ≈10% of their nominal distance, implying a large local lattice contraction. In concomitance extremely strong electric field gradients are manifest at the nn Bi nuclei. An almost temperature independent reduced relaxation rate in the temperature range from 20 K to 60 K is interpreted in terms of short range diffusion along a limited chain of alternating types of sites. Above 100 K both long range and short range diffusion are indicated.

A study of UAl2 and UAs by muon spin rotation and relaxation (μSR)

Abstract Transverse (up to 0.4 T) and zero field μSR was carried out between 300 and 4.2 K in the spin fluctuator UAl 2 and the fairly localized antiferromagnet UAs. In UAl 2 the fluctuation of uranium moments is too fast at any temperature to be seen by μSR. In UAs quite different μSR spectra were found for the two antiferromagnetically ordered spin structures. In zero field, the type I (single k ) phase showed no μ + spin rotation, while in the type IA (double k ) structure an overlay of three subspectra was observed, two of which have temperature dependent rotation frequencies.

μSR measurements on UAl2

The cubic Laves Phase UAl2 is known to exhibit unusual magnetic and transport properties at low temperatures which have been explained by a narrow 5f-band in connection with strong spin fluctuations. Transverse and longitudinal μSR spectra taken between 300 K and 2 K show little variation in damping rate and muonic Knight shift. The fact that the weak damping seen in zero field can fully be decoupled by a longitudinal field of less than 10 mT shows that its origin lies in the interactions between the muon and the27Al nuclear moments, rather than in an influence of the uranium magnetic moments.

First direct evidence for a magnetic phase transition in the heavy-electron system UCd11

Muon-spin-rotation and relaxation measurements performed on polycrystalline UCd11 give first direct evidence for an antiferromagnetic phase transition at 5 K. As far as probed by the positive muon the behaviour close to the phase transition is clearly contrasted to that in other magnetic heavy-electron materials investigated by μ+SR, e.g. U2Zn17 and UCu5, because in UCd11 longitudinal field measurements reveal dynamic relaxation effects which are attributed to the critical slowing down of the uranium 5f-moments. No signal could be observed in the ordered regime because of a large dipolar field spread.

Discovery of novel magnetic features in the heavy-electron compound U2Zn17

Abstract Transverse field μ + SR-measurements on a single crystal sample of U 2 Zn 17 reveal a strikingly non uniform magnetic behavior. While part (∼30%) of the U-5f moments remain paramagnetic below T N = 9.7 K (at least for external fields ≥0.2 T) another part (∼20%) enters into a complex order below 9.7 K which is associated with huge internal field spreads at the μ + sites. Above T N this part causes an anisotropic μ + Knight shift involving direction cosines up to 8th order. Only. the remaining fraction is compatible with the simple AF order suggested by neutron diffraction studies.

μSR in UAs

UAs has the NaCl structure and undergoes a first order transition into a type I (single k) antiferromagnetic state at 123 K, followed by a second first order transition at 62 K into a type IA (double k) antiferromagnetic structure. μSR spectra of a powder sample were taken in zero and transverse fields up to 0.3 T. They cover the paramagnetic and the two antiferromagnetic states. The most significant features of our data are: i) a first increase of relaxation rate below T=180 K; ii) a sudden jump in both, relaxation rate and frequency shift at T=123 K, together with a small decrease in initial asymmetry (≈15%); iii) no μ+ spin rotation in zero field in the type I state; iv) an overlay, of 3 spectra in the type IA state. Two of these spectra show spin rotation in zero field. Their frequencies are clearly temperature dependent. In a transverse field of 5 and 10 mT the external field adds nearly fully to the internal field.

μ+SR studies in the chevrel phase superconductors

The magnetic properties of the superconducting Chevrel compound Eu0.75Sn0.25Mo6S7.6Se0.4 have been studied by μ+SR. The measurements covered the field range 0–30 kOe at various temperatures between 0.05 K and 1.5 K as well as studied in fields between 0 and 3 kOe from 2 K up to room temperature. Strong effects due to the Eu 4f-moments are observed over the whole temperature range. The relaxation rate seems to have its origin mainly in the dynamics of the Eu spins. For comparison the nonmagnetic Chevrel phase superconductor SnMo6S4Se4 was measured too.