ß-NMR Investigation of the Depth-Dependent Magnetic Properties of an Antiferromagnetic Surface.

By measuring the prototypical antiferromagnet α-Fe_{2}O_{3}, we show that it is possible to determine the static spin orientation and dynamic spin correlations within nanometers from an antiferromagnetic surface using the nuclear spin polarization of implanted ^{8}Li^{+} ions detected with ß-NMR. Remarkably, the first-order Morin spin reorientation in single crystal α-Fe_{2}O_{3} occurs at the same temperature at all depths between 1 and 100 nm from the (110) surface; however, the implanted nuclear spin experiences an increased 1/T_{1} relaxation rate at shallow depths revealing soft-surface magnons. The surface-localized dynamics decay towards the bulk with a characteristic length of ε=11±1 nm, closely matching the finite-size thresholds of hematite nanostructures.

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers.

We report the observation of weak magnetism in superlattices of LaAlO(3)/SrTiO(3) using ß-detected nuclear magnetic resonance. The spin lattice relaxation rate of ^{8}Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO(3) exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO(3)/SrTiO(3) interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ~4-6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (~0.002µ(B)) in the two samples with a larger LaAlO(3) spacer thickness.

First experimental determination of the charge radius of 74Rb and its application in tests of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

Collinear-laser spectroscopy with the bunched-beams technique was used for the study of neutron deficient Rb isotopes, out to (74)Rb (N = Z = 37) at TRIUMF. The measured hyperfine coupling constants of (76,78m)Rb were in agreement with literature values. The nuclear spin of (75)Rb was confirmed to be I = 3/2, and its hyperfine coupling constants were measured for the first time. The mean-square charge radius of (74)Rb was determined for the first time. This result has improved the isospin symmetry breaking correction term used to calculate the Ft value, with implications for tests of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

Local magnetic properties of a monolayer of Mn12 single molecule magnets.

The magnetic properties of a monolayer of Mn12 single molecule magnets grafted onto a silicon (Si) substrate have been investigated using depth-controlled beta-detected nuclear magnetic resonance. A low-energy beam of spin-polarized radioactive 8Li was used to probe the local static magnetic field distribution near the Mn12 monolayer in the Si substrate. The resonance line width varies strongly as a function of implantation depth as a result of the magnetic dipolar fields generated by the Mn12 electronic magnetic moments. The temperature dependence of the line width indicates that the magnetic properties of the Mn12 moments in this low-dimensional configuration differ from bulk Mn12.

Magnetic-field effects on the size of vortices below the surface of NbSe2 detected using low energy beta-NMR.

A low energy radioactive beam of polarized 8Li has been used to observe the vortex lattice near the surface of superconducting NbSe2. The inhomogeneous magnetic-field distribution associated with the vortex lattice was measured using depth-resolved beta-detected NMR. Below Tc, one observes the characteristic line shape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with the magnetic field. In particular, in a low field of 10.8 mT, the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed.

beta-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.

Near-surface structural phase transition of SrTiO3 studied with zero-field beta-detected nuclear spin relaxation and resonance.

We demonstrate that zero-field beta-detected nuclear quadrupole resonance and spin relaxation of low energy (8)Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO(3) single crystal occurs at T(c) approximately 150K, i.e., approximately 45K higher than T(c)bulk, and that the tetragonal domains formed below T(c) are randomly oriented.

Nuclear charge radii of 9,11Li: the influence of halo neutrons.

The nuclear charge radius of 11Li has been determined for the first time by high-precision laser spectroscopy. On-line measurements at TRIUMF-ISAC yielded a 7Li-11Li isotope shift (IS) of 25 101.23(13) MHz for the Doppler-free [FORMULA: SEE TEXT]transition. IS accuracy for all other bound Li isotopes was also improved. Differences from calculated mass-based IS yield values for change in charge radius along the isotope chain. The charge radius decreases monotonically from 6Li to 9Li, and then increases from 2.217(35) to 2.467(37) fm for 11Li. This is compared to various models, and it is found that a combination of halo neutron correlation and intrinsic core excitation best reproduces the experimental results.

Depth-controlled beta-NMR of 8Li in a thin silver film.

Depth-controlled beta-NMR can be used to probe the magnetic properties of thin films and interfaces on a nanometer length scale. A 30 keV beam of highly spin-polarized 8Li+ ions was slowed down and implanted into a 50 nm film of Ag deposited on a SrTiO3 substrate. A novel high field beta-NMR spectrometer was used to observe two well resolved resonances which are attributed to Li occupying substitutional and octahedral interstitial sites in the Ag lattice. The temperature dependence of the Knight shifts and spin relaxation rates are consistent with the Korringa law for a simple metal, implying that the NMR of implanted 8Li reflects the spin suspectibility of bulk metallic silver.