(color online) Schematic of the exchange-spring principle from Ref. [205]. A magnetically hard phase possessing a wide hysteresis loop exchange couples to a magnetically soft phase possessing a tall hysteresis loop, forming a composite magnet that has both a wide and tall hysteresis loop. The image to the right illustrates a bilayer structure. If the soft layer thickness exceeds a critical size, the magnetization reversal happens first in the soft layer.