Fe 64 B 22.8 Nd 6.6 Y 3.9 Nb 2.7 bulk nanocomposite magnets with improved size and magnetic properties
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The Fe64B22.8Nd6.6Y3.9Nb2.7 nanocomposite permanent magnets in the form of rods of 2 mm in diameter and 25 mm in length have been prepared by annealing the amorphous precursors. The phase evolution, microstructure, and magnetic properties of Fe64B22.8Nd6.6Y3.9Nb2.7 nanocomposite permanent magnets have been investigated by x-ray diffractometry, transmission electron microscopy, and magnetometry techniques. The exchange coupling between the magnetically soft and hard magnetic phase is evidenced by the dM curves. The hard magnetic properties of the nanocomposites were found to be sensitive to the annealing process. The microstructure of the annealed nanocomposite consists of magnetically soft a-Fe (15–25 nm) and Fe3B (25–35 nm) grains and hard magnetic Nd2Fe14B (45–55 nm) grains. The optimum hard magnetic properties, such as jHc 5 961.6 kA/m (12.0 kOe), Br 5 0.65 T (6.5 kG), and BHmax 5 65.17 kJ/m3 (8.19 MGOe), were obtained by annealing the alloy at 700 °C for 15 min and are related to the more refined nanostructure leading to strong exchange coupling between the soft and hard magnetic grains. Annealing above 700 °C induces a decoupling effect due to the coarsening of soft and hard magnetic phases.
I. INTRODUCTION
In the last years, there have been constant efforts in developing high-performance isotropic nanocomposite permanent magnets with unique microstructure and magnetic properties.1–4 Recently, considerable work has been dedicated to the increase of the glass forming ability (GFA) and the enhancement of the mechanical and magnetic properties of (Nd,Pr)3–12–Febal–M–B6–30; (M 5 Co, Mo, Nb, Ti, V, and Zr) alloy systems.5–7 The synthesis of permanent magnets through copper mold casting technique was found to be facile, viable, and economical because of the single processing step and less amount of expensive rare earth elements used than in sintered magnets. The conventional way to develop a permanent magnet through copper mold casting is to make the amorphous alloy and then anneal it at 600–700 °C for 10–30 min. In fact, the nonoptimal remanence and energy product restricted the widespread applications of these materials, especially the use of small dimensional rods of 0.5–1.1 mm in diameter. This discrepancy is attributed to the difficulties in obtaining a uniform microstructure, which determines the exchange coupling between the soft and hard magnetic phases. Researchers have adopted two approaches for the development of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.413 J. Mater. Res., Vol. 27, No. 4, Feb 28, 2012
well-performing bulk nanostructured magnets: by the modification of the alloy composition and by the optimization of processing parameters. Recently, the bulk metallic glasses (BMGs) studies have shown that doping of (Nd,Pr)–Fe–B alloys with Nb,8 Y,9 or Zr10 has beneficial effects on the GFA and magnetic properties. Zhang et al.8 reported that after optimal crystallization, the bulk Nd9.5FebalNb4B22.08 alloy shows large intrinsic coercivity of 1098.5 kA
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