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 MgAl films Element-specific hysteresis curves in ErFe 2 / DyFe 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 In-plane structures on nm and  m scale Scattering length density profile: profile of absorbed gas/liquid interdiffusion magnetic structures magnetic field penetration into superconductors

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) Spin and non-spin flip reflectivity: Magnetization reversal, magnetic structure No diamagnetic background of substrate for ferromagnetic samples: Determination of absolute magnetic moment 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 )

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

Canadian Neutron Beam Centre Reflection and refraction: the critical angle reflected refracted medium 1: n 1 medium 2: n 2 90°  c 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: Materialn  c (for n 2 =1) Vacuum1.00- Water  Quartz glass  Benzene  What is the index of refraction for neutrons? Note: the index of refraction depends on the wavelength

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

Canadian Neutron Beam Centre Scattering lengths X-rays X-rays: b  Z (electron density) Neutrons 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 = nm: Materialn  b ( /nm 2 ) Vacuum Water (H 2 O) Si Quartz glass Heavy water (D 2 O) Ni Note: n  The deviation of n neutron 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 D3 S1 S2 S3 S4 sample PG filter analyzer detector Focusing PG monochromator Polarizing supermirror Spin-down neutrons spin flipper

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

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

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

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

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

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 Fe SLD Cr Fe Cr Si wafer Fe Cr } Bilayer thickness t t = 32.8 Å + 30 Å = 62.8 Å In total: 20 repetitions

Canadian Neutron Beam Centre Magnetic interaction H ext : external magnetic field B : magnetic induction µ : magnetic moment of neutrons

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

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

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

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

Canadian Neutron Beam Centre Co-sputtering of MgAl alloy films Mg Al Pd Vacuum Chamber Si Wafer

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 Reflectometry furnace: Ar atmosphere or vacuum 300 K < T < 670 K sample heater thermocouple

Canadian Neutron Beam Centre Mg 0.6 Al 0.4 at 298 K Mg 0.6 Al 0.4 Pd SiO 2 Si Fit: Pd: t = 104 Å  = 4.4 Å MgAl: t = 520 Å  = 15.7 Å

Canadian Neutron Beam Centre Absorption in Mg 0.6 Al 0.4 increase of film thickness by about 20% hydrogen content is 83 at.% = 3.2 weight %  SLD b H < 0  t 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 MgAl layer

Canadian Neutron Beam Centre DyFe 2 / ErFe 2 multilayer: element-specific hysteresis Magnetization reversal at 100 K After saturation at µ 0 H = –6 T (6 nm DyFe 2 / 6 nm ErFe 2 ) 40 ErFe 2 and DyFe 2 magnetizations are not parallel DyFe 2 : easy-axis loop ErFe 2 : hard-axis loop

Canadian Neutron Beam Centre PNR is element-specific ErFe 2 DyFe 2 R + = R - nonmagnetic layers

Canadian Neutron Beam Centre PNR is element-specific ErFe 2 DyFe 2 ErFe 2 DyFe 2 R + = R - ~R + ~R - nonmagnetic layers

Canadian Neutron Beam Centre PNR is element-specific ErFe 2 DyFe 2 ErFe 2 DyFe 2 R + = R - ~R + ~R - nonmagnetic layers

Canadian Neutron Beam Centre PNR is element-specific ErFe 2 DyFe 2 R + = R - ErFe 2 DyFe 2 ~R - ~R + ErFe 2 DyFe 2 ~R + ~R - nonmagnetic layers

Canadian Neutron Beam Centre PNR is element-specific ErFe 2 DyFe 2 ErFe 2 DyFe 2 R + = R - ~R + ~R - ErFe 2 DyFe 2 ~R - ~R + nonmagnetic layers

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, q c = 2 x q c, Ni

Canadian Neutron Beam Centre supermirror m-value: m = q c / q c, Ni Ni SLD Ni Ti Ni z

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 Fe/Co SLD spin-up neutrons Si Fe/Co Si spin-down neutrons Fe/Co SLD Si Fe/Co Si Index matching

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

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