Role of interfaces in the permittivity tensor of thin layers of a ferromagnetic metal

It is known from experimental studies that the components of the permittivity tensor $\varepsilon$ depend on layer thicknesses of multilayer thin films, and for nanometer layers, it is necessary to additionally consider the interlayer interfaces. This study provides an answer to the question of what is the reason for the influence of these interfaces on film properties. It is shown that the contribution of interband matrix elements for ferromagnetic films with off-diagonal components of the permittivity tensor determines the ratio between the diagonal and off-diagonal components of the tensor $\varepsilon$ at a ferromagnetic layer thickness of about 10 nm.