Search

The significance of neutron spin adiabaticity in the data analysis of polarized Grazing Incidence Small Angle Neutron Scattering (GISANS) is discussed, with the aim of minimizing the number of simulation parameters of complex magnetic models within the neutron scattering cross-section. We illustrate how an estimate of the neutron polarization direction and adiabaticity can be obtained by magnetic field and neutron ray-tracing simulations and compare the results with measurements of the magnetic field map for the beamline used in the GISANS experiment. We show how small deviations from the neutron polarization direction with respect to the magnetic field vector at the sample position significantly affect the data analysis, and how this can be considered within the magnetic model of the scattering cross section using the Distorted Wave Born Approximation.

Polarized neutrons will be made available on many European Spallation Source (ESS) instruments. There are a number of technologies available for polarizers and polarization analyzers which will be used at the ESS. The selection of the technology for an instrument is based on the performance and the constraints of the instrument. We will focus on the design of polarizing supermirror devices using Monte Carlo ray tracing simulation as an integral part of instrument design process. A McStas module has been developed to simulate a multichannel V-cavity polarizer, seeking the appropriate parameters to be incorporated into the respective instrument. The performance of such polarizers is studied for three instruments at ESS (MIRACLES (backscattering spectrometer), BIFROST (indirect geometry spectrometer) and ODIN (imaging)) with different requirements and constraints, where the suitability of this kind of devices can be assessed. For the first two instruments, where there is no strong constraint on the placement of the polarizer, the optimal configurations show excellent performance over the whole required wavelength ranges. However, in ODIN, due to more strict constraints in the placement of the polarizer, the performance is more dependent on the wavelength in the required wavelength range and other options may need to be considered.

Polarized neutron imaging brings the great advantage of analyzing bulk magnetic properties with good spatial resolution. The technique is based on the interaction of the neutron spin with magnetic samples or free magnetic fields and observing the changes to a spin-polarized neutron beam. The high sensitivity to even small magnetic fields is a benefit in obtaining magnetization information but simultaneously a challenge in instrumentation, since magnetic environments for the polarized neutron beam and for the sample, as well as the fringe field from the magnetic sample itself all affect the measurement and can give rise to unwanted effects. We have used finite element simulations and ray tracing simulations, to design and analyze a magnetic sample environment devised for the measurement of ferromagnetic metal sheets. Here we show an analysis of performance of the experimental setup based on the simulation results and compare them to first experimental results on a grain oriented silicon steel sample.

To meet the ever-increasing user demand, eleven of the fifteen European Spallation Source (ESS) instruments under construction aim to offer polarised neutrons for user experiments. They include an imaging instrument, a SANS instruments, two reflectometers, three diffractometers, and four spectrometers. In conjunction with in-kind contributions and instrumentation grants, the ESS Polarisation Project will support the incorporation of polarisation analysis on eight of the eleven instruments. The project aims to deliver polarised neutrons for first-science experiments as instruments enter operation. Different polariser and polarisation analyser techniques will be available to accommodate the specifics of experiments on a given instrument. Polarised 3He neutron spin filter using either Metastable Optical Pumping (MEOP) or Spin-Exchange Optical Pumping (SEOP) techniques will provide shared-use equipment among many instruments, with SEOP’s main application being in situ beam-polarisation. Several instruments will also use polarising-supermirror devices. To provide wide-bandwidth spin-flipping capability to the time-of-flight instruments, Adiabatic Fast Passage (AFP) neutron spin flippers, also known as gradient-field radiofrequency spin flippers will be the main method of choice. Devices based on the same AFP principle will also be used to flip 3He nuclear spins. We are constructing our first 3He polariser setup, including field coils to produce highly uniform magnetic field. Monte Carlo simulations are being done for the supermirror polarisers. To ensure science-focused development, we are working with university partners in doing scientific experiments with polarised neutrons. These are some of the activities developing polarisation analysis for ESS instruments in our project.

High magnetization materials are in great demand for the fabrication of advanced multifunctional magnetic devices. Notwithstanding this demand, the development of new materials with these attributes has been relatively slow. In this work, we propose a new strategy to achieve high magnetic moments above room temperature. Our material engineering approach invoked the embedding of magnetic nanoclusters in an oxide matrix. By precisely controlling pulsed laser deposition parameters, Co nanoclusters are formed in a 5 at % Co-TiO 2 film. The presence of these nanoclusters was confirmed using transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption fine structure. The film exhibits a very high saturation magnetization of 99 emu/cm 3 . Detailed studies using X-ray magnetic circular dichroism confirm that Co has an enhanced magnetic moment of 3.5 μ B /atom, while the Ti and O also contribute to the magnetic moments. First-principles calculations supported our hypothesis that the metallic Co nanoclusters surrounded by a TiO 2 matrix can exhibit both large spin and orbital moments. Moreover, a quantum confinement effect results in a high Curie temperature for the embedded Co nanoclusters. These findings reveal that 1-2 nm nanoclusters that are quantum confined can exhibit very large magnetic moments above room temperature, representing a promising advance for the design of new high magnetization materials.

Five percent Fe-doped In 2 O 3 films were deposited using a pulsed laser deposition system. X-ray diffraction and transmission electron microscopy analysis show that the films deposited under oxygen partial pressures of 10 -3 and 10 -5 Torr are uniform without clusters or secondary phases. However, the film deposited under 10 -7 Torr has a Fe-rich phase at the interface. Magnetic measurements demonstrate that the magnetization of the films increases with decreasing oxygen partial pressure. Muon spin relaxation (μSR) analysis indicates that the volume fractions of the ferromagnetic phases in PO 2 = 10 -3 , 10 -5 , and 10 -7 Torr-deposited samples are 23, 49, and 68%, respectively, suggesting that clusters or secondary phases may not be the origin of the ferromagnetism and that the ferromagnetism is not carrier-mediated. We propose that the formation of magnetic bound polarons is the origin of the ferromagnetism. In addition, both μSR and polarized neutron scattering demonstrate that the Fe-rich phase at the interface has a lower magnetization compared to the uniformly distributed phases.