Measurements of Gilbert damping parameter α for CoPt-based and CoFe-based films for magnetic recording applications

Abstract

Time-resolved MOKE (TRMOKE) and Q-band ferromagnetic resonance (FMR) are used to evaluate the damping parameter α and the magnetic anisotropy field (where K is the uniaxial anisotropy constant and is the saturation magnetization) of CoPtX–Y and CoPtX (where X is non-magnetic element and Y denotes oxide(s)) granular alloys with perpendicular magnetic anisotropy and variable Co/Pt atomic ratio R. It is found that α monotonically increases in the range of 0.03–0.07 with decreasing R in the range of 2.5–5.5. By comparing the CoPtX films sputter-deposited in Ar and in Ar+O2 atmosphere, it is found that α is less sensitive than HK to the presence of non-magnetic additives (Cr, B) in the grain core. This suggests that α is much less sensitive than K to perturbations of d-electron band structure. FMR and TRMOKE are also used to evaluate α of magnetically soft CoFeX alloys. It is shown that adding 2 at.% Tb to CoFeX leads to an increase of α from 0.012 (undoped case) to 0.048, and a negative shift of the resonance field which is linearly dependent on the RF frequency. This effect, similar to that previously reported on Tb-doped NiFe films, is attributed to the highly anisotropic Tb 4f–5d exchange coupling. A synthetic-antiferromagnetic trilayer CoFeX (7 nm)/Ru (0.8 nm)/CoFeX (7 nm) is investigated by FMR. The interlayer RKKY coupling constant, evaluated from the frequency shift between the acoustic (symmetric) and optic (antisymmetric) resonance modes is JEX = 0.11 erg cm−2. The α value corresponding to the acoustic mode (0.016) matches that of a single-layer CoFeX film whereas the optic mode shows a higher α (0.0265). This is interpreted to be a consequence of spin-pumping through the Ru interlayer. The evaluated spin mixing conductance for the CoFeX/Ru interface is = 22.5 nm−2.