I assume you refer to figure 2 of that paper. I don't know how exactly they visualized this data, but I can tell you how to obtain it. I attach some simple example input for such a Cr layer that I once used for some tutorial. This is not converged with respect to parameters. With that input you first have to obtain a self-consistent density. Then change the iplot parameter in the inp.xml file to 2 and run FLEUR again. This will generate several XCrySDen structure output files containing data on the density, the magnetization density, the direction of the magnetization and so on. In detail you obtain the files denIn_A1.xsf denIn_A2.xsf, denIn_A3.xsf which contain the magnetization in the three directions, denIn_Aabs.xsf for the absolute value of the magnetization, denIn_Aphi.xsf for the in-plane magnetization direction in units of pi, and denIn_Atheta.xsf for the out-of-plane magnetization direction. The visualized data from figure 2 is available in the denIn_Aphi.xsf (for the arrows) and denIn_Aabs.xsf (for the gray-level coding) files. I attach an image with a separate visualization of the data in two plots. The direction here is color-coded on a linear scale and the absolute value of the magnetization density on a logarithmic scale. These are simple visualizations that you can obtain with XCrySDen. The figure in the paper is probably hand-crafted and not just output from some visualization tool. To create a similar plot you might want to look at the Python library matplotlib or even try to create something with Povray or a similar tool allowing a lot of user control. Interpreting the data in an xsf file is not too difficult. This is a simple and well-documented text format and thus directly readable.