a Study of R(2) IRON(14) Alpha-Iron Nanocomposite Permanent Magnets.
Abstract
The magnetic and structural properties of melt -spun nanocomposite rm R_6Fe_ {87}Nb_1B_6 and rm R_{3.5}Fe_{91}Nb _2B_{3.5} magnets (R=Nd, Pr, Dy, and Tb) consisting of a mixture of soft magnetic alpha-Fe and hard magnetic rm R_2Fe_{14}B phases have been investigated using calorimetry, x-ray diffractometry, magnetometry, and electron microscopy. The as-spun samples of the boron-rich composition were amorphous or non-crystalline in structure while the ribbons of low-boron alloys contained an alpha -Fe component. Crystallization studies on these ribbons revealed at least two exothermic peaks. In rm R_6Fe_{87}Nb_1B _6 samples with R=Nd, Pr, and Dy the first exotherm corresponds to the formation of metastable rm Y_3Fe_{62}B_{14 }-type structure along with alpha -Fe. The corresponding metastable phase in rm Tb_6Fe_{87}Nb_1B _6 samples was identified as TbCu _7-type. Upon crystallization, the low-boron ribbon samples consisted of predominantly alpha -Fe, and no other metastable phase was detected by the techniques used in this study. All the samples transformed into the expected mixture of rm R_2Fe_{14}B + alpha -Fe phases when annealed at higher temperatures, and the transformation temperature was found to strongly depend on the rare-earth element. Smooth loops, concealing the individual hysteresis behavior of the constituent phases of the composite magnet, were observed. High values of remanent magnetization (110 to 145 emu/g), reduced remanences, rm M_ {r}/M_{s}, in the range 0.6-0.8, and modest room-temperature coercivities of 2.7 -4.5 kOe with energy products in the range of 10-14 MGOe were obtained. The microstructure of coercivity optimized samples reveals a uniform mixture of rm R _2Fe_{14}B and alpha-Fe phases with grain sizes in the 30 -50 nm range. The presence of high reduced remanences (greater than 0.5) in the isotropic samples and the greater reversibility observed in the demagnetization curves are the result of exchange coupling among the 2:14:1 and alpha -Fe grains.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1995
- Bibcode:
- 1995PhDT.......136W
- Keywords:
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- Physics: Condensed Matter; Engineering: Materials Science