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Synthesis and Magnetic Properties of Exchange-Coupled BaFe12O19@Fe3O4 Nanocomposite
Keywords: Exchange-spring , Rare-earth free, Permanent magnets
The recent rare-earth element price crisis has prompted intensive research in the development of rare-earth free permanent magnets for the European and US industry [1]. Since 1989, when the exchange coupling effect between the soft and hard magnetic phases was first observed, much work has been done to produce hard/soft exchange- spring magnets using various preparation techniques. From the start of the 21st century, the bottom-up approaches, using magnetic nanoparticles, have become the field of interest to produce exchange-spring nanocomposite magnets. However, a challenge for bulk nanostructured magnet synthesis is maintaining the homogeneous nanoscale morphology [2]. Hexaferrites with hexagonal structure, first discovered in the 1950s, are a type of technologically important materials which constitute more than 50% of the global consumption of magnetic materials. Hexagonal barium ferrites (BaFe12O19), also known as M- type barium hexaferrite (BaM), are magnetically hard metal-oxides with the coercivity of 2-3.2 kOe and highly resistive to oxidation and corrosion [3]. Due to their low cost and impressive magnetic and microwave properties in numerous electronic devices, many researchers have been motivated to develop high-performance BaM based exchange-coupled magnets in recent years [4-6]. The present investigation explores the exchange-coupling mechanism and shows the correlation between the structural, compositional and microstructural changes, and magnetic properties of BaM hexaferrite and Fe3O4 nanocomposites. In order to get magnetic exchange interactions between the hard and soft phase, which occurs within typically nanometer size regions, flake-like BaM nanoparticles were synthesized using a sol-gel auto- combustion method and then coated with magnetite nanoparticles using a hydrothermal method. In order to prepare flake-like BaM@Fe3O4 core- shell nanocomposite, BaM nanoparticles were mixed with the ferrous/ferric solution in an autoclave. Sodium hydroxide solution was then added drop by drop to the mixture under stirring in an argon atmosphere to precipitate magnetite nanoparticles on the heterogeneous surface of the BaM nanoparticles. The exchange-spring mechanism was then explored and optimized via the control of composite phase fractions and heat treatment conditions. In this study the flake-like BaM nanoparticles with an average particle size of about 300 nm and the ultra- thin thickness (<20 nm) were achieved after annealing at 1100 C for 5 h, to obtain optimal coercivity and saturation magnetization. Electron microscopy results show that uniform magnetite nanoparticles with a diameter of about 10 nm were developed as the shell of flake-like BaM nanoparticles by using a mixture of ferric chloride hexahydrate (FeCl3 . 6H2O) and Iron(II) lactate hydrate (C6H10FeO6 . XH2O) as the ferric ion sources. We observed the exchange coupling between the hard and the soft magnetic phases after post heat treatments at 300, 400, 500 and 600 C for 2 hours. The obtained flake-like BaM@Fe3O4 nanocomposite exhibits a saturation magnetization of 60 emu/g and a coercivity of 1.2 kOe at room temperature for the sample with BaM to magnetite weight ratio of 1:1. Our results improve on recent reports [4,5]. We discuss the effect of hard to soft phase ratios on the exchange coupling behavior of the achieved nanocomposite in order to optimize and develop a high-performance rare-earth free permanent magnet.
Farzin Mohseni,
Department of Materials and Ceramic Engineering, Department of Physics, CICECO Aveiro Institute of Materials, University of Aveiro
Aveiro, Aveiro


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