Phase formation and magnetic properties of nanocomposite Nd-Fe-B adjusted by small amount of Dy and Co

In order to improve and stabilize the magnetic properties of nanocomposite Nd₂Fe₁₄B/α-Fe magnetic alloys by a compositional adjustment, small amount of Dy and/or Co was added to Nd₉Fe₈₄B₇ alloys. DTA analysis on the amorphous Nd_9-xDy_xFe₈₄B₇ (x=0~1.5) and Nd₉Fe_84-yCo_yB₇ (y=0~3.0) both melt spun at 40 m · s^-1 revealed that crystallization of the alloys took place as the soft magnetic phases were crystallized, and then the hard magnetic Nd₂Fe₁₄B was precipitated from them. While α-Fe and a metastable 1:7 (TbCu₇-type) phase were formed simultaneously in Dy and Co-free alloys, they were crystallized separately at different temperatures after Dy or Co was added. This phase separation occurred more clearly in the Dy-treated alloys and the other soft magnetic phase Fe₃B was also stabilized by Dy and/or Co. The 1:7 phase that was stabilized by Dy and/or Co was not eliminated at 700°C, decreasing magnetic properties of the alloys. It was eventually disappeared above 725°C, but Fe₃B was not eliminated even at 750°C. when Dy was added more than 0.5 at% or Co was added more than 2.0 at%. Amount of Nd₂Fe₁₄B in the alloys tended to increase as Dy addition increased, whereas Co addition did not lead to any appreciable change in the ratio of α-Fe and Nd₂Fe₁₄B. Moreover, Dy addition apparently increased coercivity of an alloy while Co addition had a beneficial effect on remanence. The grains in the Dy-treated alloys were usually finer than those in the Co-treated alloys. The grain size of both α-Fe and Nd₂Fe₁₄B in the alloys exhibiting mr ≥ 0.72 was in the range of 20~40 nm or even larger 50 nm, which is larger than the theoretical optimum size (~10 nm). Typical magnetic properties obtained from a Nd_7.5Dy_1.5 Fe_82.5Co_1.5B₇ alloy annealed for 12 min at 725°C were _iH_c= 4.85 kOe, B_r=11.32 kG, (BH)_max=15.73 MGOe, and m_r=0.73.