Structural and magnetic properties of annealed FePt/Ag/FePt thin films

摘要

Layer stacks of FePt(15 nm)/Ag(x)/FePt(15 nm) with various Ag interlayer thicknesses (x = 0,3, and 30 nm) were deposited at room temperature onto SiO_2(100nm)/Si(001) substrates. These films were post-annealed in a temperature range of 100℃-900℃ for 30s under high vacuum condition. The influence of the Ag interlayer on the Ll_O(FePt)_(FCT) phase formation and its related structural and magnetic properties were investigated. The onset of chemical ordering of FePt single layers is observed at annealing temperatures between 600 ℃ and 700 ℃. Increasing the Ag interlayer thickness to 30 nm reduces the ordering temperature of the L1_O(FePt)_(FCT) phase formation by about ～100℃. Higher annealing temperatures lead to a further increase in volume fraction of the ordered L1_O(FePt)_(FCT) phase but also to the diffusion of Ag towards the free surface as probed by secondary neutral mass spectrometry. In addition, due to the limited solvability of Ag into the FePt phase, Ag is expected to be located at the grain boundaries. The increasing volume fraction of the L1_O(FePt)_(FCT) phase is also reflected in the increasing coercivity of the films which reaches values of up to ～19 kOe after annealing at 900 ℃. Furthermore, in this case similar M-H hysteresis loop were recorded when the field is applied in the film plane and perpendicular to it. This isotropic behavior is in agreement with the coexistence of L1_O ordered grains which are randomly oriented.