The α-Fe/R2Fe14B-type exchange-coupled NdFeB nanocomposites with a coercivity (iHc) of more than 11 kOe and energy product, (BH)max, of more than 16 MGOe have been successfully obtained on melt spun (Nd0.95La0.05)7.5+xFebalCr2B10 alloy powders, where x ranged from 3 to 3.5. It was found that a slight substitution of Cr for Fe suppresses the formation of the R2Fe23B3 and Fe3B phases during crystallization and results in the formation of an α-Fe/R2Fe14B mixture. Increasing the total rare earth content was found to enhance the remanence, and the Hic of (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 ribbons was also increased drastically with increasing values of x. A Br of 9.2–9.7 kG, Hic of 11.1–13.2 kOe, and (BH)max of 16.5–18.0 MGOe have been obtained on (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 (x=3–3.5).
The phase evolution and magnetic properties of melt-spun Nd9.5(Fe1−xCox)85.5B5 (x=0, 0.05, 0.1, and 0.15) nanocomposites have been investigated. It was found that Co substitution for Fe, i.e., x=0.05–0.15, improves the Curie temperature (Tc), remanence (Br), and maximum energy product (BH)max of the materials obtained. Co substitution for Fe was found to promote grain coarsening, after thermal processing, in materials with a high Co concentration. Wohlfarth remanence analysis suggests that dilute-Co substitution for Fe increases the strength of the exchange coupling between the soft magnetic phases, α-(Fe, Co) and Nd2(Co, Fe)17, and the 2:14:1 phases. The increase in the saturation magnetization of α-(Fe, Co) and 2:14:1 phase, in conjunction with the enhanced exchanged coupling, presumably, result in an increased (BH)max of Nd9.5(Fe0.95Co0.05)85.5B5 and Nd9.5(Fe0.90Co0.10)85.5B5 samples. Once above a critical Co concentration, for the compositions studied, the strength of exchange coupling interactions between magnetically soft and hard phases does not seem to vary significantly with Co content. Moreover, both the irreversible loss of induction and reversible temperature coefficient of induction (conventionally referred to as α) of fully processed materials decrease with increasing Co concentration.
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