Observation of the Dispersion Relation for Spin Waves in Hexagonal Cobalt

The acoustic branch of the spin-wave spectrum of hexagonal cobalt has been measured for values of the magnon wave vector q up to qc/2π=0.2 by observing the inelastic scattering of polarized neutrons by the diffraction method. The results indicate a decided departure from the dispersion law expected for a Heisenberg ferromagnet. In particular, if the energy is expressed as a sum of q2 and q4 terms, then the q4 term should only contribute 3% to the energy, whereas a contribution of 35% is observed. The coefficient of the q2 term yields an exchange stiffness parameter D=490±20 meV·Å2.