Magnetism Reflectometer
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The Magnetism Reflectometer (MR) is designed mainly for reflectometry and high-angle diffraction studies of magnetic thin films, superlattices, and surfaces. The combination of the high-power SNS neutron source and the use of advanced neutron optics allow off-specular diffraction studies of in-plane structures. Today, even at the world's most advanced neutron sources, such experiments are extremely difficult to perform. The availability of polarized neutrons and polarization analysis allows MR to be used for specific studies of non-magnetic thin film samples. Examples for the latter case include contrast variation, incoherent background reduction and phase determination for direct inversion of reflectivity data into real-space scattering-length density profiles.
Neutrons from the moderator are guided to the sample position at a distance of 18.5 m via a combination of a short channel beam bender and a tapered neutron guide. A horizontal scattering plane has been chosen to allow more convenient operation of the instrument and auxiliary equipment. Neutrons that are reflected/diffracted by the sample are counted by a two-dimensional multidetector at a 20 m distance from the moderator. Polarizing neutron optical elements (polarizer, analyzer and spin flipper) determine the spin-state before and after scattering by the sample. The wavelength is determined by time of flight. The instrument is designed for 60 Hz operation. Bandwidth choppers restrict the total bandwidth of neutrons that are incident onto the sample to 3.5 Å when the instrument is collecting data in every time frame. The highest intensity will be available in the second frame (3.5 to 7.0 Å); however, by changing the phasing of the bandwidth choppers, the available 3.5 Å bandwidth can be shifted to either smaller or higher neutron wavelengths. Additionally, by changing the speed of the choppers, pseudo 30 Hz (or lower) operation can be realized with correspondingly wider wavelength bandwidth.
Besides the conventional use of a neutron reflectometer to analyze the structural and magnetic depth profiles of thin film structures, complementary kinetic studies (for example, probing the dynamic behavior of magnetic domains) are feasible. The capability of "time-tagging" pulses over a broad Q-range to study processes with longer time constants (miliseconds to hours) is unique to the time-of-flight method and is being explored. Furthermore, studies of phonons and magnons at interfaces could be possible.
MR is one of two SNS neutron reflectometry instruments to be used for (near) surface science experiments. MR is designed for magnetism studies on thin films, and the other, the Liquids Reflectometer, is dedicated to studies of liquid surfaces.
