Spin waves in the one-spin-slip phase of holmium

The dispersion relationship for spin waves propagating the c-axis in the one-spin-slip phase of holmium at 19.5 K has been measured using a neutron triple-axis spectrometer. A gap in the spin-wave spectrum of approximately 0.6 meV is resolved clearly at a reduced wave vector transfer with respect to the magnetic unit cell of qm = (5/11) c*, and the presence of a second gap at qm = (6/11) c* is inferred from the anomalous behaviour of the intrinsic spin-wave width. Our results are discussed in the light of recent calculations of the spin waves in holmium