Effect of annealing time on the exchange coupling interactions and microstruc-ture of nanocomposite Pr_7.5Dy₁Fe₇₁Co₁₅Nb₁B_4.5 ribbons

The influence of annealing time on the magnetic properties and microstructure of nanocomposite Pr_7.5Dy₁Fe₇₁Co₁₅Nb₁B_4.5 ribbons was systematically investigated by the methods of vibrating sample magnetometer (VSM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Interaction domains derived from strong exchange coupling interactions between hard and soft magnetic grains were imaged using magnetic force microscopy (MFM). Maximum remanence, intrinsic coercivity, and maximum energy product values were obtained in the ribbons annealed at 700℃ for 15 min, which were composed of Pr₂(Fe, Co)₁₄B, α-(Fe, Co), and slight Pr₂(Fe, CO)₁₇ phases. Although J_r, H_ci, and (BH)_max decreased gradually with further increase of annealing time, it is emphasized that comparatively high J_r and H_ci and (BH)_max were obtained in a wide annealing time period of 15 to 360 min. The shape of initial magnetization curves and hysteresis loops change as a function of annealing time, indicating different magnetization reversal routes, which can be fully explained by the corresponding microstructure.