S.R. Kreitzman

Low-energy spin-polarized radioactive beams as a nano-scale probe of matter

Abstract We have commissioned a polarized low-energy 8 Li ion beam line, which together with a high-field β-NMR spectrometer, can act as sensitive new probe of thin films and interfaces. The implantation energy can be continuously adjusted from 1 to 90 keV and the maximum polarization achieved thus far is 80%. This instrument opens up new applications for β-NMR which parallel and complement efforts with low-energy muons. For example, it is possible to probe the magnetic field distribution near the surface of a material by stopping a polarized 8 Li beam in a thin overlayer of Ag. Since the 8 Li adopts a site with cubic symmetry in Ag there is no quadrupolar splitting of the resonance, and the 8 Li acts as a purely magnetic sensor.

β-NMR of isolated lithium in nearly ferromagnetic palladium

The temperature dependence of the frequency shift and spin-lattice relaxation rate of isolated, nonmagnetic (8)Li impurities implanted in a nearly ferromagnetic host (Pd) are measured by means of beta-detected nuclear magnetic resonance (beta-NMR). The shift is negative, very large, and increases monotonically with decreasing T in proportion to the bulk susceptibility of Pd for T > T* approximately 100 K. Below T*, an additional shift occurs which we attribute to the response of Pd to the defect. The relaxation rate is much slower than expected for the large shift and is linear with T below T*, showing no sign of additional relaxation mechanisms associated with the defect.

A μSR Study of Magnetic Correlations in CeNiSn

The formation of magnetic correlations in CeNiSn has been studied by muon spin rotation and relaxation (μSR) spectroscopy covering the temperature range from 250 K to 33 mK. Mainly a single crystalline sample was used. At high temperatures (≥ 2 K) a behaviour similar to that of a spin fluctuator like UAl2 is observed, meaning that over the whole temperature range the paramagnetic moments fluctuate extremely fast (≥ 31014 Hz). Below 1 K the material exhibits properties typical for a paramagnet moving towards magnetic order, but no transition into long-range order could be observed down to the lowest measured temperature. The dependences of muon spin relaxation rate and muon spin precession frequency on external field (applied parallel to the a-axis) are unusual and indicate the formation of extended spin correlations up to short-range order.

Magnetic behavior of CeTSn (T = Ni, Pt) from μSR and Mössbauer spectroscopy

Abstract Between 1 K and 33 mK CeNiSn behaves as a paramagnet approaching magnetic order, but no transition is observed. A strong reduction in the moment participating in the magnetic correlations is likely. The La-doped compounds give no magnetic response down to 1.5 K. CePtSn shows unusual spin freezing just above T N ≈8K and sharp transition into a more complex spin order around 5.2 K.

8Li β-NMR in thin metal films

Abstract We have measured the temperature dependence of the β-NMR resonances of 8 Li implanted in thin films of Au and Ag. At room temperature single narrow resonances are observed. The absence of appreciable quadrupolar splitting or broadening indicates that the Li is isolated and at a site of cubic symmetry. As the temperature is lowered, a second resonance appears which is shifted higher in frequency by ∼100 ppm . We attribute the two lines to two distinct cubic sites with slightly different Knight shifts and discuss the application of this result to study fundamental phenomena exhibited by nano-confined metallic electrons.

Local magnetic susceptibility of the positive muon in the quasi-1D S ¼ 1=2 antiferromagnet KCuF3

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet KCuF3: Signals from two distinct sites are resolved which have a temperature-dependent frequency shift which is different than the magnetic susceptibility. This difference is attributed to a muon-induced perturbation of the spin 1/2 chain. r 2002 Elsevier Science B.V. All rights reserved.

The interplay of charge order and magnetism in the one-dimensional quantum spin system Sr14Cu24O41

Abstract We report muon Knight shift measurements on single crystals of Sr14Cu24O41 and Ca9La5Cu24O41 to determine the muon stopping site in the isostructural compounds of the (Sr,Ca,La)14Cu24O41 series. A site close to the ladder oxygen (0, 0.29, 0) is found. In addition, the magnetic correlations in the system Sr14Cu24O41 have been studied by zero field μ+SR between room temperature and 2 K . Quasi-static muon spin relaxation is found below 10 K , whereas thermodynamic and neutron scattering reveal a singlet ground state.

Avoided level crossing measurements of electric field enhanced diamagnetic states in gallium arsenide

Recently, we started developing a new technique that makes possible spectroscopic studies of electric field enhanced diamagnetic states in semiconductors and other materials. This novel approach combines avoided level crossing with electric-field μSR. The first results, obtained in semi-insulating GaAs, are described in this paper.