Presentation Information
[O12-6]Tailoring the Magnetic Properties of Nanostructured Ce- and Mn-substituted Sr-Hexaferrite
*Adrian Fernandez-Calzado1, Ester M. Palmero1, Darko Makovec2, Alberto Bollero1 (1. IMDEA Nanoscience (Spain), 2. Jožef Stefan Institute (Slovenia))
Keywords:
Strontium hexaferrite,Magnetic properties,Ce substitution,Permanent magnet,Sustainability
As global demand for high-performance permanent magnets grows, the need to enhance the magnetic properties of materials like Sr-hexaferrite (SrFe12O19) has become essential for meeting the evolving needs in a wide range of technological applications (from sensors to electric motors) [1]. An efficient approach to increase the permanent magnet properties is through the combination of micro-/nanostructuring and elemental doping to modify the intrinsic magnetic properties and induce the formation of secondary phases that can have an impact (e.g., pinning) on the extrinsic magnetic properties through an optimized processing. The most studied doping elements to enhance the magnetic performance of these systems are cobalt (Co) and rare-earth elements (REEs). Among them, cerium (Ce) stands out due to its relative abundance, lower cost, and potential to reduce reliance on more expensive elements like neodymium (Nd) and dysprosium (Dy) [2].
In this study, Sr-hexaferrite powder has been synthesized with enhanced magnetic performance by doping with Ce and manganese (Mn) using the ceramic synthesis route. Cobalt was not used. SrCO3, Fe2O3, CeO2 and Mn3O4 were mixed by planetary ball milling. The powders were prepared according to the formula Sr1-xCe3+xFe12-xMn2+xO19 with x = 0, 0.03, 0.06 and 0.1. The dopants were introduced during the crystallization process of the Sr-hexaferrite by controlled thermal treatments after the milling process.
In Sr-hexaferrite, the incorporation of Ce faces challenges due to its higher oxidation state (4+) and larger atomic radius compared to iron (Fe3+). Mn serves as a facilitator, promoting Ce integration into the hexaferrite structure [3]. After crystallization, flash-milling was used to increase the homogeneity of the material and refine the microstructure by application of extremely short milling times (maximum of 45 min) [4, 5]. Due to the high energy associated with this milling process, amorphization of the material occurs over extended milling times, which requires a post-heating process for recrystallization. Heat treatment at different temperatures (1000-1200 ºC) enabled a precise control over morphology, microstructure and phase purity.
Structural and magnetic characterization, including X-ray diffraction (XRD) and room temperature vibrating sample magnetometry (VSM), confirmed significant improvements in coercivity (Hc) for all the different dopant concentrations after processing. Particularly, for the Sr0.94Ce0.06Fe11.94Mn0.06O19 powders it is observed an increase in coercivity from 80 to 500 kA/m after 30 min of flash-milling and recrystallization at 1000 ºC for 1 hour. This process granted a successful particle size refinement by reducing the mean size, starting from a few micrometers to end with tens of nanometers.
The improvement in the magnetic properties obtained in this study are attributed to dopant-induced changes in crystal structure, microstructural refinement and formation of secondary phases, such as hematite (α-Fe2O3) [5]. The present study provides a valuable framework for the development of high-performance hexaferrite materials made of abundant elements and requiring short processing times. This work offers a significant potential for a range of advanced applications and contributes to the sustainable development of a next-generation of permanent magnets.
Acknowledgements Authors acknowledge support from EU through the H2020 PASSENGER project (Ref. 101003914). Authors from IMDEA thank support from MICINN through the RETAIN project (Ref. TED2021-132490B-I00). E.M.P. acknowledges support from AEI through Juan de la Cierva Incorporación program (IJC2020-043011-I/MCIN/AEI/10.13039/501100011033) and EU by NextGeneration EU/PRTR.
Present address (A. Bollero): Advanced Technologies and Micro Systems, Robert Bosch GmbH, 70839 Stuttgart, Germany
References
[1] A. Bollero and E.M. Palmero, "Recent advances in hard-ferrite magnets" in Mod. Perm. Magn., J.J. Croat and J. Ormerod (Eds., Elsevier (2022) pp. 65-112.
