Effect of surface morphology on magnetization dynamics of cobalt ultrathin films: An in-situ investigation

Growth of Co film on SiO2/Si substrates with surface roughness of 0.5 nm and 1.6 nm have been studied in-situ using magneto-optical Kerr effect (MOKE) and transport measurements. In-situ measurements jointly suggest that the films grow via Volmer-Weber growth process and proceed via a nonmagnetic, superparamagnetic and a ferromagnetic phase formation on both the substrates. Islands are found to coalesce at film thicknesses ~ 0.6 nm and at ~ 1.5 nm with continuous film formation around film thickness ~1.5 nm and ~ 3.0 nm for smooth and rough substrates, respectively. Ferromagnetic long-range ordering i.e., appearance of magnetic hysteresis loop in both films is observed just after coalesce stage. Observed azimuthal angular dependence of coercivity confirmed the presence of a weak uniaxial magnetic anisotropy (UMA) in both the films, whereas difference in UMA with substrate roughness is interpreted in terms of combined effect of domain wall pinning and internal stresses in the films. Origin of much higher UMA in case of the Co film deposited on ripple patterned substrate of similar root mean square roughness is attributed to the modified long range dipolar stray fields on the surface.