High saturation magnetization soft magnetic iron-cobalt-nickel films for GHz applications

In this work, Fe-Co-N alloy films with the thickness of 100 nm with a Permalloy nano-underlayer (<5 nm) have been synthesized and characterized. The Fe-Co-N films have a high saturation magnetization of 2.4 T (24 kG), a low coercivity of less than 80 A/m (1 Oe), a resistivity of 50 μΩ·cm, and an initial permeability of 1000 with a 3dB bandwidth of 1.5 GHz. The combination of high saturation magnetization, low coercivity, and high bandwidth makes the Fe-Co-N thin film a very promising candidate material for magnetic recording heads. An exchange induced ripple reduction mechanism is proposed to explain the observed magnetic softness in the Fe-Co-N films with a Permalloy nano-underlayer. Pulsed inductive magnetometer studies show that the Fe-Co-N films exhibit a ferromagnetic resonance frequency of 2 GHz. The Fe-Co-N films with larger dispersion angles generally show higher damping constants. The susceptibility spectrum of a magnetic thin film is obtained from the Landau-Lifshitz-Gilbert (L-L-G) Equation and is analyzed to obtain the damping criteria for magnetic thin films. The damping criteria for a magnetic thin film with the relative dc susceptibility χ₀ = χ(ω = 0) = M_s/(H_k+ H_appl) are α > 2χ−1/2, over damped; α = 2χ−1/2, critically damped; and 0 < α < 2χ−1/2, under damped. Since the actual damping constants for the magnetic thin films are far less than 2χ−1/2, the magnetic thin films are generally observed to be under-damped. A general method of solving the L-L-G equation and the Landau-Lifshitz (L-L) equation in the frequency domain is proposed, and is used to solve the L-L-G equation and the L-L equation for a magnetic sphere and a magnetic thin film. The different harmonic terms of the magnetization are also predicted for a magnetic sphere and a thin film from the L-L-G equation and the L-L equation.