# High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ under pressure

## TL;DR

This study introduces a high-resolution neutron depolarization microscopy technique using Wolter mirrors, revealing magnetic inhomogeneities and domain formation in ferromagnetic materials under pressure with enhanced spatial resolution.

## Contribution

The paper presents a novel neutron microscopy method with ~100 μm resolution, enabling detailed imaging of ferromagnetic transitions and inhomogeneities in small samples under pressure.

## Key findings

- Identified non-uniform ferromagnetic transition in Ni3Al due to grain distribution.
- Demonstrated imaging of magnetic domains in HgCr2Se4 under pressure.
- Achieved faster imaging with high spatial resolution despite sample environment constraints.

## Abstract

We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed bulk magnetic inhomogeneities in the ferromagnetic transition temperature with spatial resolution of about 100~$\mu$m. To obtain such spatial resolution, we employed a novel neutron microscope equipped with Wolter mirrors as a neutron image-forming lens and a focusing neutron guide as a neutron condenser lens. The images of Ni$_3$Al show that the sample does not homogeneously go through the ferromagnetic transition; the improved resolution allowed us to identify a distribution of small grains with slightly off-stoichiometric composition. Additionally, neutron depolarization imaging experiments on the chrome spinel, HgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the new technique especially for small samples or sample environments with restricted sample space. The improved spatial resolution enables one to observe domain formation in the sample while decreasing the acquisition time despite having a bulky pressure cell in the beam.