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CRG - Advanced Reflectometer for the Analysis of Materials

The reflectometer SuperADAM is an angle dispersive fixed wavelength machine which combines high flux due to a focussing monochromator with a high Q resolution.

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Applications


Due to the flexible construction and extensive peripheral equipment available for the users of the instrument, ADAM offers an ideal environment for investigations from an explorative level to a highly advanced and sophisticated level.

The main topics, which have been investigated with the ADAM instrument, are briefly reviewed below.

Many more and different types of experiments can be envisioned which may be performed with the ADAM reflectometer, depending only on the ideas and demands of the users.

In principle the following areas are:

1. Solid Films and Superlattices

Layer magnetizations and hysteresis

Polarised neutron reflectivities are shown for a ferromagnetic 2 nm thick Fe layer on a non-magnetic Nb film.

The intensity splitting between the R(+,+) and R(-,-) reflectivities is a clear sign for the ferromagnetic state of the very thin Fe layer sandwiched between Nb films.

The green dots show the spin flip reflectivity R(+,-). (Courtesy of K. Theis-Bröhl).

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Lateral patterns

Polarised neutron scattering from an array of CoFe-stripes. The neutron maps for up and down polarised neutrons clearly show a magnetic signal at the position of the first and second order Bragg peak. The intensity asymmetry is used to measure the magnetic hysteresis of the stripe pattern.
Magnetisation reversal measurements (see fig.) of laterally patterned magnetic systems are currently of great interest because of potential applications in the area of magneto-electronics.

(Courtesy of K. Theis-Bröhl).

GISANS

By restricting the divergence of the incoming beam in the direction perpendicular to the scattering plane three dimensional information becomes accessible.

The picture shows the arrangement of polymer micelles with a diameter of about 180 Å close to a hydrophilic and a hydrophobic coated silicon wafer.

For the top panels the sample is in a liquid phase.

The ring of increased intensity corresponds to the liquid structure factor and the distance between two micelles. The bottom panels show the sample in a crystalline cubic phase.

Close to the hydrophilic interface a cubic close packed structure is formed whereas close to the hydrophobic interface a three dimensional powder with a cubic face centred structure is formed.

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Antiferromagnetic exchange coupling of Heusler alloys

Heusler alloys promise a 100% spin polarisation at the Fermi surface and are therefore highly interesting for spin electronic device applications.

The thin film fabrication of these ternary alloys is not trivial since the correct stochiometry and the sublattice order (right panel) must be guaranteed for magnetic polarization.

The polarised neutron reflectivity from a double Heusler alloy Co2MnGe with a V spacer layer exhibits an antiferromagnetic exchange coupling at low temperatures.

The coupling is weak and could only be discerned with the use of polarised neutron reflection.

(Courtesy A. Bergmann and K. Westerholt).

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Exchange bias effects

The exchange bias (EB) phenomenon is associated with an exchange coupling between ferromagnetic (F) and antiferromagnetic (AF) layers across their common interface,  resulting in a unidirectional  magnetic anisotropy and a shift of the magnetic hysteresis by an exchange bias field HB, after field cooling the F/AF bilayer below the blocking temperature TB < TN  of the AF layer.

The upper panel shows a magnetic hysteresis, measured by the magneto-optic Kerr effect after field cooling in an external field below the TN.

The lower panels reproduce the spin-flip (SF) and non-spin-flip (NSF) intensities for the descending (left) and ascending (right) part of the hysteresis loop.

Note the different SF intensities at both coercive fields (indicated by the arrows), being characteristic for domain wall motion (left) and coherent rotation (right) (Courtesy of F. Radu).

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Isotopic superlattices

Isotopic superlattices are chemically not visible and therefore yield no x-ray contrast. However, due to the isotope dependence of the cross sections, neutrons are sensitive to different isotopes in the sample.

The unpolarised neutron reflectivity from a 56Fe/57Fe superlattice clearly shows the layering of the sample with sharp interfaces.

