Recoil hysteresis of Sm–Co∕Fe exchange-spring bilayers

The exchange-spring behavior found in Sm–Co (20nm)∕Fe epitaxial bilayer films was investigated by analyzing major hysteresis and recoil curves as a function of anneal conditions. The hard layer consists of nanocrystalline intermetallic Sm–Co hexagonal phases (majority phase Sm2Co7 with SmCo3 and SmCo5). Recoil curves, obtained from the successive removal to remanence and reapplication of an increasingly negative field from the major demagnetization curve, reveal the reversible and irreversible components of the magnetization. The Sm–Co thickness was fixed at 20nm while the Fe thicknesses of 10 and 20nm were studied, with ex situ annealing carried out in evacuated, sealed silica tubes at different temperatures. The peak in the recoil curve area is associated with the coercivity of the hard phase. The development of the soft component magnetization is revealed by the departure of the recoil area from zero with application of a reverse field. These two features together confirm that annealing stabilizes the 10nm Fe bilayer sample against local magnetic reversal while it weakens the 20nm bilayer sample. Furthermore, in both its as-deposited and annealed states the Sm–Co∕Fe bilayer of 10nm Fe thickness always displays a higher exchange field and smaller recoil loop areas than the bilayer of 20nm Fe thickness, consistent with a stronger exchange response and more reversible magnetization in the former.