m at . o th er ] 1 8 Fe b 20 04 Interface - Localized Mode In Bilayer Film Ferromagnetic Resonance Spectrum

Ferromagnetic resonance (FMR) in exchange-coupled bilayer films has been the subject of intensive studies in recent years. From the experimental viewpoint, a characteristic feature of this FMR is that some specimens show single resonance, whereas others show double resonance. Moreover, double resonance can exhibit a regular pattern , in which the high-field (HF) line intensity surpasses that of the low-field (LF) line, or it can exhibit an inverted pattern with the HF line less intense than the LF line. There is a general agreement that the inverted FMR pattern occurs when the HF line is an 'optic mode', i.e. an out-of-phase composition of individual sublayer modes, and the LF line is an 'acoustic mode', or an in-phase mode composition. The existing theoretical explanations of bilayer ferromagnetic resonance are, as a rule, based on phenomenological equation of motion of the magnetization vector. In this paper, we propose a theory of FMR in ultrathin bilayers based on an entirely microscopic approach, using the Heisenberg model of localized spins and assuming that the two fer-romagnetic sublayers are exchange-coupled through their interface. Both the strength and the sign of this interface coupling (J int) is arbitrary (we admit ferromagnetic or antiferromagnetic interface coupling). The Hamiltonian contains an exchange energy and a Zeeman energy terms; the external field is assumed to be applied obliquely to the film surface. We focus on the effects originating from the interface coupling, though the system is assumed to exhibit also pinning effects, originating from surface anisotropy on the outer surfaces of the film as well as from intrinsic interface anisotropy present on internal interfaces. The latter anisotropy is assumed to consist of two components: uni-directional (K int) and uni-axial (D int). We show that the resonance spectrum in symmetric bilayer is completely independent of J int , but depends strongly on the applied static field configuration with respect to the interface normal (angle θ). A critical angle θ crit is found to exist (as in the case of single-layer film) for which the multipeak spectrum reduces to a single-peak one. This rigorous microscopic FMR theory does explain the inverted pattern of the bilayer FMR spectrum by assuming the HF line to correspond to an in-phase mode, but of interface-localized nature. The intensity of such localized mode decreases with growing strength of its localization at the interface and, when the localization becomes sufficiently strong, becomes lower than the intensity of …