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Magnetic Excitations in the Spin-Spiral State of TbMnO$_3$ and DyMnO$_3$

We calculate spectra of magnetic excitations in the spin-spiral state of perovskite manganates. The spectra consist of several branches corresponding to different polarizations and different ways of diffraction from the static magnetic order. Goldstone modes and opening of gaps at zero and non-zero energies due to the crystal field and the Dzyaloshinski-Moriya anisotropies are discussed. Comparing results of the calculation with available experimental data we determine values of effective exchange parameters and anisotropies. To simplify the spin-wave calculation and to get a more clear physical insight in the structure of excitations we use the σ-model-like effective field theory to analyze the Heisenberg Hamiltonian and to derive the spectra.