Synthesis and magnetic properties of bulk α″-Fe16N2/SrAl2Fe10O19 composite magnets

•α″-Fe16N2/SrAl2Fe10O19 composite magnets were synthesized for the first time.•α″-Fe16N2 enhances saturation magnetization, but lowers coercivity.•High milling energy deteriorates magnetic properties in SrAl2Fe10O19.•Poor thermal stability of α″-Fe16N2 limits the consolidation temperature.•Oxidation of α″-Fe16N2 and partial density of compacts hinder exchange coupling. In the quest for hard-soft composite magnets, we present here the interesting case of mixing iron nitride and Sr-hexaferrite, where the uniaxial anisotropy of the ‘soft’ phase is larger than that of the hard phase. We have synthesized bulk hard-soft composite magnets from Al-doped Sr-hexaferrite SrAl2Fe10O19 sub-micron- and iron nitride α″-Fe16N2 nano-particles. Both powders have been mixed via ball milling, followed by compaction at low temperatures. The resultant microstructure and magnetic properties are investigated. Higher iron nitride fraction results in increased saturation magnetization, but lower coercivity. Remanence can be slightly enhanced, from 22.4 A·m2/kg in the initial ferrite powder to 26.4 A·m2/kg for 15 wt% α″-Fe16N2. However, the squareness is deteriorated significantly. Microstructural investigations reveal low density, high porosity and partial oxidation of the α″-Fe16N2 phase which are the reasons for the poor coupling. Micromagnetic modelling is used to unravel the decisive role of different microstructural features on the coercivity and energy product of the composites.