Towards bi-magnetic nanocomposites as permanent magnets through the optimization of the synthesis and magnetic properties of SrFe₁₂O₁₉ nanocrystallites

Abstract

Sol–gel synthesis was used in order to obtain nanocrystallites of the SrFe12O19 (SFO) hexaferrite in an efficient and reliable way. By optimizing the initial synthetic conditions, we were able to control the size of the nanoparticles (NPs), at lower annealing temperature. The x-ray powder diffraction, transmission electron microscopy (TEM), and magnetic measurements have demonstrated a significant relation between the morphology, size, and magnetic properties of the nanoscale SFO, revealing a definite dependence on the crystallite size along the c-axis. The obtained NPs appear almost isotropic, in the form of platelets and exhibit similar magnetic performance, in terms of the energy product (BH)MAX, thus, demonstrating the suitability of reducing the annealing temperature without any deterioration in the magnetic properties. Additionally, this work illustrates the feasibility of the sol–gel bottom-up approach to employ magnetic NPs as building-blocks for designing hard/soft exchange-coupled bi-magnetic nanocomposites, combining the high coercivity of a hard phase (SFO) and the high saturation magnetization of a soft phase (CoFe2O4); in this regard, we discuss the tunability of the magnetic anisotropy by symbiotically restricting the growth of both phases.

Complex Correlations between Microstructure and Magnetic Behavior in SrFe12O19 Hexaferrite Nanoparticles

“…These results are promising for the application of such line-profile analysis to more complex systems, where several different crystal phases can overlap, without losing the information of respective structures. 6) and 67( 5) nm [6].…”

“…Furthermore, the agreement between the average diameter from Rietveld analysis and Williamson-Hall plot (<DXRD> and <DXRD WH > in Table S1) suggests almost strainfree crystallites, with concomitant absence of structural distortions. The evolution of crystallite size distributions along different orientations focused our attention on the resulting particle size, whose analysis is not straightforward as the samples obtained by sol-gel self-combustion consist of particles´ aggregates with a porous structure 6,9,20 . Therefore, a thorough analysis of such systems is required.…”

“…From the analysis of peak broadening, it is clear that there is a specific dependence of the size along [001] on the synthesis approach, resulting in the formation of platelet-like crystallites for sol-gel synthesized SFO 6,20 . This feature opens the way for the use of such a method in the design of nanocrystallites with preferential shape.…”

“…Among nanostructured materials, M-type hexaferrite SrFe12O19 (SFO) has become extremely appealing as promising candidate material for the renewal of permanent magnets applications 5 . As a matter of fact, over the recent years, an intense research effort has been carried out to tailor SFO-based materials at the nanoscale through several synthetic and processing approaches to optimize size, morphology, as well as magnetic properties [6][7][8] . Moreover, it has received significant attention for the study and development of exchange coupled systems 9,10 .…”

“…On this basis, a correct understanding of the correlation between magnetic volume and physical one is rather important. Therefore, as a model system, we use a set of SFOs prepared by bottom up sol-gel approach and recently investigated 6 . Previous results indicate that the crystallites' size lies in the nanoscale range and that it depends, together with the crystallites' shape, on the employed thermal treatment.…”