High coercivity in Sm(FeT)/sub 12/ type magnets

Synthesis of mesoscopic particles of multi-component rare earth permanent magnet compounds

“…NdFe Firstly, the size and shape of RT 12 MMPs have not been well optimized, especially in the sub-micrometer range, as shown in Figure 2b [40][41][42][43][44][45][46][47][48][49][50]111,112,124] anisotropic bulk magnets with a large ( ) max , which will be reported in the forthcoming paper. Taking these potentials into consideration, synthetic prospects will move a step forward in the size control through kinetics, which is advanced by the use of encapsulated nanostructure as precursors, to obtain their particle size in the range below 1 m.…”

“…3/2 , reaches the maximum at the single-domain critical size ( sd ) given by sd ≈ 72√ ex 1 μ 0 s 2 ⁄ where ex is the exchange stiffness and 1 is the first anisotropy constant, and then decreases as c ~ 1 n ⁄ , provided that the grain has a strong cubic anisotropy [33][34][35][36]. The grainsize dependent coercivities of representative R 2 T 17 and RT 12 compounds are named a few and shown in Figure 2 [37][38][39][40][41][42][43][44][45][46][47][48][49][50]. Owing to the phase-stabilization challenges, control over the microstructure of R-T multielement materials is still a non-trivial task, though the R-T permanent magnetic materials have been established since 1960s [51] and the Nd 2 Fe 14 B compound has been utilized since its discovery in 1984 [52][53][54][55].…”

“…In contrast, the nonequilibrium processes are appropriate for synthesizing not only metastable compounds but also the intermetallics with desired crystal structures free from impurities and a fine grain microstructure [61]. The most typical method is first to form the amorphous phase, followed by annealing at an appropriate temperature [14,[40][41][42][43][44][45][46][47][48][49][50].…”

“…In contrast, the nonequilibrium processes are appropriate for synthesizing not only metastable compounds but also the intermetallics with desired crystal structures free from impurities and a fine grain microstructure [ 61 ]. The most typical method is first to form the amorphous phase, followed by annealing at an appropriate temperature [ 14 , 40–50 ]. The annealing evolves the formation of metastable phases, which can be produced at various extreme nonequilibrium conditions, and the dynamical transformation between the metastable and the equilibrium phases, corresponding to the local free-energy minima.…”

“…For most of the R–T compounds, the metastable CaCu 5 -type phases may form in a narrow temperature range and, thus, are hardly observed experimentally. The formation of the CaCu 5 -type phases can be observed by careful annealing with a very slow heating rate for a short time [ 46 ], whereas the metastable TbCu 7 -type phases form in a sufficiently wide temperature range to be observed in various synthetic processes [ 14 , 40 , 43 ]. In practice, the metastable compounds usually crystallize at annealing temperatures slightly higher than the crystallization temperature of the amorphous phase, and subsequently can be dynamically transformed into more stable compounds at higher annealing temperatures [ 61 ].…”