Abstract:
In the last years, much effort has again been devoted to the research of ferrite-based permanent magnets due to the so-called rare-earth crisis.[1],[2] In particular, a quest to enhance ferrites' BHmax, is still underway.[3] Large BHmax values are found in magnets combining substantial magnetisation at remanence (Mr) with high coercivity. Both parameters are influenced by materials properties, such as crystalline and shape anisotropy and particle' size.
Here, the influence of composition, particle size, sintering conditions, and exposure to the external magnetic field before compaction on microstructure and consequently, magnetic properties of strontium ferrite (SFO)-based hybrid composites will be presented.
Powders' mixtures consisted of commercial SFO powder consisting of micron-sized, isotropic particles, or hydrothermally (HT) synthesised SFO with hexagonally-shaped platelets with a diameter of 1 micron and thickness up to 90 nm, and a soft magnetic phase in various ratios. Powders were sintered with spark plasma sintering (SPS) furnace. Starting powders and hybrid magnets were examined by means of phase composition (XRD) and microstructure (TEM, SEM). Their magnetic properties were evaluated with vibrating sample magnetometer (VSM), permeameter and by single-point-detection (SPD) measurements.
Depending on the concentration and composition of the soft phase, the MR of the composite can be altered. Application of the external magnetic field before the consolidation induces the anisotropy in commercial, and HT synthesised SFO, leading to the increase in the Mr of hybrid magnets [4]. Moreover, sintering with SPS promotes the alignment of HT synthesised SFO particles in the direction of the applied pressure, which is also the direction of SFOs' easy axis. Thus the enhancement in MR is perceived leading to the Mr/Ms higher than 0.8. Besides, after SPS, almost no grain growth was observed, which is beneficial for exploiting advantages of nanosized-induced phenomena also in bulk sintered samples.
This work received financial support from the European Commission through the project AMPHIBIAN (H2020-NMBP-2016-720853).
References:[1] J. M. D. Coey, Permanent magnets: Plugging the gap, Scr. Mater., vol. 67, no. 6, pp. 524-529, 2012.
[2] M. Stingaciu, M. Topole, P. McGuiness, and M. Christensen, Magnetic properties of ball-milled SrFe12O19 particles consolidated by Spark-Plasma Sintering, Sci. Rep., vol. 5, p. 14112, 2015.
[3] R. Skomski and J. M. D. Coey, Magnetic anisotropy: How much is enough for a permanent magnet? Scr. Mater., vol. 112, pp. 3-8, 2016.
[4] P. Jenuš et al., Ferrite-Based Exchange-Coupled Hard-Soft Magnets Fabricated by Spark Plasma Sintering, J. Am. Ceram. Soc., vol. 8, no. 37805, p. n/a-n/a, 2016
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