Coercivity enhancement of strontium hexaferrite nano-crystallites through morphology controlled annealing

• Published in: Materialia Vol. 4; pp. 203 - 210
• Main Authors: Gjørup, Frederik Holm, Saura-Múzquiz, Matilde, Ahlburg, Jakob Voldum, Andersen, Henrik Lyder, Christensen, Mogens
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-12-2018
• Subjects: Coercivity; Ferrite; Nanoparticles; Stable single-domain; X-ray diffraction (XRD); Nanoparticles; X-ray diffraction (XRD); Ferrite; Stable single-domain; Coercivity
• ISSN: 2589-1529; 2589-1529
• DOI: 10.1016/j.mtla.2018.09.017

In order to improve the performance of ferrite based permanent magnet materials, the evolution of magnetic properties with increasing particle size of anisotropic SrFe12O19 nano-platelets is investigated, allowing elucidation of the stable single domain (SSD) size. Phase-pure, ultra-thin platelet-shaped strontium hexaferrite (SrFe12O19) nano-crystallites have been successfully synthesized using a simple, green and scalable supercritical hydrothermal flow method. The flow reactor precursor is prepared from aqueous solutions of strontium nitrate, iron nitrate and sodium hydroxide. The samples were subsequently annealed at different temperatures (600–1100 °C) in intervals between 1 and 68 h in order to tune size and morphology. The nano-platelets grow anisotropically, with pronounced growth along the c-axis, which coincides with the magnetic easy axis. The annealing treatment gives rise to variations in platelet aspect ratio. Microstructural investigations by transmission electron microscopy (TEM) and Rietveld refinement of powder X-ray diffraction (PXRD) data, combined with vibrating sample magnetometry (VSM), reveal a significant correlation between size and morphology, on the macroscopic magnetic properties of the produced powders. The optimal single-domain crystallite dimensions are found from PXRD to be 62 nm thick and 76 nm wide, attaining a coercivity (Hc) of 459(10) kA/m (5.77 kOe), and a saturation magnetization (Ms) of 74.0(1) Am2/kg. A theoretical single domain critical size for SrFe12O19 particles is proposed, based on a disc-shaped ellipsoidal particle model in combination with the obtained experimental results. Particle shape is revealed to have an enormous influence on the SSD size, varying significantly for anisotropic particles compared to isotropic particles, and thus underlying the need for meticulous characterization in order to optimize the magnetic performance of specific systems.
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