Effect of high-magnetic-field annealing on the magnetic properties of Nd-Co-B nanocomposite ribbons

In this work, the effect of high magnetic fields applied during the annealing process in the as-spun ribbons of nanocomposite magnets is studied. Ribbon samples of Nd₁₂Co₇₉NbB₈ are prepared by a single-roller melt-spinning method. The annealing temperature T_a is selected between 675°C and 825°C while the duration is fixed to be 10 min. X-ray diffraction (XRD) patterns at room temperature reveal the coexistence of mainly hard magnetic Nd₂Co₁₄B and soft magnetic fcc-Co phases. For T_a=675°C, demagnetization curves exhibit a double step behavior, suggesting that the soft phase is not properly exchange-coupled with the hard phase. Magnetic-field annealing at T_a which is smaller than Curie temperature T_C (∼730°C) of Nd₂Co₁₄B phase gives rise to higher volume fraction of the hard Nd₂Co₁₄B phase and hence the higher coercivity H_c values. Since the amorphous Nd-Co-B phase is considered to be paramagnetic at T_a, the magnetization M_amorphous is very small even in the high field, as compared with the ferromagnetic magnetization M_crystal for the crystalline Nd₂Co₁₄B phase.