Origin of magnetic anisotropy in thin films deposited on step-bunched substrates

Abstract Ferromagnetic thin films of cobalt and permalloy have been grown on a non-planar Si(1 1 1) substrate presenting the step-bunching phenomenon. Room temperature hysteresis loop measurements show a strong uniaxial magnetic anisotropy, which decreases as the magnetic layer thickness increases. In the case of permalloy, the measured anisotropy is of magnetostatic origin and results from the non-planar nature of the substrate. The effective demagnetising field induced by the substrate is determined using a simple model based on the surface magnetic-charge density. The obtained values of the demagnetising field are then used to show that in the cobalt layers, the effective anisotropy is given by the substrate-induced shape anisotropy and an additional magneto-elastic contribution, which scales as the inverse of the thickness, consistent with the transition from FCC to HCP cobalt above a critical thickness close to 20 A.