Magnons in ferromagnetic terbium under high pressure

We have performed an inelastic neutron scattering experiment in order to investigate the pressure dependence of the magnon dispersion of terbium in its ferromagnetic phase. Our measurements were performed along the crystal c axis at 90 K (well below the Curie temperature T_c=220 K) at ambient pressure, and at 4.3 and 15.2 kbar of applied hydrostatic pressure. The difference between the magnon dispersion curves at ambient pressure and at 4.3 kbar is small, while at 15.2 kbar the dispersion curve shifts appreciably towards higher energies. The measured magnon dispersion curves have been analyzed using a Hamiltonian that includes Heisenberg exchange as well as single-ion anisotropy terms. For each dispersion curve we have calculated five exchange constants and two anisotropy terms. The energy gap at q=0 due to the anisotropies is enhanced with pressure and exchange interactions acting between c planes appear to decrease with the application of high pressure.