Spin Waves in 2D ferromagnetic square lattice stripe

Abstract

In this work, the area and edges spin wave calculations were carried out using the Heisenberg Hamiltonian and the tridiagonal method for the 2D ferromagnetic square lattice stripe, where the SW modes are characterized by a 1D in-plane wave vector $q_x$. The results show a general and an unexpected feature that the area and edge spin waves only exist as optic modes. This behavior is also seen in 2D Heisenberg antiferromagnetic square lattice. This absence of the acoustic modes in the 2D square lattice is explained by the fact that the geometry constrains for NN exchange inside the square lattice allow only optical modes. We suggest that this unexpected behavior of spin waves in the 2D square lattice may be useful in realizing an explanation for HTS.