[2] M. A. Almessiere et al., Ceram. Int. 44(8), 2018, pp. 9000-9008.
[3] Y.-M. Kang, K.S. Moon, Ceram. Int. 41(10), 2015, pp. 12828-12834.
[4] J. Pedrosa et al., RSC Adv. 6(90), 2016, pp. 87282-87287.
[5] A. Bollero et al., Publication number: WO/2018/211121. Int. Application nº: PCT/EP2018/063222, 2018.
In this study, Sr-hexaferrite powder has been synthesized with enhanced magnetic performance by doping with Ce and manganese (Mn) using the ceramic synthesis route. Cobalt was not used. SrCO3, Fe2O3, CeO2 and Mn3O4 were mixed by planetary ball milling. The powders were prepared according to the formula Sr1-xCe3+xFe12-xMn2+xO19 with x = 0, 0.03, 0.06 and 0.1. The dopants were introduced during the crystallization process of the Sr-hexaferrite by controlled thermal treatments after the milling process.
In Sr-hexaferrite, the incorporation of Ce faces challenges due to its higher oxidation state (4+) and larger atomic radius compared to iron (Fe3+). Mn serves as a facilitator, promoting Ce integration into the hexaferrite structure [3]. After crystallization, flash-milling was used to increase the homogeneity of the material and refine the microstructure by application of extremely short milling times (maximum of 45 min) [4, 5]. Due to the high energy associated with this milling process, amorphization of the material occurs over extended milling times, which requires a post-heating process for recrystallization. Heat treatment at different temperatures (1000-1200 ºC) enabled a precise control over morphology, microstructure and phase purity.
Structural and magnetic characterization, including X-ray diffraction (XRD) and room temperature vibrating sample magnetometry (VSM), confirmed significant improvements in coercivity (Hc) for all the different dopant concentrations after processing. Particularly, for the Sr0.94Ce0.06Fe11.94Mn0.06O19 powders it is observed an increase in coercivity from 80 to 500 kA/m after 30 min of flash-milling and recrystallization at 1000 ºC for 1 hour. This process granted a successful particle size refinement by reducing the mean size, starting from a few micrometers to end with tens of nanometers.
The improvement in the magnetic properties obtained in this study are attributed to dopant-induced changes in crystal structure, microstructural refinement and formation of secondary phases, such as hematite (α-Fe2O3) [5]. The present study provides a valuable framework for the development of high-performance hexaferrite materials made of abundant elements and requiring short processing times. This work offers a significant potential for a range of advanced applications and contributes to the sustainable development of a next-generation of permanent magnets.
Acknowledgements Authors acknowledge support from EU through the H2020 PASSENGER project (Ref. 101003914). Authors from IMDEA thank support from MICINN through the RETAIN project (Ref. TED2021-132490B-I00). E.M.P. acknowledges support from AEI through Juan de la Cierva Incorporación program (IJC2020-043011-I/MCIN/AEI/10.13039/501100011033) and EU by NextGeneration EU/PRTR.
Present address (A. Bollero): Advanced Technologies and Micro Systems, Robert Bosch GmbH, 70839 Stuttgart, Germany
References
[1] A. Bollero and E.M. Palmero, "Recent advances in hard-ferrite magnets" in Mod. Perm. Magn., J.J. Croat and J. Ormerod (Eds., Elsevier (2022) pp. 65-112.
[2] M. A. Almessiere et al., Ceram. Int. 44(8), 2018, pp. 9000-9008.
[3] Y.-M. Kang, K.S. Moon, Ceram. Int. 41(10), 2015, pp. 12828-12834.
[4] J. Pedrosa et al., RSC Adv. 6(90), 2016, pp. 87282-87287.
[5] A. Bollero et al., Publication number: WO/2018/211121. Int. Application nº: PCT/EP2018/063222, 2018.
