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Optimizing the energy product of exchange-coupled soft/hard Zn0.2Fe2.8O4/SrFe12O19magnets

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Permanent magnets based on ferrites are currently studied as possible alternatives, in several application areas, to rare-earth-based magnets to overcome the barriers of high costs, unavailability, and environmental impact. Their attractiveness lies in the large crystalline anisotropy, ensuring resistance to demagnetization, and the possibility of having their modest saturation magnetization enhanced through exchange-coupling with a compatible soft magnetic material of higher saturation magnetization. Using analytical calculations, a micromagnetic finite element model, and comparison with measurements on a produced sample, the conditions that give the highest possible maximum energy product are determined for ferrite-based exchange-coupled Zn0.2Fe2.8O4/SrFe12O19 soft/hard nanocomposite magnets. Two geometries are considered: A spherical core-shell geometry and a composite granular microstructure. Two sets of material parameters are considered for the granular structure, one from the literature and one obtained by fitting to the measured magnetization data. The results show that it is important to have a well-aligned easy axis of hard grains and that the optimal amount of the soft material depends on the alignment of the hard grains as well as their size, with smaller grains yielding larger (BH)max values. The core-shell model shows that the maximum (BH)max can be strongly enhanced, from ∼40 to ∼60 kJ/m3, by using a hard core diameter of <30 nm and a soft shell thickness of <7 nm. The composite granular structure yields a maximum (BH)max of ∼50 kJ/m3 for a soft volume fraction of 43%.

Original languageEnglish
Article number163904
JournalJournal of Applied Physics
Number of pages11
Publication statusPublished - Oct 2022

Bibliographical note

Funding Information:
This study was supported financially by the Innovation Fund Denmark (Project “MAGFLY-Novel Magnets for Flywheel Energy Storage,” No. 7046-00015B), and the Department of Mechanical and Production Engineering at Aarhus University, Denmark. We would like to thank Søren Dahl and Jakob Weiland Høj from Topsoe A/S for providing the SrFe12O19 sample. Aref Hasen Mamakhel is acknowledged for help with acquiring electron microscopy images and data. Affiliation with Center for Integrated Materials Research (iMAT) at Aarhus University is gratefully acknowledged.

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