Origin of ‘in-plane’ and ‘out-of-plane’ magnetic anisotropies in as-deposited and annealed CoFeB ferromagnetic thin films

A detailed comparative Ferromagnetic resonance study of pulsed laser deposited Co₄₀Fe₄₀B₂₀ thin films, before and after annealing, was under taken. The dependence of resonance field (H_res) and peak-to-peak linewidth (ΔH_pp) on film thickness, annealing temperature, and magnetic field orientation is examined. ‘In-plane’ (IP) and ‘out-of-plane’ (OP) angular dependence of the resonance fields, (IP:H_res(ψ); OP:H_res(α)), were measured at T = 150 and 295 K for the as deposited (as-) to annealed (an-) thin film samples to determine IP $( H K ∥ )$ and OP $( H K ⊥ )$ uniaxial anisotropy fields. Variation of H_res(ψ) and H_res(α) on sample geometry demonstrate that the uniaxial magnetic anisotropy is present in as- and an-thin films of Co₄₀Fe₄₀B₂₀. The effective magnetic anisotropy $( K u e f f )$ increases after nanocrystallization in CoFeB films indicates that the exchange interactions are unable to average out the local-magnetocrystalline anisotropy of the nanocrystalline grains and thereby lead to magnetic hardening in the early stages of crystallization.