  Nanoscale structural and electronic heterogeneities are prevalent in
condensed matter physics. Investigating these heterogeneities in three
dimensions (3D) has become an important task for understanding their material
properties. To provide a tool to unravel the connection between nanoscale
heterogeneity and macroscopic emergent properties in magnetic materials,
scanning transmission X-ray microscopy (STXM) is combined with X-ray magnetic
circular dichroism (XMCD). A vector tomography algorithm has been developed to
reconstruct the full 3D magnetic vector field without any prior assumptions or
knowledge. 2D STXM projections of single crystalline \ndfeb\ pillars are
recorded for two different sample tilt angles while rotating around the
vertical tomographic axis using $120$ nm X-ray beams with left and right
circular polarization. Image alignment and iterative registration has been
implemented, based on the 2D STXM projections for the two tilts. Dichroic
projections obtained from difference images are used for the tomographic
reconstruction to obtain the 3D magnetization distribution at the nanoscale.
