Thomas Krist

Substrate-stress-induced magnetic and nonmagnetic structural correlations in Fe/Si multilayers

Application of a bias voltage can influence the growth kinetics and thereby the stress in a magnetic multilayer. The inherent structural correlations in turn can influence the magnetic domain structures and thereby the overall device performance. Here, prototypical Fe/Si supermirrors are subjected to stress relaxation during the growth of sequential layers by applying a sufficient substrate bias voltage. A change in the coercivity associated with the grain size variation upon biasing is found. Most interestingly, using polarized neutron scattering, it is possible to identify that the conformal roughness becomes nonconformal with the relaxation of stress within the multilayers. The magnetic domains, on the other hand, always remain nonconformal (independent of the structural change) as they undergo spatial fluctuations around a mean magnetization. This study underscores the importance of the substrate biasing in affecting the structural correlation, which is detrimental to the resultant optical (e.g. supermirror) quality.

A white beam neutron spin splitter

The polarization of a narrow, highly collimated polychromatic neutron beam is tested by a neutron spin splitter that permits the simultaneous measurement of both spin states. The device consists of a Si-Co{sub 0.11} Fe{sub 0.89} supermirror, which totally reflects one spin state up to a momentum transfer q=0.04 {angstrom}{sup -1}, whilst transmits neutrons of the opposite spin state. The supermirror is sandwitched between two thick silicon wafers and is magnetically saturated by a magnetic field of 400 Oe parallel to its surface. The neutron beam enters through the edge of one of the two silicon wavers, its spin components are split by the supermirror and exit from the opposite edges of the two silicon wafers and are recorded at different channels of a position-sensitive detector. The device is shown to have excellent efficiency over a broad range of wavelengths.

Upgrade Program for the Cold Neutron Instrumentation of the Helmholtz-Zentrum Berlin

The Helmholtz-Zentrum Berlin (HZB, former Hahn-Meitner-Institut) operates more than 20 instruments at the medium-flux research reactor BER II, a modern neutron source which was completely renewed during the years 1985–1992. The creation of the Berlin Neutron Scattering Center (BENSC) in 1993 marked the beginning of an extended user program, offering access to a great variety of new instruments with some unique features suited for research in many fields of science. In the course of time the Helmholtz-Zentrum Berlin has established itself as a major neutron science facility with an exceptionally high demand from international, in particular European users.

Solid state neutron polarizers and collimators

In most cases, neutron optical elements like polarisers or collimators use coated surfaces which define the neutron flight path in air or vacuum. To reduce the size of such elements silicon single crystals can be used as the medium in which the neutrons travel. We have built and tested a neutron polarising bender which consists of a stack of thin silicon wafers. The neutrons enter at the front side. Inside the wafers the spin up component is reflected from the supermirror coated side and can leave the wafers while the spin down component passes the supermirror and is absorbed in the Gd layer of the adjacent wafer. Other neutron optical element, we tested for the first time are several collimators made from silicon wafers coated with either Gd or reflecting coatings below an absorbing Gd layer. These collimators produce a beam with quasi-rectangular distribution of angles, which represents an intensity gain at equal resolution compared to the triangular distribution in conventional Soller collimators. Finally, we report on the first test of a solid state radial collimator. Some general aspects of sold state neutron optical elements are also discussed.

Stress Reduction in Multilayers Used for X-Ray and Neutron Optics

Multilayer systems have important applications in many areas of X-ray and neutron optics. For some applications the positions of the optical surfaces have to be controlled with accuracies in the sub-nanometre range. For neutron supermirrors with over a thousand layers, stresses above 1000MPa can occur. In addition to bending the substrate such stresses can lead to the films peeling from the substrate, or even to the destruction of the substrate surface, and so must be avoided. After an introduction to stress, this chapter describes how stresses can be reduced to acceptable values and discusses two examples – FeCo/Si polarizing neutron supermirrors and Mo/Si multilayer mirrors for extreme ultraviolet lithography.

Stress dependence in Fe89Co11Fe89Co11–Si multilayers on layer thicknesses

We report on a study of the stress developing in materials which are used for polarising neutron supermirrors. The stress was examined as function of the thickness of Si and Fe89Co11Fe89Co11 layers in multilayer systems. The samples were produced in a triode sputter machine. The bending of the samples was measured on a profilometer and the stress was calculated with the Stoney formula. The samples were characterized with polarized neutron and X-ray reflectometry and XRD. It was found that an increase in layer thickness leads to decreasing compressive stress for FeCo layers and to decreasing tensile stress for Si layers. A formula is given which allows to estimate the resulting stress.

Stress dependence in Fe89Co11–Si multilayers on layer thicknesses

We report on a study of the stress developing in materials which are used for polarising neutron supermirrors. The stress was examined as function of the thickness of Si and Fe89Co11Fe89Co11 layers in multilayer systems. The samples were produced in a triode sputter machine. The bending of the samples was measured on a profilometer and the stress was calculated with the Stoney formula. The samples were characterized with polarized neutron and X-ray reflectometry and XRD. It was found that an increase in layer thickness leads to decreasing compressive stress for FeCo layers and to decreasing tensile stress for Si layers. A formula is given which allows to estimate the resulting stress.

Stress dependence in –Si multilayers on layer thicknesses

We report on a study of the stress developing in materials which are used for polarising neutron supermirrors. The stress was examined as function of the thickness of Si and Fe89Co11Fe89Co11 layers in multilayer systems. The samples were produced in a triode sputter machine. The bending of the samples was measured on a profilometer and the stress was calculated with the Stoney formula. The samples were characterized with polarized neutron and X-ray reflectometry and XRD. It was found that an increase in layer thickness leads to decreasing compressive stress for FeCo layers and to decreasing tensile stress for Si layers. A formula is given which allows to estimate the resulting stress.