Thickness dependent magneto-static and magneto-dynamic properties of CoFeB thin films

The authors report thickness dependent magneto-static and magneto-dynamic properties of amorphous Co₄₀Fe₄₀B₂₀ films with thickness, t = 10–200 nm deposited on thermally oxidized Si substrates using magnetron sputtering technique at ambient temperature. Magnetic hysteresis loops of films with t ≤ 20 nm transformed from rectangular or flat type shape to transcritical nature for t ≥ 50 nm. This is attributed to a change in the magnetic domain structure from in-plane magnetization with uniaxial anisotropy in thinner films to stripe domains with effective magnetic anisotropy in thicker films. Magnetization dynamics in the films has been investigated using ferromagnetic resonance spectra. Out-of-plane angular dependent resonance field (H_r) and linewidth (ΔH) data have been interpreted using the Smit–Beljers formula. It is found that ΔH contains large extrinsic contribution with the major one being two-magnon scattering. An inverse scaling of Gilbert damping constant (α) with the film thickness over the entire thickness range of 10–200 nm is observed with the lowest α value of 0.004. The results reveal a cost effective strategy for developing films for spintronics based microwave devices and spin-transfer-torque based magnetic devices.