Magnetic properties of chromium-doped Ni₈₀Fe₂₀ thin films

This paper investigates the properties of thin films of chromium-doped Ni₈₀Fe₂₀ (Permalloy) that could potentially be useful in future low-power magnetic memory technologies. The addition of chromium reduces the saturation magnetization, M_s, which is useful for low-energy switching, but does not significantly degrade the excellent switching properties of the host material even down to 10 K, the lowest temperature measured, in films as thin as 2.5 nm. As an example, an alloy film composed of 15% chromium and 85% Ni₈₀Fe₂₀ has an M_s just over half that of pure Ni₈₀Fe₂₀, with a coercivity H_c less than 4 Oe, an anisotropy field H_k less than 1 Oe, and an easy-axis remanent squareness M_r/M_s of 0.9 (where M_r is the remanent magnetization). Magnetodynamical measurements using a pulsed inductive microwave magnetometer showed that the average Landau Lifshitz damping λ was relatively constant with changing Cr content, but increased significantly for thinner films (λ ≈150 MHz for 11 nm, λ ≈250 MHz for 2.5 nm), and at low bias fields likely due to increased magnetic dispersion. Density functional theory calculations show that chromium reduces M_s by entering the lattice antiferromagnetically; it also increases scattering in the majority spin channel, while adding almost insignificant scattering to the minority channel.