ON TILTED MAGNETIZATION IN THIN-FILMS

We suggest a new mechanism for explaining the tilt of the magnetizatio n away from the surface normal in certain magnetic ultra-thin films. O ur arguments are based on a simple classical spin Hamiltonian in which the magnetocrystalline surface anisotropy is described by H-a = -Sigm a(i) lambda(i)(s(i)(z))(2) + Sigma(i) gamma(i)(s(i)(z))(4), where lamb da(i) and gamma(i) are non-negative phenomenological constants, s(i)(z ) denotes the z-component (normal to the surface) of the spin at the s ite labelled by i. In this paper we study only the ground state. In co ntrast to the usual explanation which attributes the experimentally ob served tilted magnetization to the fourth-older term involving gamma(i ), we show that the second-order term alone can lead to this interesti ng phenomenon. Our explanation implies that the magnetization of the s uccessive layers are not collinear. As an illustration of our argument s we discuss the experimentally observed orientational transition of t he Co Au(1 1 1) system in quantitative details. (C) 1998 Elsevier Scie nce B.V. All rights reserved.