2. By Abdullah Framalawi Aly Abouhaswa Polarized Neutron Spectrometry : Studying nanostructure magnetism with the use of polarized neutron reflectometry The project supervisor Dr. Yuriy Nikitenko Head of the Polarized Neutron Spectrometry group at the Department of Neutron Scattering

3. Outline  Reflectivity.  General principles.  Polarised neutrons.  The functional scheme of the REMUR spectrometer.  Data analyses.

4. Neutron Reflectometry Neutron Reflectometry is a technique for measuring the structure of thin film. Reflectivity profile give information about: 1- Structure of surface. 2- Thickness of each layer. 3- Density of each layer. 4- Roughness of thin film layered on the substrate.

5. Measurement of Specular Reflectivity Measurement of Off-Specular Reflectivity The incident and exit angles are equal (θ = θi = θf ) The wave vector Q z is defined as 4 π sin θ / λ, where λ is the neutron wavelength. θi ≠θf

6. Why Use Neutron Reflectivity? Advantages of neutrons include: – Contrast variation (using H and D, for example). – Low absorption – probe buried interfaces, solid/liquid interfaces etc – Non-destructive. – Sensitive to magnetism, allows the study of the magnetic configuration of a multilayer system. – Thickness length scale 10 – 5000 Å.

7. 2 ways of varying the scattering wave-vector Angular scan θ – 2θ. (glancing angle of incidence θ at constant wavelength) Time-of-flight. (varying wavelength, at constant θ)

8. Reflection on a thin film deposited on a substrate If we add a thin layer on top of the substrate we get interference fringes & the reflectance is given by: and we measure the reflectivity R = r.r* If the film has a higher scattering length density than the substrate we get the green curve (if the film scattering is weaker than the substance, the green curve is below the red one) Reflection on a substrate t

9. Reflectivity on a multilayer system d (n)

10. Example Cu(500 Å)/Cr(90 Å) on silicon

14. IBR-2M Water and cold moderator

15. Reflectometry experiment The main parameters of the REMUR spectrometer Sample plane: vertical Scattering plane: horizontal Neutron wavelength: 0.9 – 10 Å Wavelength resolution: δλ = 0,011 Å Scattering angle range: 1 – 100 mrad Sample – detector distance: 4.9 m Detector's spatial resolution: 1.5 mm.

16. PolarizerFlipper 1sampleFlipper 2 Analyzer Detector + - R+R+ R-R- R + + R + - R - + R - - Standard polarized neutron reflectometry

17. Polarized neutron reflectometry (non-spin-flip scattering)

18. Analysing Reflectivity Data Simulation of reflectivity profiles using SimulReflec. You can see the effect you want to see. What is the best substrate? Surface roughness. Layer thicknesses between 10 Å and 5000 Å