Abstract
We have theoretically investigated the influence of the anisotropy field and the Dzyaloshinsky–Moriya type coupling on the bulk magnetic excitations in a noncolinear frustrated structure. Application to the cubic frustrated weak ferromagnet FeF3 is presented. Using a Green function formalism and matching technique procedure, the dispersion curves of the bulk magnons are obtained within a single framework by matching the evanescent and travelling solutions, respectively, of the secular equation, satisfying surface boundary conditions. The calculation concerns, in particular, the spin fluctuation dynamics in the bulk of an antiferromagnetic lattice, the spin order being in the direction normal to the surface boundary. The competing nearest J1AB and next nearest neighbours exchange interactions (J2AA(BB)) are considered. The theoretical approach determines the evanescent spin fluctuation field in the two-dimensional plane parallel to the surface. It is shown that the propagating and evanescent spin-waves from a surface are strongly dependent on the values of ratio ε=J1AB/J2AA(BB). The presence of anisotropy field and antisymmetric exchange term induces an energy gap on the bulk spin-wave continuum which involves a new ground state of spin quantum.
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