Neutron Depolarization Microscope for Imaging of Ferromagnetic Phase Transitions: Ni₃Al and HgCr₂Se₄ under pressure

We performed spatially resolved neutron depolarization imaging of a large Ni₃Al crystal, and a small HgCr₂Se₄ spinel under pressure, to probe bulk magnetic inhomogeneities in the ferromagnetic phase and the transition temperature with the spatial resolution of 100 μm. To obtain such resolution, we employed a new technique, a neutron microscope based on image-forming Wolter optics and a focusing guide. The depolarization images of Ni₃Al show that the sample doesn't homogeneously go through the ferromagnetic transition; the improved resolution allows us to identify previously unidentified high-T_C regions. The results on HgCr₂Se₄ highlight the advantage of this technique especially for complex sample environments such as pressure cells. The improved resolution allows to image domain formation in the sample while decreasing the acquisition time. The novel optical design that enabled acquisition of the high spatial resolution neutron depolarization images is described in detail and image results are compared to a conventional radiography setup without a lens.