The inset reproduces the film thickness oscillations from the superlattice with a total thickness of 164 nm, demonstrating the high resolution of the ADAM reflectometer.

(Courtesy B. Hjörvarsson).

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Phase transitions

Radial neutron scans of the (00.0+τ) magnetic satellite peak for the 4.6 nm thick single epitaxial Ho(00.1) film for different temperatures.

The peak shifts to higher scattering vectors due to an increasing turn angle of the Ho spin helix with increasing temperature.

Simultaneously, the  intensity decreases due to a loss of long range magnetic order with increasing temperature.

In the inset the magnetic order parameter is plotted as a function of temperature.

The solid line shows a fit to the data  in the temperature region close to TN yielding a critical parameter β ≈ 0.5 and TN ≈ 105 K. Courtesy V. Leiner).

Hydrogen and oxygen profiles in metal superlattices



Due to the high sensitivity of neutrons to deuterium, the density profile of deuterium in metal superlattice can be determined with very high accuracy.

  • Left panel: a schematic outline of the Mo0.5V0.5/V superlattice in terms of the coherent neutron scattering length profile: (a) superlattice before loading the V layer with deuterium; (b) the V layer is filled with a deuterium concentration as to cancel the neutron optical contrast between both layers; (c) depending on the interfacial properties, a depletion zone for deuterium may occur at the interface between both metal layers.
  • Right panel: experimental and fitted neutron reflectivity curves of the superlattice: (a) before and (b) after loading with deuterium.

The insets to (a) and (b) show the scattering length profiles obtained from the respective fits. In (b) slight overloading and the formation of deuterium-depleted layers at the MoV/V interfaces are observed.

(From V. Leiner et al.)

Superconductivity

Flux-line row-transitions determine the magnetic behaviour of thin superconducting films in the mixed state in the configuration of an external field parallel to the film surface, as shown schematically in the upper panel for increasing field values.

Polarised neutron reflectometry reveals the number of flux-line rows and yields the magnetisation curve, which exhibits unusual behaviour arising from the reduced dimensionality.

The filled circles in the lower panel mark the positions of the flux-line rows in an external magnetic field .

a) Schematic representation of the arrangement of 1 to 3 flux-line rows inside a superconducting thin film in an external magnetic field.

b) The ideal profile of the magnetic scattering-length density (= magnetisation profile) from the flux-line rows as seen by polarised neutron reflectometry. The two spin states of the neutrons in the external field yield the two magnetic scattering length densities marked with green and pink lines. The nuclear scattering-length density is omitted.

c) The experimentally deduced magnetic scattering-length density (= magnetisation profile) in a step-function presentation. The data have been taken at a temperature of 4 K and the sample was cooled in zero field (Courtesy V. Lauter-Pasyuk).

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2. Soft films and multilayers

Multilamellar membranes supported on solid silicon

Neutron-scattering of multilamellar membranes supported on solid silicon substrates exhibit a high degree of orientational alignment, which allows for a clear distinction between specular and nonspecular reflectivity contributions.

In particular, the nonspecular diffuse scattering can be mapped over a wide range of reciprocal space, which is usually not accessible by x-rays.

This opens up the possibility to study fluctuations and lateral structure parameters of membranes on length scales between a few Å up to several µm.

The left panel shows a one-dimensional cross-section of the diffuse scattering as a function of parallel momentum-transfer.

The arrow marks the position of the evanescent wave passing over the film.

The right panel reproduces a reciprocal space mapping around the first Bragg peak of a stack of multilamellar phospholipid membranes (DMPC) (Courtesy T. Salditt).

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Langmuir-Blodgett films

The layering and temperature dependence of multilayers consisting alternately of deuterided and non-deuterided Cd-stearat layers and separated by the co-polymer MSA22 have been studied with unpolarised neutron reflectivity.

The temperature dependence of the first and second order Bragg peak (right panel) can be described by an order parameter, describing the tilt angle of the fatty acid salt molecules with respect to the surface normal (Courtesy U. Pietsch).

