Ferromagnetic resonance damping mechanisms in CoFeB thin films with Cr substitution

The substitution of iron atoms by chromium in CoFeB films has been proposed to reduce the critical current density ${J}_c$ for current-driven magnetization switching in spin transfer torque devices by reducing its magnetization. Yet the effect of Cr on the magnetic damping has remained elusive. Ferromagnetic resonance (FMR) measurements up to 110 GHz and 5 T, as well as angle-dependent FMR measurements at 9.5 and 17 GHz, are used to discriminate between different mechanisms of FMR damping in ${\mathrm{Co}}₄₀{\mathrm{Fe}}_{40\ensuremath{-}x}{\mathrm{Cr}}_x{\mathrm{B}}₂₀$ thin films ($x=0$, 4, 8, and 18). A constant isotropic Gilbert parameter is used to phenomenologically describe the magnetization damping for each composition, increasing from 0.005 to 0.022 as the amount of Cr increases from 0% to 18%. Two thirds of this contribution appear to be accounted for by intrinsic spin-orbit damping, and the balance is mostly due to extrinsic two-magnon scattering. While the introduction of Cr increases the spin-orbit damping, it nevertheless reduces the corresponding magnetic relaxation rate. The amorphous films with a high scattering rate exhibit a so-called conductivitylike damping at room temperature, in agreement with the breathing Fermi surface model. The presence of Cr also seems to inhibit internal fluctuations of the magnetization, reducing the two-magnon damping. The reduction of ${J}_c$ due to lower magnetization is partly mitigated by the overall increase in magnetic damping.