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 Fe-57 Mossbauer measurements. Cr content dependence of magnetic properties and phase composition of Cr-added Nd-4.5(Fet-xCrx)(77)B-18.5 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 Nr(4.5)(Fe1-xCrx)(77)Br-18.5 optimally boat-treated ribbon increases, bur remanence decreases. In all the specimens, hard magnet Nd3Fe14B 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 alpha-Fe depending an Cr content. Hyperfine magnetic field of Fe-B compound decreases as a function of Cr content, while those of Nd2Fe14B and alpha-Fe are independent of Cr content. Therefore, Cr atoms mainly exist in Fe-B compound, not in Nd2Fe14B and alpha-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.