MODIFICATION OF THE CLASSICAL HEISENBERG HELIMAGNET BY WEAK UNIAXIAL ANISOTROPY AND MAGNETIC-FIELD

A classical ground State of the isotropic Heisenberg Hamiltonian on a primitive Bravais lattice is known to be a single-Q plane spin helix. Additional uniaxial anisotropy and external magnetic field can signifi cantly distort this structure by generating higher-order Fourier harmo nics at wave vectors no in the spatial spin configuration. These featu res am not captured within the formalism based on the Luttinger-Tisza theorem, when the classical ground state energy is minimized under the <<weak>> condition on the length of spins. We discuss why the correct solution is lost in that approach and present another microscopic tre atment of the problem. For easy-axis and easy-plane quadratic uniaxial anisotropy it allows to find the classical ground state for general Q and for any orientation of the magnetic field considering the effect of anisotropy (but not the field) as a perturbation of the exchange st ructure. As a result, the classical ground state energy, the uniform m agnetization, and the magnetic Bragg peak intensities, that are measur ed in the experiments, are calculated.