M. Schmolz

Zero-field muon spin rotation in monocrystalline chromium

Spin precession of positive muons in chromium in zero applied magnetic field is reported for the first time. The observations cover the temperature range from about 2.5 K to 10 K and thus pertain to the so-called longitudinal spin-density wave (LSDW) state of antiferromagnetic Cr. The conclusions that may be drawn from the existence of one rather sharp spin precession line are discussed, among them the estimateDμ=2.4·10−14 m2 s−1 for the muon diffusivity at 4 K. Considerable evidence exists for a strong interactions of μ+ with the charge-density waves that are likely to accompany the LSDWs in Cr.

μ+SR study of vacancies in thermal equilibrium in ferromagnets

Muon spin precession frequencies and transverse relaxation rates have been measured on demagnetized iron, cobalt, and FeCo alloys (3 at%–50 at% Co) between room temperature and the Curie temperatureTc. The increase of the relaxation rate in iron between 930 K and 1010 K could be quantitatively attributed to the trapping of positive muons by vacancies in thermal equilibrium, resulting in an enthalpy of monovacancy formation ofH1VF=(1.7±0.1) eV. the smallest vacancy concentrations detected are = 10−8.

Positive mouns in iron: Dipolar fields at tetrahedral sites and jump frequencies at low temperatures

On polycrystalline and monocrystalline iron muon-spin precession frequencies and transverse relaxation rates have been measured down to 0.5 K. In the polycrystalline sample two distinct precession frequencies were observed at and below 1.4 K. They are attributed to the different dipolar fields at magnetically inequivalent tetrahedral interstices seen by muons moving locally around impurities. By contrast, in monocrystalline iron we observed only one precession frequency in monocrystalline iron with a damping rate which increased with decreasing temperature down to 0.5 K. We attribute the difference between the monocrystalline and the polycrystalline sample to different impurity contents. The single-crystal data are discussed in terms of μ+ diffusion by hopping between interstitial sites of tetragonal symmetry. The answer to several open questions is expected from an extension of the measurements to lower temperatures.

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityDμ of positive muons (μ+) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ+SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsDμ through the variation of the magnetic field felt by the μ+ during their diffusion through the crystals. For μ+ near the centres of the flux lines inNb it givesDμ(4.6 K)=(8±2)·10−11m2S−1. The positive temperature coefficient ofDμ indicates that at liquid-helium temperatures the diffusivity of μ+ inNb is mainly due to phonon-assisted tunnelling processes.

Muon trapping and diffusion in Al and In after electron irradiation at 9 K

The transverse spin relaxation of positive muons has been measured on an Al single crystal and on polycrystalline In after irradiation with 2 MeV electrons at 9 K or 11 K, sample transfer at 4.2 K, and various subsequent annealing treatments. The Al data are analysed in terms of diffusion-limited trapping by vacancies. This yields a muon diffusivityDμ which within experimental accuracy is proportional toT between 4 K and 50 K, indicating that in this temperature intervalDμ is dominated by one-phonon-assisted incoherent tunnelling. In In only very small effects due to the irradiation could be observed. The muons appear to be localized in octahedral interstices. From the motional averaging taking place above about 20 K the diffusivity ofDμ in In is deduced.