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Neutron reflectometry

Neutron reflectometry. Helmut Fritzsche NRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada. Application/advantages of neutron reflectometry Theoretical background Instrumental setup Experiments: Photoactive azobenzene films Hydrogen storage in MgAl films

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Neutron reflectometry

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  1. Neutron reflectometry Helmut Fritzsche NRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada

  2. Application/advantages of neutron reflectometry • Theoretical background • Instrumental setup • Experiments: • Photoactive azobenzene films • Hydrogen storage in MgAl films • Element-specific hysteresis curves in ErFe2 / DyFe2 multilayers • Supermirrors (non-polarizing and polarizing) Canadian Neutron Beam Centre Outlook

  3. Canadian Neutron Beam Centre What can be measured with neutron reflectometry? Film thickness (2 – 200 nm): swelling of polymer films due to water uptake film expansion during illumination of photoactive films film expansion during hydrogen absorption growth of oxide layer Scattering length density profile: profile of absorbed gas/liquid interdiffusion magnetic structures magnetic field penetration into superconductors In-plane structures on nm and m scale

  4. Canadian Neutron Beam Centre Specific advantages of neutron reflectometry Large penetration depth (for most materials): Buried layers In-situ measurements (cryostats, cryomagnets, high-pressure cells, furnaces) High sensitivity to hydrogen: Determine hydrogen profile in hydrogen storage materials Change of contrast by using isotopes: swelling of films during water (vapor or liquid) uptake (H2O / D2O) expansion of films during hydrogen absorption (H2 / D2) No diamagnetic background of substrate for ferromagnetic samples: Determination of absolute magnetic moment Spin and non-spin flip reflectivity: Magnetization reversal, magnetic structure

  5. specularly reflected refracted Canadian Neutron Beam Centre Reflection and refraction incoming wave  1  r medium 1: n1  2 medium 2: n2 Physical origin: different index of refraction for two media Reflection: r = 1 Refraction: Snell‘s law n1 sin 1 = n2 sin 2

  6. Critical angle: n1 sin 1 = n2 sin 90°  sin c = n2 / n1 For 1 > c : no refracted beam exists, only a reflected beam Total reflection(100% reflectivity) occurs in the medium with the larger n Canadian Neutron Beam Centre Reflection and refraction:the critical angle  c reflected medium 1: n1 refracted 90° medium 2: n2

  7. For light with = 656 nm: Material n c (for n2=1) Vacuum 1.00 - Water 1.33 48.8 Quartz glass 1.46 43.2 Benzene 1.50 41.8 Note: the index of refraction depends on the wavelength Canadian Neutron Beam Centre Index of refraction for light What is the index of refraction for neutrons?

  8. Fermi’s pseudopotential: Canadian Neutron Beam Centre Index of refraction for neutrons Ez   Ekin,2 Ekin,1 } V SLD z m: neutron mass : neutron wavelength b: nuclear scattering length : density of atoms b: scattering length density (SLD)

  9. X-rays Neutrons Canadian Neutron Beam Centre Scattering lengths X-rays: b  Z (electron density) Neutrons: no systematics Important: not absolute number but contrast of SL X-rays and neutrons are complementary probes

  10. For neutrons with = 0.237 nm: Material n b (10-4 1/nm2) Vacuum 1.00 0 Water (H2O) 1.000001 -0.561 Si 0.999998 2.073 Quartz glass 0.999997 4.185 Heavy water (D2O) 0.999994 6.366 58Ni 0.999988 13.16 Note: n  1-10-5 The deviation of nneutron from 1 is much smaller than for light, because the interaction of neutrons with matter is much weaker Canadian Neutron Beam Centre Index of refraction for neutrons:some examples

  11. Canadian Neutron Beam Centre Reflectometry setup on D3 Focusing PG monochromator Spin-down neutrons S1 PG filter spin flipper Polarizing supermirror sample S2 analyzer S3 S4 detector

  12. Canadian Neutron Beam Centre Reflectometry setup on D3 Focusing PG monochromator Spin-down neutrons S1 Spin-up neutrons PG filter spin flipper Polarizing supermirror sample S2 analyzer S3 S4 detector

  13. q-2q geometry: sample moves by q detector moves by 2q q: scattering vector 2q: scattering angle Canadian Neutron Beam Centre The reflectometry experiment detector q 2  sample slit system

  14. Reflectometry: Measuring the reflected intensity as a function of q Canadian Neutron Beam Centre The reflectometry experiment detector q sample slit system

  15. Si: c=0.11º (for =2.37 Å) 58Ni: c=0.28º (for =2.37 Å) Canadian Neutron Beam Centre Visualization of a reflectivity curve (Si wafer) Ez } z reflectivity qc q

