THEORY OF INTERFACE MAGNONS IN MAGNETIC MULTILAYER FILMS

A review is given of the properties of interface spin-waves in ferromagnetic multilayer films. We start from the general model assumptions made when dealing with thin magnetic films; complete generality is ensured by assuming the spin quantization axes in each monolayer as different. The Hamiltonian diagonalization procedure is based on the Tyablikov-Bogolyubov scheme, appropriately modified for the film structure considered; the diagonalization leads to elementary magnetic excitations, i.e., spin-waves of the multilayer system. We concentrate specifically on the spin-wave spectrum of exchange-coupled bilayer films, for which the appropriate Surface-Interface Inhomogeneity Model is elaborated and the respective Heisenberg Hamiltonian is established and diagonalized. The theory is valid for Bravais lattices with arbitrary surface/interface orientation, and holds for arbitrary (with respect to the film normal z) configurations of the bilayer film magnetization, arbitrary ferro/antiferromagnetic interface exchange coupling, and arbitrary (easy-axis/easy-plane) uniaxial interface anisotropy taken as a single-ion anisotropy of the DS(z)2 type. When exact Hamiltonian diagonalization is achieved, the microscopic theory of bilayer spin-wave resonance (SWR) is developed in detail. Particular attention is given to the conditions for the occurrence of interface-mode (IM) peaks in the bilayer resonance spectrum, and the use of these peaks when determining the interface parameters (interface pinning anisotropy and interface coupling), First, the essential features of the bilayer SWR spectrum are discussed for the case when the external static field is applied perpendicularly to the film surface and intrinsic surface and interface anisotropies are absent. We show that the pattern of the SWR spectrum is deter-mined by the nature of the interface exchange-coupling integral, with the first two (high-field side) lines exhibiting pronounced intensities if the interface coupling is antiferromagnetic, and that the highest-field line arises by excitation of the mode which is localized on the bilayer interface. When the bilayer Hamiltonian includes both surface and interface pinning anisotropies (of uniaxial type) we establish conditions for the coexistence of surface and interface modes, and moreover predict the possibility of creation of bilocalized surface-interface hybrids of these two modes. The general bilocalization conditions involving the interfacial coupling and surface/interface anisotropies are tabulated separately for symmetrical and antisymmetrical modes. Next, in the more general case when the external field is tilted to the bilayer film surface, we predict the existence of a ''critical configuration angle theta(c)'' for IM emergence at film magnetization rotation. For antiferromagnetic interface coupling, theta(c) is a function of the interface parameters. We stress the importance of a formula permitting the determination of the interface parameters from the experimental resonance peaks intensity ratio R = I(BM) I(IM) (where I(IM) is the intensity of the IM mode existing in the respective configuration and I(BM) is that of an appropriately chosen bulk mode). A separate problem concerns the existence of propagating interface-localized spin-waves in bilayer films. At fixed static interface conditions, an interface spin-wave can propagate only in certain permitted directions in the plane of the film. This restriction is visualized graphically by plotting the existence regions of the interface spin-waves in the two-dimensional Brillouin zone. These regions are then studied as to their form and size versus the interface parameters. Our general mathematical analysis of the bilayer spin-wave spectrum is based on the exact solution of the respective eigenvalue problem as enabled by our newly invented interface-rescaling approach. Finally, we propose an exact solution of the eigenvalue problem for triple-layer films. Therefore, our stepwise interface-rescaling procedure is shown to be well adapted for research on more highly complex multilayer magnetic films.