Effects of HDDR treatment conditions on magnetic properties of Nd-Fe-B anisotropic powders

It is reported that Nd-Fe-B magnetic powders prepared by utilizing the HDDR (hydrogenation, disproportionation, desorption, and recombination) phenomena exhibit high coercivity, and the addition of Co, Ga, and Zr induces magnetic anisotropy in these powders. HDDR phenomena are caused by the heat treatment in hydrogen (H treatment) and subsequently in vacuum (V treatment). Present works describe the effects of V-treatment conditions on magnetic properties of Nd _12.6Fe_bal.Co_xB_6.0 (x=0-17.4) alloys. The powders V-treated at lower temperature show lower remanence and no noticeable magnetic anisotropy. V treatment at higher temperature enhances remanence and increases the differences of remanence between the powders aligned with and without magnetic field. Higher values are obtained in Co added alloys. This result suggests that a selective grain growth of Nd₂Fe ₁₄B grains during V treatment plays an important role for the inducement of magnetic anisotropy in HDDR-treated powders. The temperature for complete recombination reaction under evacuating condition decreases with increasing Co content. It can be said that Co addition enhances recombination reaction and results in acceleration of grain growth.