Highlights from the magnetism reflectometer at the SNS

doi:10.1016/j.physb.2009.06.021

Physica B: Condensed Matter

Volume 404, Issue 17, 1 September 2009, Pages 2543-2546

https://doi.org/10.1016/j.physb.2009.06.021 Get rights and content

Abstract

The magnetism reflectometer at the SNS has passed the phase of commissioning and is in operation for users. The high power of the neutron source demands that special attention be paid to the optimization of the background in order to be able to measure the two-dimensional maps of reflected and scattered intensities with polarized neutrons in a broad range of momentum transfer. It implies an effective separation of the magnetic and non-magnetic reflectivity, off-specular scattering and the grazing incidence SANS in a high range of momentum transfer. Therefore the polarizing and the analyzing efficiencies are of particular importance on this time-of-flight instrument. At the beginning of July 2008 the world's first ³He neutron analyzer with on-line pump-up polarization was successfully installed and the tests with a magnetic multilayer film showing a strong off-specular spin-flip scattering were made. The performance of the instrument is under constant improvement in order to make it an effective and optimal instrument for the applications in nanosciences.

Introduction

Rapid advances in nanotechnology and nanoscience require development of various methods for the characterization of nanostructured materials [1]. Neutron scattering and in particular polarized neutron reflectometry serves this purpose and continuously develops [2], [3]. The development goes in several directions. Amongst them the increasing power of the neutron sources plays a very important role [4]. Nonetheless, the effective use of the produced neutrons is a no less important element in this development. It includes the improvement of the background and with this the extending of the measured range of momentum transfer, raising the polarizing and analyzing efficiency of the specific elements of the instrument and increasing the resolution. This allows improving the sensitivity to specific parameters like the absolute value and direction of the layer magnetization vector across the film thickness, and interfacial magnetic and non magnetic roughness. Lateral nanostructures, magnetic and structural inhomogeneities create off-specular scattering in addition to the specular reflection. Therefore the registration and the polarization analysis of the off-specular scattering are of particular importance. Here we present the performance of the magnetism reflectometer at SNS which we obtained after a considerable and successful improvement of the background and of different elements of the instrument.

Section snippets

Polarizer

The actual polarizer used on the reflectometer is a bender-type polarizer [5]. It is composed of 200 thin Si blades (each 0.15 mm wide) covered with a Fe/Si reflecting super-mirror coating (m=2.8) and a Gd absorbing layer on the back. The bender is 75 mm long and covers a beam with a cross-section of 29×30 mm². It is placed at the exit of the tapered guide and is aligned to the maximum intensity with the incident grazing angle of 0.4° to allow for polarized neutrons above a wavelength of 2 Å. The

Magnetic off-specular scattering: measurement of the two-dimensional maps

The short wavelength band available on the magnetism reflectometer allows us to use the highest intensity of the incident beam and to obtain a high polarization. The disadvantage of the short wavelength band is the necessity to measure the reflected intensity at several incident scattering angles in order to gain a large range in momentum transfer. This requires the merging of several two-dimensional (2D) intensity maps of reflected and scattered neutrons measured at different incident

First test results of the online polarized ³He neutron polarization analyzer

At the beginning of July 2008, we installed the online polarized-³He neutron polarization analyzer. We did a first test of the system reported here.

The setup uses a cell called “Barbera”, constructed by Wangchun Chen and Tom Gentile at NIST [8] It has a “thickness” nσl=4.12 at 4.96 Å and was tested to have reached 76% ³He-polarization. In the setup for the reflectometer, the cell was placed in a hot-air oven. The cell was heated to about 200 °C to control the alkali (Rb, K) vapor density in the

Acknowledgment

This Research at Oak Ridge National Laboratory's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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Published by Elsevier B.V.

Highlights from the magnetism reflectometer at the SNS

Abstract

Introduction

Section snippets

Polarizer

Magnetic off-specular scattering: measurement of the two-dimensional maps

First test results of the online polarized 3He neutron polarization analyzer

Acknowledgment

Phys. Rev. Lett.

J. Nanosci. Nanotechnol.

J. Phys. Condens. Matter

J. Physique, collection SFN

IEEE Trans. Magn.

First test results of the online polarized ³He neutron polarization analyzer