Nanoengineered High-Performance Hexaferrite Magnets by Morphology-Induced Alignment of Tailored Nanoplatelets

Abstract

Magnetic materials are ubiquitous in electric devices and motors making them indispensable for modern-day society. The hexaferrites currently constitute the most widely used permanent magnets (PMs), accounting for 85% (by weight) of the global sales of PMs. This work presents a complete bottom-up nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite PMs with a record-high (BH)max for dry-processed ferrites. The procedure includes the supercritical hydrothermal flow synthesis of anisotropic magnetic-single-domain strontium hexaferrite (SrFe12O19) nanocrystallites of various sizes, and their subsequent compaction into bulk magnets by spark plasma sintering (SPS). Interestingly, Rietveld modeling of neutron powder diffraction data reveals a significant difference between the magnetic structure of the thinnest nanoplatelets and the bulk compound, indicating the Sr-containing atomic layer to be the termination layer. Subsequently, high-density SrFe12O19 magnets (>95% of the theoretical density) are produced by SPS of the flow-synthesized nanoplatelets. Texture analysis by X-ray pole figure measurements demonstrates how the anisotropic shape of the nanoplatelets causes a self-induced alignment during SPS, without application of an external magnetic field. The self-induced texture is accompanied by crystallite growth along the magnetic easy-axis, i.e., the thickness of the platelets, resulting in high-performance PMs with square hysteresis curves and (BH)max of 30 kJ/m3. The (BH)max is further enhanced by annealing, reaching 36 kJ/m3 after 4 h at 850 °C, which exceeds the (BH)max of the highest grade of dry-processed commercial ferrites worldwide.

Boosting the coercivity of SrFe12O19 nanocrystalline powders obtained using the citrate combustion synthesis method

“…In turn, the specimens prepared at 1000 ºC (red and blue colors) show the lowest HC (<5800 Oe), while all the samples synthesized in the range 800 -900 ºC (black, orange and green colors in Figure 2a-b) display a HC 6000 Oe. This is likely explained by the fact that higher temperatures lead to an increase in particle size which is known to reduce the HC in the multidomain magnetic region [25][26][27][28]. In particular, the sequence 300 -900 ºC yields to HC ~6350 Oe, which is one of the highest value reported for SFO nanocrystalline powders synthesized by a citrate combustion [14].…”

Optimization of magnetic properties in fast consolidated SrFe12O19nanocrystallites

“…In a recent study by Saura-Múzquiz et al it was demonstrated, how the initial size and shape of the nanocrystalline powder was hugely affecting the texture. 40 Neutron powder diffraction was also used to infer information about the magnetic structure at the atomic scale. Magnetic interactions between the crystallites, as well as the role of grain boundaries on the evolution of H c and M r is complicated 41 and have not been addressed in this work.…”

An ab initio study of the magnetic properties of strontium hexaferrite

“…Still, important properties such as the electronic gap have not yet been determined. In the last 2 decades, significant effort has been devoted to improve its magnetic performance, particularly for permanent magnet applications, by chemical substitution and nanostructuring [3][4][5][6][7] . Nanometric thin SFO platelets have been recently characterized combining fundamental experimental techniques, as X-ray absortion based and Mössbauer spectroscopies, to density functional theory (DFT) calculations intended to rationalize Oxygen K absortion edge spectrum measurements 8 .…”