M.D. Hossain

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.

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers.

We report the observation of weak magnetism in superlattices of LaAlO(3)/SrTiO(3) using β-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.

Near-Surface Structural Phase Transition of SrTiO~3 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.

Search for broken time-reversal symmetry near the surface of superconducting YBa2Cu3O7-δ films using β-detected nuclear magnetic resonance

H. Saadaoui, ∗ G. D. Morris, Z. Salman, ∗ Q. Song, K. H. Chow, M. D. Hossain, C. D. P. Levy, T. J. Parolin, M. R. Pearson, M. Smadella, D. Wang, L. H. Greene, P. J. Hentges, R. F. Kiefl, 2, 7 and W. A. MacFarlane Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T 2A3, Canada Clarendon Laboratory, Department of Physics, Oxford University, Parks Road, Oxford, OX1 3PU, UK Department of Physics, University of Alberta, Edmonton, AB, T6G 2G7, Canada Chemistry Department, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801, USA Canadian Institute for Advanced Research, Toronto, ON, M5G 1Z8, Canada (Dated: January 6, 2011)

Nanoscale β-nuclear magnetic resonance depth imaging of topological insulators

Significance The surface states of topological insulators (TIs) and magnetically doped TIs exhibit considerable inhomogeneities at the nanoscale. Methods are needed to probe the degree of heterogeneity as a function of depth in nanoscale layers. We present a method that can directly visualize TIs in a depth-resolved manner and report on their electronic and magnetic properties. For example, in epitaxial thin films we demonstrate an increase in the density of states, a weakening of the ferromagnetic order when approaching the TI edges, as detected by measurements of the electron–nuclear hyperfine interaction, the effective s–d exchange integral, and local moment density. Depth profiling is expected to help uncover exotic physics of pure and ferromagnetic TIs and TI heterostructures. Considerable evidence suggests that variations in the properties of topological insulators (TIs) at the nanoscale and at interfaces can strongly affect the physics of topological materials. Therefore, a detailed understanding of surface states and interface coupling is crucial to the search for and applications of new topological phases of matter. Currently, no methods can provide depth profiling near surfaces or at interfaces of topologically inequivalent materials. Such a method could advance the study of interactions. Herein, we present a noninvasive depth-profiling technique based on β-detected NMR (β-NMR) spectroscopy of radioactive 8Li+ ions that can provide “one-dimensional imaging” in films of fixed thickness and generates nanoscale views of the electronic wavefunctions and magnetic order at topological surfaces and interfaces. By mapping the 8Li nuclear resonance near the surface and 10-nm deep into the bulk of pure and Cr-doped bismuth antimony telluride films, we provide signatures related to the TI properties and their topological nontrivial characteristics that affect the electron–nuclear hyperfine field, the metallic shift, and magnetic order. These nanoscale variations in β-NMR parameters reflect the unconventional properties of the topological materials under study, and understanding the role of heterogeneities is expected to lead to the discovery of novel phenomena involving quantum materials.

8Li+ β-NMR in the Cubic Insulator MgO

We present extensive high magnetic field β-NMR measurements of 8Li+ implanted in single crystals of MgO. The narrow resonance, consistent with a cubic 8Li+ site, likely the tetrahedral interstitital, is used routinely as a reference for shift measurements. We show the intrinsic linewidth is on the order of 200 Hz, allowing a frequency determination to an accuracy of a few Hz. We find no implantation energy dependence of the resonance within a few ppm, but there is evidence of slow spin dynamics in hole-burning measurements. The spin lattice relaxation is slow. The temperature dependence reveals interesting changes at low temperature whose origin remains uncertain.

Depth dependence of the structural phase transition of SrTiO3studied withβ-NMR and grazing incidence x-ray diffraction

We present an investigation of the near-surface tetragonal phase transition in SrTiO3, using the complementary techniques of beta-detected nuclear magnetic resonance and grazing-incidence X-ray diffraction. The results show a clear depth dependence of the phase transition on scales of a few microns. The measurements support a model in which there are tetragonal domains forming in the sample at temperatures much higher than the bulk phase transition temperature. Moreover, we find that these domains tend to form at higher temperatures preferentially near the free surface of the crystal. The details of the tetragonal domain formation and their depth/lateral dependencies are discussed.

Vortex lattice disorder inYBa2Cu3O7−δprobed usingβ-NMR

Beta-NMR has been used to study vortex lattice disorder near the surface of the high-Tc superconductor YBCO. The magnetic field distribution from the vortex lattice was detected by implanting a low energy beam of highly polarized 8Li into a thin overlayer of silver on optimally doped, twinned and detwinned YBCO samples. The resonance in Ag broadens significantly below the transition temperature Tc as expected from the emerging field lines of the vortex lattice in YBCO. However, the lineshape is more symmetric and the dependence on the applied magnetic field is much weaker than expected from an ideal vortex lattice, indicating that the vortex density varies across the face of the sample, likely due to pinning at twin boundaries. At low temperatures the broadening from such disorder does not scale with the superfluid density.

Low-energy muons at PSI: examples of investigations of superconducting properties in near-surface regions and heterostuctures

With the advent of polarized low-energy muons, with tunable energy in the kiloelectronvolt range, it is possible to use the sensitivity and the local-probe character of µSR to perform depth-dependent investigations on the nanometer scale of magnetic and superconducting properties of materials. Here, after a brief summary of the present status of LE-µSR at PSI, we give some examples of investigations of superconducting properties in the near-surface regions of single crystals and thin-film materials.

Slow order-parameter fluctuations in superconducting Pb and Ag/Nb films observed usingβ-detected nuclear magnetic resonance

E. Morenzoni, ∗ H. Saadaoui, D. Wang, M. Horisberger, E. Kirk, W.A. MacFarlane, G. D. Morris, K.H. Chow, M.D. Hossain, C.P. Levy, T.J. Parolin, M.R. Pearson, Q. Song, and R. F. Kiefl 6 Paul Scherrer Institut, Laboratory for Muon Spin Spectroscopy, 5232 Villigen PSI, Switzerland Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada, V6T Paul Scherrer Institut, Laboratory for Developments and Methods, 5232 Villigen PSI, Switzerland Paul Scherrer Institut, Labor für Mikround Nanotechnologie, CH-5232 Villigen PSI, Switzerland Chemistry Department, University of British Columbia, Vancouver, BC, Canada, V6T 1Z1 TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada, V6T 2A3 Department of Physics, University of Alberta, Edmonton, AB, Canada, T6G 2G7 (Dated: September 22, 2011)