In situ polarization cell installed on Magnetism Reflectometer
The neutron optics are shown in blue, the coil & shield in gray, and the laser optics in in tan.
Polarized neutron scattering is a powerful tool for studying the structure of magnetic materials and soft matter. Polarized reflectrometry is used to explore spintronic and magnetronic structures and periodic magnetic arrays such as nanodots, stripes, and islands at submicrometer scale. The complicated distribution of the magnetization vectors in those systems during the magnetization reversal and at remanence creates spin-flip specular reflection and off-specular scattering of the neutron beam. Polarization analysis is needed for the reconstruction of the depth profile and the lateral distribution of the magnetization vectors.
A 3He polarization cell was installed and tested successfully on the Magnetism Reflectometer at the Spallation Neutron Source. The cell is used for analysis of the polarization of the reflected and scattered neutron beam. The cell is polarized by in situ laser pumping, which keeps it at a constant polarization. The entire assembly can be moved into and out of the beam vertically without disrupting the polarization. The analyzing efficiency of the cell is from 95% at 2 Å to 99% from 3 to 5 Å.
An experiment performed with full polarization analysis on a reference sample of Fe/Cr multilayer with giant magnetoresistance and a strong magnetic off-specular scattering shows a high quality of the analyzed intensity for the four spin states. The next step is to modify the software to enable automatic reverse polarization.
The combination of a 2-dimensional position-sensitive detector, the high polarization of the neutron beam, and low background allows users to measure magnetic off-specular scattering to extract more information than is possible with unpolarized scattering.