Systematic growth of a soft-magnet Co shell (0.6 to 2.7 nm thick) around a hard-magnet Fe0.65Pt0.35 core (5 nm in diameter) has been achieved in a one-pot microwave synthesis. This controlled growth led to a 4-fold enhancement in the energy product of the core–shell assembly as compared to the energy product of bare Fe0.65Pt0.35 nanoparticles. The simultaneous enhancement of coercivity and saturation moment reflects the onset of theoretically predicted exchange-spring behavior. The demonstration of nanoscale exchange-spring magnets can lead to improved high-performance magnet design for energy applications.