Imaging framework for identifying Three-Dimensional Magnetic Structures using Hard X-Rays

Nanoscale electronic and structural heterogeneities are prevalent in condensed matter physics. Their imaging in 3 dimensions has become an important task for the understanding of material properties. To unravel the connection between nanoscale heterogeneity and macroscopic emergent properties, polarization dependent resonant X-ray spectroscopy and scattering methods have been developed [1]. The experiments require scanning the sample with a coherent beam of focused X-rays with left and right circular polarized X-rays in a nano ptycho-tomographic setup. One key issue is that jitter and misalignment of the rotation axis caused by vibration or drift in the experimental setup lead to misalignment between reconstructed ptychographic images for different tomographic projections or between two circular polarization states. In order to correct this, we develop an image registration and reconstruction framework that can align both left and right circular polarized projection images while reconstructing the magnetic vector tomographically. Experiments were carried out using coherent X-rays at the Velociprobe instrument [2] at the Advanced Photon Source (APS). In the experiment, circularly polarized coherent hard X-rays at photon energies corresponding to the L₂ absorption edge of Nd are incident on a cylindrical shaped Nd₂Fe₁₄B single crystal. With projection data at 2 different sample tilts, the full 3D magnetization vector field is reconstructed without any prior knowledge or assumptions.