Magnetic Properties of Nd–Fe–B–Cr Nanocrystalline Composite Magnets

Abstract

Magnetic properties and phase composition of Nd 4.5 Fe 77 B 18.5 exchange-coupled nanocrystalline composite magnet have been investigated as a function of heat treatment by means of differential thermal analysis, magnetization, X-ray diffraction, and 57 Fe Mossbauer measurements. Cr content dependence of magnetic properties and phase composition of Cr-added Nd 4.5 (Fe 1-x Cr x ) 77 B 18.5 has also been investigated. By annealing up to 853 K, Nd 4.5 Fe 77 B 18.5 amorphous ribbon crystallizes in both t-Fe 3 B and Nd 2 Fe 23 B 3 . These two compounds are soft magnets and the specimen does not have coercivity. Hard magnetic Nd 2 Fe 14 B is precipitated by annealing up to 943 K, and the specimen becomes hard magnetic. Nd 4.5 Fe 77 B 18.5 nanocrystalline composite magnet annealed up to 943 K consists of 10 mole% Nd 2 Fe 14 B and 90 mol% t-Fe 3 B, and the small amount of hard magnetic phase results in large coercivity of this material. With increasing Cr content, coercivity of Nd 4.5 (Fe 1-x Cr x ) 77 B 18.5 optimally heat-treated ribbon increases, but remanence decreases. In all the specimens, hard magnet Nd 2 Fe 14 B is formed and the amount is found to decrease with increasing Cr content. This is one of the reasons for the reduction of remanence. Soft magnetic phases which crystallize in the specimen changes from t-Fe 3 B to Fe 2 B and α-Fe depending on Cr content. Hyperfine magnetic field of Fe-B compound decreases as a function of Cr content, while those of Nd 2 Fe 14 B and α-Fe are independent of Cr content. Therefore, Cr atoms mainly exist in Fe-B compound, not in Nd 2 Fe 14 B and α-Fe, and reduce the magnetization and Curie temperature of soft magnetic phases. In addition, the total amount of soft magnetic compounds decreases by the Cr-substitution for Fe. These influences of Cr atoms result in the decrease of the remanence of this material.