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Neutron Refractometry - B Kreimer


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Neutron Refractometry - B Kreimer

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Neutron Refractometry - B Kreimer

  1. 1. Neutron reflectometryIntroduction & application to oxide interfaces B. Keimer Max-Planck-Institute for Solid State Researchmotivation • neutron reflectometry: part of “interface toolbox” • state-of-the-art instrument available for Max Planck users & collaborators outline • self-contained introduction: neutron scattering & reflection • small selection of applications to (oxide) interfaces
  2. 2. Neutron scattering neutronE1 q1 excitation: E= E2-E1 E2 q2 q=q2-q1 interaction strong (nuclear) interaction elastic lattice structure inelastic lattice dynamics magnetic (dipole-dipole) interaction elastic magnetic structure inelastic magnetic excitations
  3. 3. Neutron sourcesresearch reactor FRM-II Garching, Germanycontinuous spectrum neutron Maxwellian flux profile ~ 30 meV energy
  4. 4. Neutron sourcesspallation source pulsed beam SNS Oak Ridge, USA
  5. 5. Elastic neutron scattering
  6. 6. Elastic neutron scattering Born approximation
  7. 7. Elastic nuclear neutron scattering scattering length b ~ size of nucleus ~ 10-15 m depends on isotope Bragg peaks at reciprocal lattice vectors K nuclear structure factor
  8. 8. Scattering cross section: x-rays versus neutrons N.B. b for deuterium is negative
  9. 9. Neutron radiography two metallic cylinders attached by an adhesive only the adhesive is seen on the neutron radiograph
  10. 10. Elastic magnetic neutron scattering
  11. 11. Elastic magnetic neutron scattering
  12. 12. Elastic magnetic neutron scatteringone electron “classical electron radius” magnitude comparable to b non-spin-flip average for unpolarized beamσz → σx , σy spin-flip (not possible for nuclear scattering)
  13. 13. Elastic magnetic neutron scatteringone atomapproximated as magnetized sphere, magnetization density M(r)
  14. 14. Elastic magnetic neutron scattering generalization for collinear magnets Bragg peaks polarization factor magnetic structure factor magnetic reciprocal lattice vectorsfrom here on, assume collinear magnetism, one atom per unit cell for simplicity
  15. 15. Example one-dimensional antiferromagnet
  16. 16. Example one-dimensional ferromagnetinterference between nuclear and magnetic scattering
  17. 17. Nuclear-magnetic interferencecross section depends on spin directionuse nuclear-magnetic interference to create spin-polarized neutron beam ferromagnetic Bragg peak with
  18. 18. Reflection from interfacesconveniently discussed in terms of classical ray opticsindex of refraction for neutron wave inside material example natural Ni similar to x-rays but δ can be negative for neutronsexample natural Tiexample isotopically pure 62Ni can drastically change scattering power without changing chemistry & physics perspectives not yet explored for hard materials
  19. 19. Reflection from interfaces
  20. 20. Reflection from interfaces
  21. 21. Reflection from interfaces
  22. 22. Reflection from interfacesFresnel reflectivity
  23. 23. Reflection from interfacescontrast matchingimportant for soft matterbut also:hydrogen profiles in hard materials
  24. 24. Neutron guidessupermirrorengineer layer sequence such that effective critical angle increases
  25. 25. Neutron guidesneutron guide hall @ FRM-II
  26. 26. NREX reflectometer Thomas Keller +49-89-289-12164 Thomas.Keller@frm2.tum.destate of the art instrumentowned and operated by Max Planck Societyprivileged access to beamtime
  27. 27. Nonuniform density distribution“kinematic” approximation ignore multiple reflections contribution to R whenever density changes analog of magnetic form factor in diffraction example film on substrate
  28. 28. Multiple reflections
  29. 29. Multiple reflectionskinematic approximation recovered 0waveguide effect resonant enhancement of neutron wavefunction inside layercan use this effect to enhance contribution of single buried layer to reflectivity
  30. 30. Multiple reflectionsmultilayers image adapted from Hoppler et al., Nature Materials 2009numerical calculations: Parratt formalism
  31. 31. Reflection from graded interfaces analogous to Debye-Waller factor in diffraction
  32. 32. Reflection from graded interfaces quality of surfaces, buried interfaces can be determined by reflectometry example Nb film Fresnel 70 Å surface roughness Felcher et al. PRL 1984
  33. 33. Reflection from ferromagnets M || H η H || zmagnetic scattering amplitude neutron spin operator electronic magnetic moment determined by magnetic field component Q-vectorordinary ferromagnet no neutron spin flip
