B. Toperverg

Specular reflection and off-specular scattering of polarized neutrons

The general situation with polarized neutron reflectivity, diffraction on a crystalline structure and off-specular scattering from magnetic inhomogeneities in thin films is thoroughly discussed. It is argued that only a combination of birefringence (spin-dependent refraction) of a neutron wave in the mean magnetic field of the film and spin-flip magnetic diffraction or scattering can lead to the splitting of the outgoing (diffracted or scattered) beam into two beams with different wave vectors and spin states. An effect of interplay between the lateral projection of the coherence length and the in-plane extension of magnetic inhomogeneities (size of domains, roughness correlation length etc.) is outlined.

Towards 3D polarization analysis in neutron reflectometry

General equations for spin-flip and non-spin-flip specular reflection and off-specular scattering are analyzed for the case of multidomain states of magnetic multilayers. The results are illustrated by numerical calculations performed for Fe/Cr multilayers using an original software developed on the basis of the supermatrix formalism. This routine allows to calculate the reflectivities and scattering cross sections from the layered structures of any complexity for any directions of the incident polarization vector and directions of the polarization analysis.

Simulations of off-specular scattering of polarized neutrons from laterally patterned magnetic multilayers

We put forward a new presentation of the theory of off-specular scattering of polarized neutrons from magnetic multilayers within the distorted wave Born approximation developed by one of the authors. In this presentation, a clear separation is made between (1) the structure factors, model dependent, and (2) the optical coefficients and the efficiencies of the polarizer and the analyzer, model independent. It therefore permits an easy plugging in of any particular model of structural and magnetic correlations. For a test, we show simulations of off-specular scattering from magnetic domains in multilayers exhibiting anti-ferromagnetic coupling, a situation encountered in the literature.

Implementation of a zero-field spin-echo option at the three-axis spectrometer IN3 (ILL, Grenoble) and first application for measurements of phonon line widths in superfluid 4He

Abstract We report on a new zero-field spin-echo spectrometer (ZETA), installed at the thermal three-axis spectrometer IN3 at ILL, Grenoble. In this technique the two long precession solenoids from neutron spin echo are replaced by two pairs of magnetic resonance coils in a zero-field region. This allows easy adaptation of tilted field geometries, necessary for measurements of phonon line widths with μeV energy resolution. First test measurements of the roton line width and energy shift as well as line width of the low- Q phonon and in the “region beyond the roton” in superfluid 4 He are described. The results are in agreement with the literature and show the good operation of ZETA. The implementation of ZETA at the high-flux three-axis spectrometer IN20, foreseen in the near future, will increase the count rate by typically one order of magnitude.

Magnetic neutron off-specular scattering for the direct determination of the coupling angle in exchange-coupled multilayers

Abstract A new method is presented for the direct determination of the coupling angle in magnetic exchange multilayers. Strong anomalies in the off-specular neutron spin–flip scattering intensity at critical angles give a new tool, which is tested with Fe/Cr multilayers. In addition, a complete two-dimensional data analysis of specular reflection and off-specular scattering has been employed to verify atomic spin correlation in Fe/Cr multilayers, a typical system showing the GMR effect.

Determination of the magnetic fluctuations in an Fe/Cr/Fe trilayer exhibiting a neutron resonance state

We show a measurement of off-specular polarized neutron scattering from an Fe/Cr/Fe trilayer inside which a second-order neutron resonance has been excited, i.e. where the probability to find a neutron was maximum at both Fe/Cr interfaces. Simulating the data in the framework of the Distorted Wave Born Approximation showed the presence of structural and magnetic fluctuations. Magnetic fluctuations in the vicinity of the Fe/Cr interfaces can be explained if one considers the anti-ferromagnetic structure of Cr and structural roughness.

Polarized neutron grazing angle birefrigent diffraction from magnetic stratified media

Abstract Polarized neutron diffraction in multilayers is considered in the framework of the Supermatrix formulation of the distorted wave born approximation (DWBA). The general equations for the polarized neutron scattering cross sections are derived and illustrated by the example of diffraction from the regular ferromagnetic multilayered structure. It is shown that, due to the resonance enhancement of the neutron wave field inside the films, the intensity of birefrigent spin-flip and non-spin-flip diffraction is significantly amplified, if the wavelength is matched with the multilayers period.

Magnetic properties of laterally structured Fe/Cr multilayers

The surface layer of an epitaxial Fe/Cr multilayer system has been structured lithographically into stripes with 1 μm period. Polarized neutron reflectometry and diffuse scattering under grazing incidence measurements show strongly enhanced domain formation compared to the unstructured sample. The domains are correlated throughout the layers. The lateral domain size is partly the period of the stripe pattern of the surface layer, and partly the double.

Ordering in magnetic multilayers by off-specular neutron scattering

The combination of specular reflection and off-specular scattering with polarized neutrons gives a unique detailed information on the lateral and transverse spin-configuration of a magnetic multilayer stack. We present the first direct experimental observation of a phenomenon predicted theoretically for antiferromagnetically coupled multilayers: the twisted ground-state configuration in an external magnetic field. This twisted configuration arises due to the reduced symmetry of magnetic moments of the end layers having only one neighbor. This fact can be established directly via the qualitative analysis of the line shape of the superstructure peaks on the specular line and the related off-specular Bragg sheet. Additional quantitative evaluation of both specular reflection and off-specular scattering allows one to deduce the layer-by-layer spin configuration through the multilayer stack, as well as within the plane of the layer. The presence of spin-flip off-specular scattering means that the layer magnetization is laterally not homogeneous but is decomposed into a set of domains. The distribution of magnetic moments within each domain, the domains size and their distributions are obtained.

Optical theorem, depolarization and vector tomography

Abstract A law of the total flux conservation is formulated in the form of the optical theorem. It is employed to explicitly derive equations for the description of the neutron polarization within the range of the direct beam defined by its angular divergence. General considerations are illustrated by calculations using the Born and Eikonal approximations. Results are briefly discussed as applied to Larmor–Fourier tomography.