Spin-Wave Theory of Magnetic Resonance in Spiral Spin Structures: Effect of an Applied Field

The possibility of performing magnetic resonance experiments in substances with spiral spin structures is studied. Particular attention is paid to the variation of spin arrangement and spin-wave spectrum with an applied magnetic field in the plane of the spiral. Perturbation techniques give resonance frequencies for small fields and for fields near the value, Hf, required for ferromagnetic alignment. These results then allow a realistic extrapolation for the whole range of H. The effects of planar anisotropy and demagnetizing fields on the variation of spin arrangement and on the spin-wave frequencies are discussed. The actual conditions of resonance are examined for dysprosium and erbium. There is a discussion of the information that can be obtained from polycrystalline specimens with particular reference to the experimental results for MnAu₂.