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

Magnetic properties and phase composition of Nd4.5Fe77B18.5 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 57Fe Mössbauer measurements. Cr content dependence of magnetic properties and phase composition of Cr-added Nd4.5(Fe1−xCrx)77B18.5 has also been investigated. By annealing up to 853 K, Nd4.5Fe77B18.5 amorphous ribbon crystallizes in both t-Fe3B and Nd2Fe23B3. These two compounds are soft magnets and the specimen does not have coercivity. Hard magnetic Nd2Fe14B is precipitated by annealing up to 943 K, and the specimen becomes hard magnetic. Nd4.5Fe77B18.5 nanocrystalline composite magnet annealed up to 943 K consists of 10 mole% Nd2Fe14B and 90 mol% t-Fe3B, and the small amount of hard magnetic phase results in large coercivity of this material. With increasing Cr content, coercivity of Nd4.5(Fe1−xCrx)77B18.5 optimally heat-treated ribbon increases, but remanence decreases. In all the specimens, hard magnet Nd2Fe14B 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-Fe3B to Fe2B and α-Fe depending on Cr content. Hyperfine magnetic field of Fe–B compound decreases as a function of Cr content, while those of Nd2Fe14B and α-Fe are independent of Cr content. Therefore, Cr atoms mainly exist in Fe–B compound, not in Nd2Fe14B 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.