Elementary excitations in anisotropic nanofilms of multiferroics with competing interactions at the interface

Abstract

In this work we study, using the of two-time Green's functions method, the spectrum of spin waves in a monolayer and bilayer of anisotropic magnetic film sandwiched between the ferroelectric layers in a magnetoelectric superlattice. Surface spin configuration is calculated by minimizing the interaction energy. It is shown that the angles between spins near the surface are strongly modified with respect to the bulk configuration. We include the anisotropy energy between the spin vectors at sites i and j in the Hamiltonian of multiferroic superlattice. The anisotropy energy stabilizes the angle between local quantization axes. The anisotropy parameter K i,j supposed positive, and nonzero only for the nearest interacting neighbors. It is shown that the magnetoelectric interaction strongly affects the long-wavelength mode of spin waves. The magnetization of the magnetic film at low temperatures was also calculated. The magnetization M is strongly dependent on temperatures: at higher T, the larger the magnetoelectric interaction, the greater is M. However, at T = 0, the spin length becomes smaller for large values of the magnetoelectric interaction due to the so-called spin reduction that takes place in antiferromagnets. As a result, the crossover (intersection) of the magnetization curves occurs for different values of θ at low T.