Magnetic excitations in commensurable periodic structures

The magnetic excitations of a ${\mathrm{Ho}}_{90}$ ${\mathrm{Tb}}_{10}$ single crystal have been studied by inelastic neutron scattering. In the commensurable helical structure the bunching of the moments about the easy directions of magnetization induces an energy gap at long wavelengths whose magnitude, compared to that in the ferromagnetic phase, implies an anisotropic two-ion coupling. The strong dipolar forces give rise to a discontinuity in the dispersion relations at q=0, and resolve the longstanding mystery of the stability of the conical structure in Ho at low temperatures.