H. Fredrikze

ROG, the neutron reflectometer at IRI, Delft

Abstract A description is given of an optimally designed neutron reflectometer. The background to the design is explained, calibration measurements are discussed and data handling procedures are described. Finally, the possibilities of the reflectometer are shown with several reflectivity measurements.

Polarized neutron reflectometry studies of magnetic oxidic Fe3O4/NiO and Fe3O4/CoO multilayers

The magnetic properties of [1 0 0] oriented Fe3O4/NiO and Fe3O4/CoO multilayers, MBE-grown on MgO(0 0 1) substrates, have been studied by polarized neutron reflectometry. In both samples, the Fe3O4 layer exhibits a depth-dependent magnetic profile characterized by a reduction in the magnetization near the interfaces. The possible origins of this behaviour, such as domain wall formation in the ferrimagnetic layer and deviations in stoichiometry, are discussed.

ROG, the new neutron reflectometer at IRI, Delft

Abstract A description is given of the neutron reflectometer ROG, recently installed at IRI. The neutron wavelength is determined by time of flight. Wavelengths from 0.08 to 0.8 nm can be used. First experimental results are presented.

Test of adiabatic spin flippers for application at pulsed neutron sources

Abstract Experimental results on the flipping efficiency are shown for a set of 2 V -coils as spin flipper and for a high-frequency flipper with adiabatic transition. The influence of the adiabaticity parameter is discussed. The merits of these adiabatic flippers are compared with the use of “monochromatic” flippers, when operated in a beam from a pulsed neutron source. It is concluded that for “long pulse” sources adiabatic flippers will be superior.

Non-specular spin-flipped neutron reflectivity from a cobalt film on glass

From polarized neutron reflection experiments on a thin Co layer on glass at various applied fields it is shown beyond doubt that neutron spin-flip reflection in an applied magnetic field is non-specular in agreement with theory. It is shown that the deviations from the specular position at moderate fields is very substantial for wavelengths larger than a few tenths of a nm. The advantages of a curved stacked-mirror polarization analyzer, which recently has been installed on ROG at IRI, are discussed.

Neutron reflectometry on an exchange biased Ni80Fe20/Fe50Mn50 bilayer

Polarized neutron reflectometry experiments were performed on a thin MBE-grown [111]-oriented exchange biased Ni 80 Fe 20 /Fe 50 Mn 50 bilayer. Analysis of the data shows that the magnetic moment of the ferromagnetic Ni 80 Fe 20 layer appears constant throughout the layer. In addition, this experiment restricts the possible spin arrangements or domains that may influence the magnitude of the exchange biasing.

Calibration of a polarized neutron reflectometer

Abstract In a polarized neutron reflection experiment four reflectivities R kl are determined, where k and l denote the spin state of the incoming and reflected neutrons, respectively, either parallel or anti-parallel to the guide fields along the incoming and reflected beam. A calibration of the setup is needed because the polarizer nor the analyzer are perfect and moreover the effective polarizing power of the polarizer and analyzer may depend on the nominal spin state of the incoming and reflected neutrons. It is shown that four measurements without a sample and four measurements using a magnetically saturated sample are needed to determine the five instrumental parameters that are relevant to interpret the results of a polarized neutron reflection experiment properly. For a time-of-flight reflectometer these parameters are wavelength dependent.

Polarization analysis of neutron reflectometry on non-collinear magnetic media: polarized neutron reflectometry experiments on a thin cobalt film

Abstract In this work we present a full data analysis of polarized neutron reflectometry experiments on a thin magnetic film. A magnetic field was applied perpendicular to the layer magnetization, resulting in non spin-flip and spin-flip reflectivities. For moderate external fields, the gain or loss of Zeeman energy for spin-flipped neutrons results in off-specular reflected spin-flipped beams. The theoretical model used to explain all data simultaneously consists of a bulk cobalt layer, together with interfacial layers with low magnetization values.

On the use of a multilayer monochromator in neutron reflectometry

Abstract In neutron reflectometry experiments, the intensity reflected from the sample under investigation is determined as a function of the wave-vector transfer. One way of doing this is to start with a monochromatic beam and perform the so-called θ/2θ scans. The monochromatic beam can be produced by Bragg reflection from a single crystal, or from a multilayer system. In this paper, we experimentally investigate the performance of a multilayer monochromator. We demonstrate that the large-wavelength contamination, intrinsically present, may lead to large errors in the obtained reflectivity at large values of the wave-vector transfer.

Probing magnetic structures by neutron reflectometry: Off-specular scattering from interfaces and domains in FeCoV/TiZr multilayers

Polarized neutron reflectometry (PNR) is commonly used to characterize the depth-dependent nuclear density and magnetization profiles of thin films and multilayers. While specular reflectivity is connected with characteristics perpendicular to the sample surface, in-plane features cause off-specular scattering. In this work we present time-of-flight PNR results on (FeCoV/TiZr) multilayers with applied fields parallel to the in-plane easy axis of magnetization. Our measurements at low applied fields clearly show diffuse scattering. We attribute this scattering to randomly oriented magnetic domains, with no coupling between the magnetic FeCoV layers. With increasing applied field the domain scattering decreases. At the same time, off-specular scattering with a well-defined relation between the wavelength and the scattering angle increases with increasing applied field. This type of scattering can be attributed to correlated interfaces.