Critical Fe thickness for effective coercivity reduction in FePt/Fe exchange-coupled bilayer

FePt/Fe perpendicular exchange-coupled bilayers with different Fe thicknesses were prepared to study the exchange coupling effect and the magnetization switching mechanism. An Fe thickness of 3 nm was found to be the critical point where the coercivity reduction became saturated and had the largest thermal stability gain factor of 2.25. This thickness was close to the exchange length between the magnetically hard and soft layers. Within the exchange length the soft phase strongly coupled to the hard phase and the magnetization of the bilayer processed single switching; beyond the exchange length reversible magnetization increased with the Fe thickness and exchange spring effect was found. Our simulation results also revealed that the exchange length was the critical Fe thickness for effective coercivity reduction and for maintaining high remanence.