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Synthesis of strontium ferrite/iron oxide exchange coupled nano-powders with improved energy product for rare earth free permanent magnet applications

We present a simple, scalable synthesis route for producing exchange coupled soft/hard magnetic composite powder that outperforms pure soft and hard phase constituents. Importantly, the composites are iron oxide based (SrFe12O19 and Fe3O4) and contain no rare earth or precious metals. The two step synthesis process consists of first precipitating an iron oxide/hydroxide precursor directly on top of SrFe12O19 nano-flakes, ensuring a very fine degree of mixing between the hard and the soft magnetic phases. The second step involves the reduction of the precursor to create the appropriate soft magnetic phase and create the intimate interface necessary for exchange coupling. We establish a clear processing window; at temperatures below this window the desired soft phase is not produced, while the use of higher temperatures results in deleterious reaction at the soft/hard phase interfaces, causing an improper ratio of soft to hard phases. Improvements in Mr, Ms, and (BH)max are 42%, 29% and 37%, respectively, in the SrFe12O19/Fe3O4 composite compared to the pure hard phase (SrFe12O19). We provide evidence of coupling (exchange spring behavior) with hysteresis curves, first order reversal curve (FORC) analysis and recoil measurements.