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It is well-known that strongly nonlinear topologically nontrivial excitations (solitons) play a special role in low-dimensional magnetic systems. For example, kinks in 1D systems are responsible for the destruction of long-range order at finite temperatures, and the presence of vortices in 2D systems gives rise to a special type of phase transition -- the Berezinskii-Kosterlitz-Thouless transition[1]. Experimental observation of soliton signatures in the response functions is usually based on the fact that translational motion of solitons leads to the the so-called soliton central peak, for review see [2],[3],[4]. Another possibility is to look for internal degrees of freedom of solitons, such as magnon modes localized on a soliton. Resonances at characteristic frequencies of an internal motion can be observed in electron spin resonance or inelastic neutron scattering (INS) experiments. In 1D such effects of ``soliton magnetic resonance'' were detected in the Ising-type antiferromagnet (AFM) CsCoCl [5], and were predicted theoretically for Heisenberg AFMs [6]. In 3D, magnon modes localized on domain walls in AFMs were observed in thulium orthoferrite [7]. Presently, no experimentally testable examples of internal soliton dynamics in 2D are known. Moreover, only a quasi-local mode has been predicted for solitons in isotropic [8], XY-type[9], and easy-plane Heisenberg[10],[11],[12] 2D ferromagnets (FM).

The aim of the present Letter is to draw attention to the fact that in classical easy-plane Heisenberg 2D AFMs ``out-of-plane'' vortices have finite-frequency truly localized internal modes which may be detectable in resonance or INS experiments. We also numerically obtain the vortex-magnon S-matrix in the long-wavelength region, analyzing linearized perturbations of the vortex structure, for continuum and discrete lattice models.

Gary M Wysin
Wed Sep 6 18:51:57 CDT 1995