Presentation Information
[P2-64]Tailoring the morphology and magnetic properties of Strontium Hexaferrite (SFO) nanoparticles by successive cation substitution of Ca, Al and Mn for Rare Earth Free Permanent Magnet Applications
*Durgamadhab Mishra1, Nishtha Vats1, Sampat Raj Vadera1 (1. IIT Jodhpur (India))
Keywords:
rare earth free magnet,stronium hexaferrite,Ca,Mn and Al cation substitution
Hexaferrite materials with their unique hexagonal crystal structure allow moderate cation substitution to tailor the structure, morphology and magnetic properties for various applications, including permanent magnets. Particularly, cation substituted strontium hexaferrite (SFO) has the potential to replace rare earth magnets in some applications as a “gap magnet” due to its chemical stability, high resistivity and low cost. However, it is challenging to enhance their coercivity and saturation magnetization simultaneously only through morphology variation or through single/double cation substitutions. Recent work indicates that Al substitution leads to huge enhancement in coercivity, albeit with drastic reduction in magnetization. Partial replacement of nonmagnetic Al with magnetic cation is promising. The double and triple cation substitution could affect the structure, morphology (ideally single domain) and resultant magnetic properties based on the Wyckoff sites being occupied. Another challenge is hematite as a secondary phase appears in many cases and the sample requires high temperature sintering above 1100 °C to obtain the phase purity. Here, we present a novel sol gel synthesis of SFO nanoparticles using reduced molar ratio of 9.23:1 instead of usual 12:1 between Fe/Sr. This allows us to produce pure hexaferrite phase SFO nanoparticles in the size range of 100 nm without any hematite at low calcination temperature of 900 °C. We followed these optimized conditions to investigate the impact of cation substitution on the structure, morphology and magnetic properties. Two series of samples were prepared as described here. In the first series, Ca substitution was carried out at the Sr site. Based on the structural and magnetic analysis of samples Sr1-xCaxFe12O19 (x = 0, 0.1, 0.2, 0.3, 0.4) the optimum value corresponding to x = 0.2 was obtained. In the next series, Al and Mn substitutions were carried out systematically in the series Sr0.8Ca0.2Fe10Al2-yMnyO19 (y = 0, 0.5, 1, 1.5, 2). For low Mn substitution up to y = 1, the pure hexaferrite phases were obtained, whereas at higher Mn substitution hematite appeared as a secondary phase. This fascinating behaviour was attributed to the mixed valency of Mn, which transforms from Mn2+ to Mn3+ with increasing substitution. These transformations were confirmed through Raman, FTIR and electrical resistance measurements. The Mn substituted sample with y > 1 exhibited higher electrical conductivity compared to all other samples due to the increase in electron hopping in the mixed valency state. This mixed valency behaviour had direct bearing on the magnetic properties as well. The magnetization showed an identical behaviour having maximum saturation magnetization of 48.63 emu g-1 for y = 1, since Mn2+ contributes 5 µB per ion compared to 4 µB for Mn3+. The coercivity (Hc) showed a monotonic decrease with increasing Mn substitution having maximum Hc of 12.06 kOe for y = 0 (no Mn) and minimum of 7.29 kOe for y = 2 (no Al), whereas the optimized sample Sr0.8Ca0.2Fe10Al1Mn1O19 had a Hc value of 10.38 kOe. The enhanced Hc could be attributed to intrinsic parameter such as ionic radii and bond length as well as extrinsic parameters such as particle size and size distribution. Incorporation of Mn in place of Al increases the bond length and hence effective anisotropy. The morphology analysis showed that y =1 substitution has the lowest particle size of 90 nm with small size distribution having anisotropic shape. Moreover, it showed a narrow switching field distribution having a maximum effective anisotropy constant of 2.57×105 Jm-3 compared to other samples.We demonstrate a facile route to obtain Ca, Al and Mn substituted SFO nanoparticles with enhanced coercivity and saturation magnetization. A smaller Mn substitution results into phase pure hexaferrites with anisotropic nanoparticles having smallest particle size that results into optimized magnetic properties. Higher Mn substitution leads to precipitation of hematite as a secondary phase along with reduced magnetization and coercivity due to the existence of multi-valance state of Mn2+ and Mn3+.
