Magnetism at the nanoscale
The Magnetism Reflectometer is an advanced polarized neutron reflectometer dedicated for studies of magnetism and structure in nano-system samples. This instrument is used in nano-science and spintronics investigations at versatile experimental conditions of magnetic and/or electric fields, light, and temperature applied in-situ to nanosystems from a few atomic monolayers thick to complicated prototype device-like systems with multiple buried interfaces between different materials. This is possible due to the unique combination of the time-of-flight technique, the optimized parameters of the Magnetism Reflectometer, and the advanced custom sample environment.
The Magnetism Reflectometer (MR) is optimized for grazing incidence geometry experiments, namely reflectometry with off-specular scattering, GISANS and high-angle diffraction studies. Due to availability of the highly polarized beam and polarization analysis, the instrument is used for investigation of magnetic thin films, superlattices, surfaces and nanostructures. The combination of the high-power SNS and the use of advanced neutron optics allow off-specular scattering studies of in-plane structures with polarized neutrons and polarization analysis. Today, even at the world's most advanced neutron sources, such experimental conditions are rare. The availability of polarized neutrons and polarization analysis allows MR to be used also 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 for magnetic studies with auxiliary equipment. Neutrons that are reflected/scattered/diffracted by the sample are counted by a two-dimensional multidetector at a 2.5 m distance from the sample position. 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 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.
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.
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.