Magnon energy gap in a three‐layer ferromagnetic superlattice

Abstract

AbstractThe magnon energy band in a three‐layer ferromagnetic superlattice is studied by using the linear spin‐wave approach and Green's function technique. It is found that two modulated energy gaps exist in the magnon energy band along Kx direction perpendicular to the plane of the superlattice. The spin quantum numbers and the interlayer exchange couplings all affect the two energy gaps. The disappearance of one of the energy gaps corresponds to a high symmetry among the interlayer exchange couplings, while the vanishing of another energy gap corresponds to a high symmetry of the system, which belongs to the type III Shubnikov group or the polychromatic group of magnetic group. This magnetically structural symmetry includes not only the symmetry of crystallographic point‐groups or space‐groups, but also the symmetry of strength and direction of spins. Comparing with the previous results for a three‐layer ferrimagnetic superlattice, it is concluded that the energy gap of the ferromagnetic/ferrimagnetic superlattices is dominated by different symmetries originated from ferromagnetic/antiferromagnetic interlayer exchange couplings. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)