Hysteresis behavior and microstructure of exchange coupled R/sub 2/Fe/sub 14/B/sub 1///spl alpha/-Fe magnets

The hysteresis behavior of nanocomposite R/sub 2/Fe/sub 14/B//spl alpha/-Fe magnets with R=Nd and Pr was investigated at low temperatures. The room temperature coercivity depends on annealing with a peak value at 700/spl deg/C. The low coercivity of samples annealed at temperatures below the optimum is due to incomplete transformation of R/sub 3/Fe/sub 62/B/sub 14/ to R/sub 2/Fe/sub 14/B phase. Hysteresis loops of Pr/sub 2/Fe/sub 14/B//spl alpha/-Fe samples change from a smooth to a constricted loop when temperature is varied from 300 to 10 K. This may be due to the decoupling of a fraction of the larger 2:14:1 grains which reverse independently of the rest of the sample.