Ulrich Rücker

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.

Single-particle blocking and collective magnetic states in discontinuous CoFe/Al2O3 multilayers

Discontinuous metal?insulator multilayers (DMIMs) of [CoFe(tn)/Al2O3]m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness tn = 0.5?nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5?nm 1.4?nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.

Enhancement of the Curie temperature of epitaxial EuS(100) films caused by growth dislocations

Abstract We have observed a large enhancement of the Curie temperature of undoped EuS(100) epitaxial films, dependent on the growth conditions. The study performed allows us to claim that such an enhancement is associated with indirect exchange through additional charge carriers that are created in the film due to non-stoichiometry caused by dislocations.

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.

KWS-3: The New (Very) Small-Angle Neutron Scattering Instrument Based on Focusing-Mirror Optics

Ultra-small angle (U-SANS) and small angle neutron scattering (SANS) experiments are performed by two different types of instruments to cover a combined Q-range from ≈10−5Å−1 up to ≈1Å−1. Bonse-Hart cameras (double crystal diffractometers) are used for U-SANS experiments, whereas the “standard” SANS experiment is performed using a pinhole camera. In principle, the Q-range of both instrument classes overlaps. Typical U-SANS instruments like S18 (ILL), PCD (NIST), or DKD (FZJ) may reach maximum Q-vectors of ≈5 × 10−3. The disadvantage of these instruments is that they do not allow taking a full area image on a 2D position sensitive detector. On the other hand, the well-known pinhole instrument D11 at Institut Laue-Langevin (France) reaches a minimum Q-vector of 5 × 10−4Å−1 by use of large wavelengths and sample-to-detector distances (≈40m).

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.

Polarization analysis for the 2D position-sensitive detector of the HADAS reflectometer in Jülich

A neutron reflectometer with polarization analysis is being built on the basis of the HADAS spectrometer in the neutron guide hall at the research reactor FRJ-2 (DIDO) in Julich. The new instrument is optimized for reflectivity and diffuse scattering measurements under grazing incidence on layered magnetic structures with thicknesses in the nm range. In order to measure diffuse scattering with polarization analysis, the 2D position-sensitive detector has been equipped with a polarization analyser that consists of a stack of supermirrors parallel to the scattering plane. First tests have revealed that the resolution of the instrument is not reduced by the polarization analyser. A flipping ratio of 20 has been achieved already during the first experiment.

Superlattice growth and rearrangement during evaporation-induced nanoparticle self-assembly

Understanding the assembly of nanoparticles into superlattices with well-defined morphology and structure is technologically important but challenging as it requires novel combinations of in-situ methods with suitable spatial and temporal resolution. In this study, we have followed evaporation-induced assembly during drop casting of superparamagnetic, oleate-capped γ-Fe2O3 nanospheres dispersed in toluene in real time with Grazing Incidence Small Angle X-ray Scattering (GISAXS) in combination with droplet height measurements and direct observation of the dispersion. The scattering data was evaluated with a novel method that yielded time-dependent information of the relative ratio of ordered (coherent) and disordered particles (incoherent scattering intensities), superlattice tilt angles, lattice constants, and lattice constant distributions. We find that the onset of superlattice growth in the drying drop is associated with the movement of a drying front across the surface of the droplet. We couple the rapid formation of large, highly ordered superlattices to the capillary-induced fluid flow. Further evaporation of interstitital solvent results in a slow contraction of the superlattice. The distribution of lattice parameters and tilt angles was significantly larger for superlattices prepared by fast evaporation compared to slow evaporation of the solvent.

Antiferromagnetic interlayer coupling in epitaxial Fe/EuS ( 100) bilayers

Abstract The magnetic coupling of Fe/EuS (100) epitaxial bilayers has been investigated. Evidence of antiferromagnetic coupling has been found, both when the samples are cooled below the Curie temperature of the EuS layer, and in the hysteresis loops at low temperatures. The coupling strength has been measured as a function of temperature.

Proximity effects in Fe1-xCox/Mn/Fe1-xCox trilayers

We report on the first results of a structural and magnetic investigation of Fe 1-x Co x /Mn/Fe 1-x Co x trilayers (x = 0-0.75). In situ low energy electron diffraction shows that Mn grows epitaxially on bcc Fe 1-x Co x (x ≤ 0.75) with a good crystallinity up to 30 A. Reflectivity and small angle diffuse scattering of synchrotron radiation reveal sharp interfaces, each of them having two lateral characteristic lengths in the height-height correlation function that differ by one order of magnitude. The long-range height-height correlation function is vertically correlated and the short-range one is not. From magneto-optical Kerr effect measurements, evidence for non-collinear coupling between Fe 1-x Co x layers (x = 0 and 0.5) was found for Mn spacer thicknesses below 12 A. Polarized neutron reflectometry performed at 300 K reveal a net magnetization of 0.8 ± 0.3 μ B /at in the Mn layer in Fe 0.5 Co 0.5 /Mn/Fe 0.5 Co 0.5 and no net magnetization in the Mn layer in Fe/Mn/Fe.