Evidence for perpendicular anisotropy gradients in Co thin films on Pt seeds

Tailoring the magnetization dynamics and anisotropy in ferromagnetic thin films by different seed layers is of great fundamental and practical importance, e.g., different seed layer materials lead to different microstructure and magnetocrystalline anisotropy energy. We have used Ta and Pt as seed layers for thin Co films and studied their broadband ferromagnetic resonance (FMR) in out-of-plane saturation as a function of Co thickness and determined the respective perpendicular magnetic anisotropy. For Ta seeds, the magnetic anisotropy decreases and shows an inverse thickness dependence. In contrast for Pt seeds the magnetic anisotropy is no longer monotonous with thickness, but shows an initial thickness dependent decrease with a distinct minimum and subsequently a steady increase again. XRD measurements show that for Pt seeds, the Co develops a well-defined hcp texture with a thickness dependent strain relaxation. As a result of this structural evolution for Co on Pt seeds our FMR measurements reveal a strong anisotropy gradient in growth direction for this system.