Thermal stability and magnetization reversal mechanism in granular L1 FePt thin films

Abstract

The relationship between coercivity HC and magneto-crystalline anisotropy field HK of L10 FePt granular alloy thin films is investigated as a function of film thickness in the range of 3.5–12.5 nm. While HK exhibits a decrease from 82 kOe to 71 kOe with increasing film thickness, HC displays a pronounced peak at a critical film thickness of tCR ≅ 7 nm. In order to explain the non-monotonic behavior of HC as a function of film thickness, the time dependence of HC at ambient temperature (TRT = 300 K) and the temperature dependence of the AC susceptibility in the range TRT – 800 K are measured as a function of film thickness and interpreted in the frame of the Stoner–Wohlfarth model of coherent rotations. It is demonstrated that the HC decrease with increasing film thickness above tCR is a consequence of a transition from coherent to an incoherent magnetization reversal mechanism in isolated grains. For a 7 nm thick film (tCR), the average grain size of ∼7.4 nm is comparable with the film thickness, suggesting that the domain-wall (DW) width δ ≅ tCR. Previous theoretical work has demonstrated a strong dependence of δ on the orientation of the DW with respect to the (001) planes of an L10 FePt lattice. By using the values of the micromagnetic exchange coupling A theoretically evaluated for parallel and vertical DW orientation with respect to the (001) planes, one obtains δ = 5.2 nm for parallel and δ = 6.7 nm for vertical DWs. The latter is closer to the experimental value of δ, suggesting that the nucleation of vertical DWs inside the grains (probably at grain boundaries) is the dominant mechanism responsible for the incoherent magnetization reversal evidenced in the investigated films.