Effect of rare earth elements substitution in FePt-C granular films on magnetic properties and nanostructure

Investigation of nanostructure, magnetic properties, and blocking temperature (Tb) of typical FePt-C granular films substituted with rare earth (RE) elements such as Gd, Tb, and Ho is reported. The granular films consist of well separated magnetic clusters with carbon cluster boundary material, having average magnetic cluster size changes from 6.8 to 6.3, 6.7, and 9.3 nm with Gd, Tb, and Ho substitution, respectively. Based on the in-plane EDX element mapping images, most of the RE elements remain inside the magnetic clusters which is supported by unit cell volume evaluation from XRD. Perpendicular magnetic anisotropy of the film (Ku⊥film) is reduced with substitution of Gd, Tb, and Ho from 1.6 to 0.4, 0.6 and 1.2×107 erg/cm3, respectively. The reduction of Ku⊥film is likely attributable to the reduction in degree of order. Interestingly, saturation magnetization of the granular films (Msfilm) is almost constant at around 500 emu/cm3 with substitution of any RE elements. Tb of the FePt-C granular films substituted with Gd, Tb, and Ho is reduced from 690 to 620, 630, and 661 K, respectively. These results suggest that the substitution of RE elements such as Gd, Tb, and Ho to FePt-C granular films will decrease Tb while maintaining Msfilm, however will lead to a reduction in Ku⊥film. It is likely that the reduction in Ku⊥film in this study is due to the substitution of RE elements to both Fe and Pt elements. Therefore, we propose substituting RE elements only to Pt element in future studies.