  16. Canadian Neutron Beam Centre Kiessig fringes qc q=2/d Oscillations due to total film thickness q  1/d

  17. SLD Fe Fe Fe } Cr Cr Cr ••• Si wafer Bilayer thickness t t = 32.8 Å + 30 Å = 62.8 Å In total: 20 repetitions Canadian Neutron Beam Centre Multilayer Bragg peaks bilayer Bragg peaks at q=2/t q = n · 2/62.8 Å-1 = n · 0.1 Å-1 Short period oscillations: Kiessig fringes

  18. Hext: external magnetic field B : magnetic induction µ : magnetic moment of neutrons Canadian Neutron Beam Centre Magnetic interaction

  19. Canadian Neutron Beam Centre PNR: bulk Fe qc+ qc- V Vnuc Vnuc spin up (R+) spin down (R-) Different reflectivity for spin-up and spin-down neutrons  Determination of the absolute magnetic moment possible

  20. Canadian Neutron Beam Centre PNR: Fe/Cr multilayers Ferromagnetic coupling: Magnetic period = chemical period Antiferromagnetic coupling: Magnetic period = 2 x chemical period AF peak Structural peak Structural peak

  21. Canadian Neutron Beam Centre In-situ setup for photoactive films lenses shutter mirror Neutron reflectometry and Laser illumination at the same time

  22. Canadian Neutron Beam Centre Results for azobenzene films Laser irradiation time 0.0 h 0.4 h 2.5 h 8.0 h Smaller q  larger film thickness

  23. Vacuum Chamber Al Mg Pd <100> Si Wafer Canadian Neutron Beam Centre Co-sputtering of MgAl alloy films

  24. Canadian Neutron Beam Centre Hydrogen absorption Absorption cell for thin films on wafers with up to 100 mm diameter Hydrogen gas cylinder

  25. heater sample thermocouple Canadian Neutron Beam Centre Hydrogen desorptionequipment Reflectometry furnace: Ar atmosphere or vacuum 300 K < T < 670 K

  26. Canadian Neutron Beam Centre Mg0.6 Al0.4 at 298 K SiO2 Mg0.6Al0.4 Pd Si Fit: Pd: t = 104 Å  = 4.4 Å MgAl: t = 520 Å  = 15.7 Å

  27. Canadian Neutron Beam Centre Absorption in Mg0.6 Al0.4 • SLD bH < 0 t • increase of film thickness by about 20% • hydrogen content is 83 at.% = 3.2 weight %

  28. Canadian Neutron Beam Centre Annealing of a desorbed Mg0.7 Al0.3 film Pd layer does not exist anymore after 9 h: Pd diffuses into the MgAl layer

  29. Canadian Neutron Beam Centre DyFe2 / ErFe2 multilayer:element-specific hysteresis Magnetization reversal at 100 K After saturation at µ0H = –6 T (6 nm DyFe2 / 6 nm ErFe2)40 ErFe2 and DyFe2 magnetizations are not parallel DyFe2: easy-axis loop ErFe2: hard-axis loop

  30. R+ = R- DyFe2 ErFe2 Canadian Neutron Beam Centre PNR is element-specific nonmagnetic layers

  31. R+ = R- DyFe2 ErFe2 Canadian Neutron Beam Centre PNR is element-specific ~R+ ~R- DyFe2 ErFe2 nonmagnetic layers

  32. Canadian Neutron Beam Centre PNR is element-specific ~R+ ~R- DyFe2 ErFe2 R+ = R- nonmagnetic layers DyFe2 ErFe2

  33. ~R+ ~R- DyFe2 ErFe2 R+ = R- DyFe2 ErFe2 Canadian Neutron Beam Centre PNR is element-specific ~R- ~R+ DyFe2 ErFe2 nonmagnetic layers

  34. Canadian Neutron Beam Centre PNR is element-specific ~R- ~R+ DyFe2 ErFe2 ~R+ ~R- DyFe2 ErFe2 R+ = R- nonmagnetic layers DyFe2 ErFe2

  35. Canadian Neutron Beam Centre supermirror goal: Extend the range of neutron reflection beyond the regime of total reflection concept: continuous Bragg reflection from a multilayer composed of bilayers with a variation of the thickness realization: Ni/Ti multilayer, bNi = 10.3 fm, bTi = -3.4 fm 100 bilayers, qc = 2 x qc, Ni

  36. SLD Ni Ni Ni z Ti Ti Ti Canadian Neutron Beam Centre supermirror m-value: m = qc / qc, Ni

  37. spin-up neutrons spin-down neutrons SLD SLD Fe/Co Fe/Co Si Si Fe/Co Fe/Co Si Si Canadian Neutron Beam Centre Polarizing supermirror concept: Using the supermirror concept with a magnetic/non-magnetic bilayer The SLD of the bilayer is index-matched for spin-down neutrons no multilayer Bragg peaks for down-neutrons Spin-up neutrons show supermirror behavior with extended critical edge Index matching

  38. Canadian Neutron Beam Centre Polarizing supermirror:Fe-Co/Si Transmitted intensity Reflected intensity

  39. Flipping ratio = Canadian Neutron Beam Centre Flipping ratio 25 usable range

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