  Inelastic light scattering spectroscopy around the \nu=1/3 filling discloses
a novel type of cyclotron spin-flip excitation in a quantum Hall system in
addition to the excitations previously studied. The excitation energy of the
observed mode follows qualitatively the degree of electron spin polarization,
reaching a maximum value at \nu=1/3 and thus characterizing it as a \nu=1/3
ferromagnet eigenmode. Its absolute energy substantially exceeds the
theoretical prediction obtained within the renowned single-mode approximation.
Double-exciton corrections neglected utilizing the single-mode approach are
evaluated within the framework of the excitonic representation and are inferred
to be responsible for the observed effect.
