We have grown hetero-epitaxially ultrathin Co films on a 7 x 7-Si(1 1 1) surface using buffer layers of Au and Cu, where the thickness of each film is controlled to atomic-scale dimensions. The film structure and related magnetic properties are investigated. The hcp-(0 0 0 1) planes are identified in Co deposited on the Au/Cu surface, while the fee(I 1 1) planes are dominant with a Cu-buffer layer. Perpendicular magnetic anisotropy constants in Co indicate sharp thickness-independent interfaces and bulk-like crystallinity even in a monatomic thickness region of <2 monolayers of Co. Structure related inhomogeneities are quantified as a function of Co thickness for various overlayers by the spin-wave spectrum width. In the results, we show a successful fabrication of epitaxial growth of ultrathin Co films on the Au/Cu/Si surface with a well-defined magnetic structure. (C) 2002 Elsevier Science B.V. All rights reserved.