Exchange-spring permanent magnet particles of composition Pr6.93Fe85.7B5.95Nb1.39 in at. % were prepared by spark-erosion in liquid argon. The best exchange-spring properties were achieved after vacuum annealing the as-sparked powders at 700 °C for ∼5 min, and did not change significantly for anneals up to 120 min. This behavior suggests that the equilibrium dimensions of the “hard” and “soft” phases were determined by the sizes of the spark-eroded particles in which they developed. The Pr2Fe14B phase developed as spheres, likely single crystals, in an α-Fe+Fe2B matrix. © 2003 American Institute of Physics.