Multimodal behavior of the dynamic magnetic susceptibility spectrum in amorphous CoZrTaB magnetic thin films

Abstract

The origins of sub-Ferromagnetic Resonance (FMR) multimodal behavior in the magnetic susceptibility spectrum of amorphous CoZrTaB magnetic thin films are investigated using Brown's diffusion model describing continuous diffusion of magnetic spins. Brown's diffusion model is regressed onto experimental data for the amorphous CoZrTaB magnetic thin films with thicknesses spanning 80–530 nm. The mathematical model presented successfully reproduces the thickness dependent dynamic magnetic susceptibility of the amorphous CoZrTaB magnetic thin films with strong statistical significance. The model proposes the formation of additional energy wells in the uniaxial anisotropy energy plane of the material after a critical film thickness. The sub-FMR resonance peaks arise when the frequency of the external excitation field approaches the natural frequency of the well. Furthermore, the additional energy wells in the anisotropy energy plane cause a breakdown in the axial symmetry of the anisotropy energy plane. This breakdown of axial symmetry results in dynamic coupling between the transverse (χ⊥) and longitudinal (χ∥) magnetic susceptibility. This dynamic coupling results in the initial low frequency step-down in the magnetic susceptibility observed in the thicker CoZrTaB magnetic thin films. It is found that the application of an external bias magnetic field along the easy axis of the amorphous CoZrTaB magnetic thin films suppresses the sub-FMR resonance peaks by restoring the axial symmetry of the anisotropy energy plane.