Magnetic-field-dependent small-angle neutron scattering (SANS) will be employed for studying the spin microstructure of rare-earth-free Mn-based permanent magnets. This class of materials is currently heavily investigated due to its potential to fill the gap between high-performance Nd-Fe-B and low-performance Ferrite-type magnets. Most investigations employ, however, integral methods (e.g., magnetometry) in order to correlate the macroscopic magnetic properties to the details of the microstructure (e.g., grain size and shape). In this proposal we will use for the first time the SANS method to scrutinize the material on the mesoscopic length scale (about 1-300 nm), which is an important size regime for the realization of the relevant properties such as the coercivity and maximum energy product. We believe to make an important contribution to the understanding of the microstructure-property relationship in Mn-based magnets, in this way supporting the development of novel functional materials.