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Liquids under shear

Neutron reflectrometry experiments have shown that sheared polymer solutions exhibit distinct wetting behaviour on coated silicon wafers and characteristic structural changes in the bulk.

The intensity maps reproduce the specular and off-specular reflectivity of P85 molecules dissolved in heavy water under shear, which are placed in between hydrophobic and hydrophilic Si discs.

The black line indicates the specular reflectivity of the liquid.

The top panels show the reflecivity of the liquid without shear, the bottom panels with shear.

The left panels are for hydrophilic interfaces and the right panels for hydrophobic interfaces.

Without shear a Bragg sheet is visible for the hydrophilic interface, which becomes more pronounced under shear, indicative for an ordering and layering of the liquid.

In contrast, only a weak Bragg sheet is visible for the hydrophobic interface, which becomes even weaker under shear.

This is an impressive demonstration of how liquid ordering depends on the nature of the solid – liquid interface (Courtesy M. Wolff).

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3. Materials Science Topics

Polyelectrolytes

The structure of polyelectrolyte multilayers built up by alternate adsorption of polyanions and polycations was investigated by neutron reflectivity at the solid/liquid interface.

The experiment provides detailed information on the density gradient of polyelectrolyte chains across the films and shows the influence of the water content of the film on the internal structure.

The polyelectrolyte density is determined by the adsorption conditions (e.g. amount of NaCl) and cannot be changed by addition of salt after adsorption.

After drying, the film thickness is reduced by 30%. The top panel shows schematically the experimental set up and the sample design.

The bottom panel reproduces reflectivity spectra and scattering-length profiles (insets).

a) x-ray reflectivity before adsorption of the polyelectrolyte multilayer at the solid/air interface;
b) neutron reflectivity before adsorption at the solid/liquid interface;
c) as b) but after adsorption;
d) as a) but after adsorption (Courtesy R. v. Klitzing).

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Ion implantation

Ion implantation is a useful technique to obtain composite materials such as metal nanocluster composite glasses (MNCGs), formed by nanometer-size metallic clusters embedded in silicate glasses.

Due to quantum and dielectric confinement effects, these composites exhibit several effects such as an enhanced optical Kerr susceptibility, which can be important for application in all-optical switching devices. Moreover, MNCGs obtained by ion implantation of transition elements are important for their magnetic properties itself.

Neutron reflectometry was used as a non-destructive method to determine the in-depth metal cluster density distribution and to roughly estimate the cluster size.

The data analysis under a multilayer sample approximation gives realistic results: this suggests the possibility to use neutron beams to study very diluted systems such as MNCGs.

The figure shows the experimental in-depth nickel distribution measured with Rutherford backscattering spectroscopy (RBS), left, and reflectivity, right. (Courtesy C. Mondelli).

 

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4. Technical developments

Neutron waveguides as neutron optical devices

Planar neutron waveguide structures can be used as resonant beam coupling devices to efficiently produce a coherent neutron line source with cross-sections in the sub-micrometer range.

The Fraunhofer farfield diffraction pattern of the resonance modes was measured at the neutron reflectometer ADAM and found to be in excellent agreement with the theoretical model.

Inset: a sketch of a neutron resonant beam coupling waveguide device (RBC). The impinging neutron beam excites a resonance mode inside the structure.
Main figure: Linear contourplot of the calculated neutron wavefuntion |ψ(αi,z)|2 in a (52Å Ni / 1410Å C / 490Å Ni / Si) neutron waveguide as a function of the angle of incidence αi and the depth below the surface z.

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Characterization of Si perfect crystals for USANS

Left panel : Experimental setup for Ultra-Small Angle Neutron Scattering (USANS) tested on the ADAM reflectometer.

Right panel : performance of the ADAM reflectometer in comparison to other instruments. According to this chart, the ADAM reflectometer covers a q-range from 2.5 x 10-6 nm-1 to 8 x 10-4 nm-1 , corresponding to structural units from 25 mm to 0.078 mm.

 

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