  34. 34. Reflection from ferromagnets M η H || zmagnetization components H, Qe.g. spin canting at interface, strong anisotropy  neutron spin flip
  35. 35. Spin-polarized neutron reflectometrynuclear-magnetic interference effecttotal scattering amplitude four different reflectivities for single interface: R++, R--, R+-, R-+reflection, transmission amplitudes in Parratt calculations become matricespolarizing mirror
  36. 36. Spin-polarized neutron reflectometryreflectometer with spin polarization analysis allows separate measurements of R++, R--, R+-, R-+
  37. 37. Spin-polarized neutron reflectometry
  38. 38. SrRuO3 – La0.7Sr0.3MnO3 Heterostructures SrRuO3 TC = 140 K, M SL La0.7Sr0.3MnO3 TC = 320 K, M || SL Ziese, Vrejoiu et al. (Halle group) PRL 2010antiferromagnetic coupling through Mn-O-Ru bond competing interactions at interfaces
  39. 39. SrRuO3 – La0.7Sr0.3MnO3 Heterostructures M || Q inside SRO layer invisible to neutrons M Q at interface through Ru-O-Mn couplingJ.H. Kim et al. (MPI-FKF)
  40. 40. LaMnO3 – SrMnO3 HeterostructuresSantos et al. (Argonne group)arXiv:1105.0223
  41. 41. LaMnO3 – SrMnO3 Heterostructuresspin-flip scattering canted structureSantos et al. (Argonne group)arXiv:1105.0223
  42. 42. Reflection from superconductorsPb film in Meissner state Nutley et al. PRB 1994
  43. 43. Reflection from superconductorsPb film in vortex state Drew et al. PRB 2009
  44. 44. Superconductor – Ferromagnet Heterostructures inverse proximity effect at interface between superconductor and ferromagnet Bergeret et al. PRB 2004
  45. 45. Superconductor – Ferromagnet Heterostructures engineered waveguide structure to observe inverse proximity effect amplitude of waveguide resonance suggestive of inverse proximity effect Khaydukov et al. (Dubna group) arXiv:1005.0685
  46. 46. YBCO-LCMO interfaceYBa2Cu3O7 (YBCO): high-Tc superconducorLa0.7Ca0.3MnO3 (LCMO): double-exchange ferromagnetSrTiO3 (001) substrate CuO2 layers || interface coherence length interface very small  SC proximity effects not expected Zhang et al. APL 2009
  47. 47. Magnetic proximity effects?YBCO-LCO on (110) SrTiO3CuO2 layers perpendicular to interface Kim, Mustafa
  48. 48. YBCO-LCMO interface suppression of superconductivity suppression of metallicity for YBCO layers thinner than ~ 5 nmSefrioui et al., PRB 2003 Holden et al. PRB 2004
  49. 49. YBCO-LCMO charge transfercharge transfer La1-xCaxMnO3doping without chemical substitution YBa2Cu3O6+x
  50. 50. YBCO-LCMO magnetic reconstructionneutron reflectometrytwo interface models yield equivalent fits:- antiferromagnetically polarized layer- magnetically “dead” layer Stahn et al. PRB 2005 model 1 model 2 J.H. Kim NREX @ FRM-II
  51. 51. YBCO-LCMO magnetic reconstructionadditional information from XMCD Chakhalian et al., Nature Phys. 2006 • superexchange coupling• ferromagnetic polarization through Cu-O-Mn bond of Cu in YBCO• direction antiparallel to Mn Chakhalian et al. Nature Phys. 2006
  52. 52. Off-specular reflectometryspecular off-specular correlations plane  correlations || plane • in-plane domain structure • interface roughness
  53. 53. In-plane domain structure FePd films YBCO-LCMO superlattice Qx T > 100 K Qz T < 100K Chakhalian et al. Fermon et al. Nature Phys. 2006magnetic stripe domains new magneto-structural domain state periodicity ~ 1µm
  54. 54. In-plane domain structureYBCO-LCMO superlattice on SrTiO3origin: structural phase transition novel superconductivity-induced in STO substrate magnetic domain structure J. Hoppler, C. Bernhard et al. Nature Mat. 2009
  55. 55. In-plane domain structureLaNiO3-LaAlO3 superlattice on SrLaAlO4simpler structure of superlatticeno structural transitions in substratefull crystallographic description of lattice structure, A. Franostrain-induced domains
  56. 56. Magnetic depth profiling by soft x-rays 639 eV This image cannot currently be displayed. 620 eVresonant reflectometry fit intensity (arb. units)with circularly polarized x-rays element-specific magnetization profileexample CaRuO3 — CaMnO3 superlattices experiment dichroic difference model Freeland et al. PRB 2010 momentum transfer (nm-1)
  57. 57. Neutron versus resonant x-ray reflectometryneutron reflectometry advantages• yields total magnetization, independent of electronic structure• cross section completely understood, no calculation required• no beam heating  can reach mK temperatures• isotopic labeling, sensitivity to hydrogen• Larmor phase manipulation of neutron spin, spin-echo experimentsresonant x-ray reflectometry advantages• element specific• yields valence state, orbital occupation, magnetization in one shot (software available soon) S. Macke• higher intensity, dynamic range
  58. 58. Further reading