F. Klose

Metallic multilayers and hydrogen

The interaction of hydrogen with metallic layer structures and the change of properties of these structures upon hydrogen charging are reviewed. Drastic deviations from the bulk behaviour are observed. The deviations can be traced back to the elastic boundary conditions and to the electron transfer at the interfaces between the different materials. Examples of improvements of the structure or the electronic properties of the layers by hydrogen incorporation are reported. Presently, several groups are trying to change the magnetic coupling of magnetic layers by introducing hydrogen in the non-magnetic spacer layer.

Hydrogen-charging of Nb/Fe multilayers

Abstract Nb/Fe multilayers with a Pd top layer were prepared by electron beam evaporation on Si substrates. The multilayer stacks were charged with hydrogen from the gas phase at 200°C and at a gas pressure of 0.1 mbar. The resulting hydrogen concentration profiles were measured by the 15 N nuclear reaction method. We find that the multilayers take up hydrogen under these conditions, that hydrogen also reaches the low lying layers and that hydrogen is accumulated in Nb, leaving the Fe layers essentially hydrogen-free. The present work was undertaken as a preparatory study for the investigation of the magnetic coupling of the Fe layers through the Nb spacers as a function of the hydrogen content.

Magnetic and structural properties of Fe/Nb multilayers

Abstract MBE grown Fe/Nb multilayers were characterized by means of X-ray diffraction in small and high angle regions, and neutron reflectivity measurements. It can be observed that this system has an oscillating ferro/antiferromagnetic ground state depending on the Nb thickness. The coherent long range magnetic order was found to be irreversible, i.e. it gets lost if one cycles the films in an external magnetic field. The influence of hydrogen charging on the magnetic and structural properties of the multilayers was studied.

Measuring lateral magnetic structures in thin films using time-of-flight polarized neutron reflectometry

Polarized neutron reflectometry (PNR) has recently been applied to study lateral magnetic structures such as regular micron-sized magnetic arrays on a surface. To date, however, there is a lack of detailed accounts of the features observed in the scattered intensity map in the special case of time-of-flight (TOF) PNR. We present here preliminary measurement results on lithographically produced arrays of micron-sized rectangular permalloy magnetic bars. The measurements demonstrate the potential of the method to provide detailed structural information on a laterally patterned sample, as well as on its magnetic characteristics. The information can be obtained by analyzing the specular reflection along with three off-specular Bragg sheets. Most of the features seen experimentally can be interpreted by using simple heuristic arguments. In addition, we also present results of a study of lateral magnetic domains in an exchange-biased Co/CoO bilayer film to illustrate the capability of TOF PNR in the study of large lateral magnetic domains in the case when almost no off-specular scattering is detected.

Magnetic depth profile of Au/Fe/Ni trilayers

Abstract Magnetization measurements of a series of Au/Fe/Ni trilayers were carried out by spin-polarized neutron reflectometry and SQUID magnetometry. The bottom layer of Ni had the same thickness for all samples (140 A) while the top layer of Fe varied from sample to sample (16–90 A). The magnetic moment of iron was found to be dependent on the layers thickness. For Fe layers of 32 A or less, the Fe magnetization was an order of magnitude smaller than that of bulk BCC Fe, indicating that Fe is either in a non-magnetic state or that its ordering is antiferromagnetic. For Fe layers of 60 A or more, the Fe moment was found to be in the plane of the film, constant throughout its entire layer, and with a value (2.0 μ B /atom) approaching that of bulk iron. This magnetic transition probably can be attributed to a structural phase transition from the FCC to the BCC structure.

Polarized neutrons for pulsed neutron sources

Polarized neutrons provide a powerful tool for research at todays continuous neutron sources, permitting unique information to be obtained from neutron-scattering experiments and in the area of fundamental neutron physics. The information obtained is indispensable to important areas of current research such as magnetism, spin fluctuations in correlated-electron materials, nanoscience, astrophysics, and cosmology. Unfortunately, the polarized neutron capabilities that already exist at reactor neutron sources, mainly in Europe, cannot be transferred directly to pulsed spallation sources because many of the devices used at reactors to manipulate neutron spins operate only at a single neutron wavelength or with a neutron beam of limited divergence. Both of these restrictions must be overcome for devices to be useful at pulsed spallation sources. This report identifies the significant R&D efforts that will be required to achieve these goals. These efforts must be undertaken now, if they are to be useful early...

SNS magnetism reflectometer: basic design and neutron guide optimization using Monte Carlo simulations

In this contribution, we summarize the optimization of the neutron guide optics of the Magnetism Reflectometer, which is currently under construction at the Spallation Neutron Source. The guide system consists of a straight source tube, a polygonal curved multi-channel bender, and a converging guide section. The bender will be essential for high-energy neutron and g-ray background suppression, while the converging guide will focus the neutron beam onto the sample. Monte Carlo (MC) numerical methods were used to optimize the guide performance. The flux on sample and detector count rates were systematically determined for various guide configurations. The impact of guide imperfections like surface waviness and misalignment of sections are also studied.

Hydrogen-induced change of the magnetic coupling in Nb/Fe multilayers

Abstract Nb/Fe multilayers, consisting of typically 10–20 doublelayers with individual layer thicknesses in the nanometer range, were investigated by neutron reflectometry, SQUID magnetization and magnetoresistivity measurements. The main purpose of this investigation was to study the influence of hydrogen on the magnetic properties of these systems. We find that the magnetic coupling of the Fe layers over the nonmagnetic Nb layers can be switched between antiferro- and ferro-magnetic by hydrogen charging. The change of the coupling upon hydrogen charging is continuous and reversible.

Magnetic coupling in Fe/Nb multilayers

Abstract Fe/Nb multilayers are studied by neutron small-angle reflectivity measurements. It is shown that for small thicknesses of the Nb spacer layers ( t Nb A ) an oscillating behaviour of the indirect exchange coupling is present. For larger Nb thicknesses the strength of this RKKY-like interaction decays rapidly and merges into a non-oscillatory antiferromagnetic background presumably resulting from a dipole contribution to the coupling energy.

Neutron reflectivity on Tb/Fe multilayers

Abstract Temperature-dependent spin-polarized neutron reflectometry and SQUID magnetization measurements have been carried out on a [ 26 A Tb 50 A Fe ] × 10 multilayer, which shows perpendicular anisotropy. By comparing the results with previous Mossbauer spectroscopy data, we can develop a detailed model which can describe how the Fe spins turn out of the layer plane at lower temperatures. This magnetic reorientation process leads to a canted spin structure. The SQUID data indicate increasing Tb moments at low temperatures, which are coupled antiparallel to the Fe moments. The field dependence of the orientation of the Tb and Fe moments is discussed.