SPIN-WAVE EXCITATIONS OF SINGLE-DOMAIN WALLS IN ORTHORHOMBIC FERROMAGNETS

A ferromagnetic crystal with cubic and induced orthorhombic and uniaxi al anisotropies is considered. The hard uniaxial anisotropy axis is pa rallel to the [110] direction, the hard orthorhombic axis parallel to the [(1) over bar 10] direction. Localized spin waves propagating alon g a single domain wall in such materials are analyzed by numerical sol ution of the Landau-Lifshitz and magnetostatic equations. The spin wav e spectra an calculated for propagation directions either perpendicula r or parallel to the direction of the magnetization in the two adjacen t domains. There exist critical combinations of magnetic anisotropy co nstants for which the uniform Bloch wall becomes unstable before the N eel wall is preferred. In such parameter regions Goldstone modes show a gap in the wave number and a loop of the dispersion branch for the c ase of perpendicular propagation. This behavior is very similar to tha t of hydrodynamic waves excited in stratified shear flows. The existen ce of negative energy waves and the appearance of a Kelvin-Heimholtz i nstability are to be expected for spin waves propagating along an init ially plane domain wall in the absence of dissipation. All spectra for perpendicular propagation are asymmetric with respect to wave vector inversion. They also contain the unidirectional Gilinskii-type mode wh ich has a finite frequency at zero wave number. This activation freque ncy coincides with the frequency of the bottom of the bulk spin wave m anifold for propagation parallel to domain magnetizations. Measurement s of resonant excitations of single domain walls in bismuth-substitute d garnet films with in-plane magnetization are presented. The resonanc es are observed in the frequency range from 50 to 1500 MHz. They are q ualitatively discussed on the basis of the developed theory. (C) 1997 American Institute of Physics.