Magnetism Reflectometer at SNS
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 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 nonmagnetic 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 is 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.
Instrument scientist Valeria Lauter (left) and scientific associate Haile Ambaye at the Magnetism Reflectometer.
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 for studying processes with longer time constants (milliseconds 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 that can be used for (near) surface science experiments. MR is designed for magnetism studies on thin films, and the other instrument, the Liquids Reflectometer, is dedicated to studies of liquid surfaces.
The Magnetism Reflectometer is applicable primarily to studies with thin magnetic films, an increasingly important area of solid-state physics. Experiments could also benefit engineering, metallurgy, or biological problems. Instrument capabilities allow, for example, studies of magnetic recording media and magnetic sensors, as well as depth-dependent studies of structural/magnetic nanoparticles or domains. The instrument's unique capabilities provide for multilength-scale experiments, and it has sufficient beam intensity for detailed structural/magnetic phase-diagram determinations.