Neutron reflectometry Helmut Fritzsche NRCSIMS Canadian Neutron Beam
Neutron reflectometry Helmut Fritzsche NRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada
Canadian Neutron Beam Centre Outlook Application/advantages of neutron reflectometry Theoretical background Instrumental setup Experiments: • Photoactive azobenzene films • Hydrogen storage in Mg. Al films • Element-specific hysteresis curves in Er. Fe 2 / Dy. Fe 2 multilayers Supermirrors (non-polarizing and polarizing)
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
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 (H 2 O / D 2 O) expansion of films during hydrogen absorption (H 2 / D 2) No diamagnetic background of substrate for ferromagnetic samples: Determination of absolute magnetic moment Spin and non-spin flip reflectivity: Magnetization reversal, magnetic structure
Canadian Neutron Beam Centre Reflection and refraction incoming wave medium 1: n 1 medium 2: n 2 1 r 2 specularly reflected refracted Physical origin: different index of refraction for two media Reflection: Refraction: Snell‘s law r = 1 n 1 sin 1 = n 2 sin 2
Canadian Neutron Beam Centre Reflection and refraction: the critical angle medium 1: n 1 medium 2: n 2 c 90° reflected refracted Critical angle: n 1 sin 1 = n 2 sin 90° sin c = n 2 / n 1 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 Index of refraction for light For light with = 656 nm: Material n c (for n 2=1) Vacuum Water Quartz glass Benzene 1. 00 1. 33 1. 46 1. 50 48. 8 43. 2 41. 8 Note: the index of refraction depends on the wavelength What is the index of refraction for neutrons?
Canadian Neutron Beam Centre Index of refraction for neutrons Ez Ekin, 1 E kin, 2 } V SLD z m: neutron mass : neutron wavelength b: nuclear scattering length : density of atoms b: scattering length density (SLD) Fermi’s pseudopotential:
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
Canadian Neutron Beam Centre Index of refraction for neutrons: some examples For neutrons with = 0. 237 nm: Material n b (10 -4 1/nm 2) Vacuum Water (H 2 O) Si Quartz glass Heavy water (D 2 O) 58 Ni 1. 000001 0. 999998 0. 999997 0. 999994 0. 999988 0 -0. 561 2. 073 4. 185 6. 366 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 Reflectometry setup on D 3 Focusing PG monochromator Spin-down neutrons S 1 PG filter spin flipper Polarizing supermirror S 2 sample S 3 S 4 analyzer detector
Canadian Neutron Beam Centre Reflectometry setup on D 3 Focusing PG monochromator Spin-down neutrons S 1 Spin-up neutrons PG filter spin flipper Polarizing supermirror S 2 sample S 3 S 4 analyzer detector
Canadian Neutron Beam Centre The reflectometry experiment detector q 2 sample slit system -2 geometry: sample moves by detector moves by 2 q: scattering vector 2 : scattering angle
Canadian Neutron Beam Centre The reflectometry experiment detector q sample slit system Reflectometry: Measuring the reflected intensity as a function of q
Canadian Neutron Beam Centre Visualization of a reflectivity curve (Si wafer) Ez } reflectivity z qc q Si: 58 Ni: c=0. 11º (for =2. 37 Å) c=0. 28º (for =2. 37 Å)
Canadian Neutron Beam Centre Kiessig fringes qc q=2 /d Oscillations due to total film thickness q 1/d
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 SLD Fe Fe Si wafer Cr Fe Cr } Bilayer thickness t t = 32. 8 Å + 30 Å = 62. 8 Å In total: 20 repetitions • • • Cr Short period oscillations: Kiessig fringes
Canadian Neutron Beam Centre Magnetic interaction Hext: external magnetic field B : magnetic induction µ : magnetic moment of neutrons
Canadian Neutron Beam Centre PNR: bulk Fe qc- qc+ V Vnuc spin up (R+) Vnuc spin down (R-) Different reflectivity for spin-up and spin-down neutrons Determination of the absolute magnetic moment possible
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
Canadian Neutron Beam Centre In-situ setup for photoactive films lenses shutter mirror Neutron reflectometry and Laser illumination at the same time
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
Canadian Neutron Beam Centre Co-sputtering of Mg. Al alloy films Vacuum Chamber Mg Al <100> Si Wafer Pd
Canadian Neutron Beam Centre Hydrogen absorption Hydrogen gas cylinder Absorption cell for thin films on wafers with up to 100 mm diameter
Canadian Neutron Beam Centre Hydrogen desorption equipment heater sample thermocouple Reflectometry furnace: Ar atmosphere or vacuum 300 K < T < 670 K
Canadian Neutron Beam Centre Mg 0. 6 Al 0. 4 at 298 K Pd Mg 0. 6 Al 0. 4 Si. O 2 Si Fit: Pd: t = 104 Å Mg. Al: t = 520 Å = 4. 4 Å = 15. 7 Å
Canadian Neutron Beam Centre Absorption in Mg 0. 6 Al 0. 4 SLD b. H < 0 • increase of film thickness by about 20% • hydrogen content is 83 at. % = 3. 2 weight % t
Canadian Neutron Beam Centre Annealing of a desorbed Mg 0. 7 Al 0. 3 film Pd layer does not exist anymore after 9 h: Pd diffuses into the Mg. Al layer
Canadian Neutron Beam Centre Dy. Fe 2 / Er. Fe 2 multilayer: element-specific hysteresis Magnetization reversal at 100 K After saturation at µ 0 H = – 6 T (6 nm Dy. Fe 2 / 6 nm Er. Fe 2)40 Er. Fe 2 and Dy. Fe 2 magnetizations are not parallel Dy. Fe 2: easy-axis loop Er. Fe 2: hard-axis loop
Canadian Neutron Beam Centre PNR is element-specific R+ = R nonmagnetic layers Er. Fe 2 Dy. Fe 2
Canadian Neutron Beam Centre PNR is element-specific ~R+ ~R- Er. Fe 2 Dy. Fe 2 R+ = R nonmagnetic layers Er. Fe 2 Dy. Fe 2
Canadian Neutron Beam Centre PNR is element-specific ~R+ ~R- Er. Fe 2 Dy. Fe 2 R+ = R nonmagnetic layers Er. Fe 2 Dy. Fe 2
Canadian Neutron Beam Centre PNR is element-specific ~R- ~R+ Er. Fe 2 Dy. Fe 2 ~R+ ~R- Er. Fe 2 Dy. Fe 2 R+ = R nonmagnetic layers Er. Fe 2 Dy. Fe 2
Canadian Neutron Beam Centre PNR is element-specific ~R- ~R+ Er. Fe 2 Dy. Fe 2 ~R+ ~R- Er. Fe 2 Dy. Fe 2 R+ = R nonmagnetic layers Er. Fe 2 Dy. Fe 2
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, b. Ni = 10. 3 fm, b. Ti = -3. 4 fm 100 bilayers, qc = 2 x qc, Ni
Canadian Neutron Beam Centre supermirror SLD Ni Ni Ni Ti Ti Ti z m-value: m = qc / qc, Ni
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 spin-up neutrons spin-down neutrons SLD Index matching Fe/Co Si Si Fe/Co Si
Canadian Neutron Beam Centre Polarizing supermirror: Fe-Co/Si Reflected intensity Transmitted intensity
Canadian Neutron Beam Centre Flipping ratio = 25 usable range
- Slides: 39