Magnetic Reversal Behaviors of Perpendicular Exchange-Coupled Fe/FePt Bilayer Films

Fe top layers with a thickness 4 between 1 and 15 nm were deposited on textured FePt(001)/Pt(001)/Cr(002) layers to study the associated exchange coupling effect and their switching behaviors. The deposition of a Fe layer does not change the crystalline structure of the FePt layer. Microstructural analysis reveals that the Fe(200) texture is formed epitaxially on the FePt(001) surface. When t(Fe) = 1 nm, the remanent coercivity (H-cr) remains the same as that without a Fe top layer. The lack of reversible magnetization reveals that the Fe layer is thinner than the critical thickness; the strong coupling force throughout the whole Fe layer facilitates the cooperatively magnetic rotation of Fe and FePt layers. Significant exchange spring was observed in the sample with t(Fe) = 3 and 5 nm, reducing H-cr by 14% and 43% (the maximum reduction), respectively. The results are consistent with the theoretical prediction. As t(Fe) was increased up to 8 and 12 nm, the exchange spring effect disappeared. Although the two films exhibited similarly reduced H., values of about 1.5 kOe, the reduction of H-cr is caused by the tilt of the magnetic moments rather than the exchange spring. This incline of the moments is caused by the weakened exchange coupling and the complex domain structure. The magnetic reversal was further examined by measuring the variation in longitudinal and transverse magnetizations during magnetic field sweeping. The results suggest that at t(Fe) = 1 and 3 nm, the reversal occurs cooperatively in a single direction. As t(Fe) is increased to 5 and 8 nm, the moments switch randomly, in a manner that may be related to the increase in the density of the domain wall, observed by MFM. This work demonstrates the feasibility of reducing coercivity through exchange-spring mechanism and provides information on detailed magnetic reversal mechanism in perpendicular exchange coupled FePt.