Abstract:
The influence of a weak external field on spiral magnets is investigated
for a fairly general operator structure of the Hamiltonian of the
interaction of the system with the external field. Equations of motion
for the parameters of the reduced description are obtained, and the
specific structure of the sources in these equations due to the external
field is found. The asymptotic behaviors of the Green's functions
$G_{ab}(k,\omega)$ are calculated for arbitrary quasilocal operators $\hat a$, $\hat b$ in the
low-frequency region. A model of a weakly nonideal gas of magnons is
investigated by means of the analogy between superfluid and magnetic
systems with spiral structure. It is shown that the spectrum of
elementary excitations of the spiral magnet arises due to the phenomenon
of Bose condensation of spin waves. The Green's functions are found in
the approximation of an ideal gas of quasiparticles. It is shown that
the spectrum of a weakly nonideal gas of magnons goes over into a hydrodynamic
spectrum if the internal energy is taken in the self-consistent field approximation.
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