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

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 4; no. 24; pp. 5593 - 561
• Main Authors: Volodchenkov, A. D, Kodera, Y, Garay, J. E
• Format: Journal Article
• Language: English
• Published: 01.01.2016
• Subjects: Coupling; Exchange; Iron oxides; Joining; Nanostructure; Precursors; Rare earth metals; Synthesis
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c6tc01300g

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 (SrFe 12 O 19 and Fe 3 O 4 ) 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 SrFe 12 O 19 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 M r , M s , and (BH) max are 42%, 29% and 37%, respectively, in the SrFe 12 O 19 /Fe 3 O 4 composite compared to the pure hard phase (SrFe 12 O 19 ). We provide evidence of coupling (exchange spring behavior) with hysteresis curves, first order reversal curve (FORC) analysis and recoil measurements. We present a simple, scalable synthesis route for producing exchange coupled soft/hard magnetic composite powder that outperforms pure soft and hard phase constituents.