Effective Fields in Magnetic Thin Films: Application to the Co/Cu and Fe/Cr Systems

Abstract

The description of magnetic properties associated with the behavior of ultrathin structures can be simplified significantly compared with that for bulk materials. Ultrathin layers lose their internal magnetic degrees of freedom. All atomic magnetic moments across the film thickness are parallel because of the very strong exchange interactions between spins, and consequently the total magnetic moment of the film is given by a simple algebraic sum of all of its constituent atomic moments. Ultrathin films are essentially giant magnetic molecules which can have magnetic properties different from those in the bulk. This simple picture can be quantified by employing the Landau-Lifshitz (L-L) equations of motion to describe the behaviour of the magnetization. Since a system consisting of only a few atomic layers is being considered, it is useful to start with the L-L equation of motion for a single atomic magnetic moment which is located in the ultrathin film, $$ - \frac{1}{\gamma }\frac{{dm}}{{dt}} = [mxH_{eff}^a] $$ (1.1) where m is the atomic magnetic moment, H eff a is the effective field acting on the atomic moment m and γ=glel/(2mc) is the gyromagnetic ratio. γ is the spectroscopic splitting factor; for a free electron g=2 and γ=1.75888x107 Hz/Oersted. the left hand side describes the time evolution of the atomic mechanical angular momentum and the right hand side represents the total torque acting on the atomic magnetic moment. In ultrathin films all atomic magnetic moments are parallel and therefore one can sum up all moments across the film thickness; it is convenient to carry out the sum normalized to a unit area.