Abstract: A one-pot microwave synthesis of Fe0.65Pt0.35@Co allows systematic growth of the soft-magnet Co shell (0.6 nm to 2.7 nm thick) around the hard-magnet Fe0.65Pt0.35 core (5 nm in diameter). Controlled growth leads to a four-fold enhancement in energy product of the core-shell assembly as compared to the energy product of bare Fe0.65Pt0.35 cores. The simultaneous enhancement of coercivity and saturation moment reflects the onset of theoretically predicted exchange spring behavior. The demonstration of nanoscale exchange-spring magnets will result in improved high-performance magnet design for energy applications.