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 is iron oxide based (SrFe12O19 and Fe3O4) and contain no rare earth or precious metal. 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. We then use a second step that serves to reduce the precursor to create the proper 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 higher temperatures result in deleterious reaction at the soft/hard phase interfaces, causing an improper ratio of soft to hard phases. Improvements of Mr, Ms, and (BH)max are 42%, 29% and 37% respectively in the SrFe12O19/Fe3O4 composite compared to pure hard phase (SrFe12O19). We provide evidence of coupling (exchange spring behavior) with hysteresis curves, first order reversal curve (FORC) analysis and recoil measurements. 
A. D. Volodchenkov, Y. Kodera and J. E. Garay
cond-mat.mtrl-sci