Isotropic bulk nanocomposite permanent magnets were produced with strontium hexaferrite, SrO·6FeO, and magnetite, Fe O, as the magnetically hard and soft components. A novels synthesis scheme based on the partial reduction of SrO·6Fe O was employed. In two parallel experiments, nano- and microcrystalline SrO·6FeO particles were compacted into pellets along with a controlled, understoichiometric amount of potato starch as a reducing agent. The pellets were then sintered in a passive atmosphere. Based on XRD and room temperature magnetic hysteresis measurements, it was concluded that a fraction of the SrO·6FeO input material had been reduced into FeO. In comparison with pure SrO·6FeO control pellets, these composites exhibited maximum energy product increases in excess of 5 % due to remanence boosting. The improvement of magnetic properties was attributed to an efficient exchange spring coupling between the magnetic phases. Interestingly, as the synthesis scheme also worked for microcrystalline SrO·6Fe O, the method could presumably be adapted to yield crystallographically oriented bulk nanocomposite magnets. © Owned by the authors, published by EDP Sciences, 2014.