Excitations in KCoF3-I. Experimental

Dispersion curves for magnons and phonons propagating in KCoF 3 at low temperatures have been determined by means of neutron inelastic scattering techniques. Two branches of magnetic excitations have been observed corresponding to a transition between the two members of the ground state doublet (j= 1 / 2 ) of the Co 2+ ion and a transition between the ground state and the lowest member of the j= 3 / 2 spin-orbit level. Neither branch shows evidence for the directional anisotropy which has been suggested for the exchange interactions in KCoF 3 ; any splitting of the doubly degenerate magnon branch is shown to be less than 8% in frequency. The magnon branch of lowest frequency interacts with a phonon branch in the vicinity of the magnetic zone centre. The strength of the magnon-phonon interaction is found to be 0.35+or-0.10 THz and from the temperature dependence of the modes the magnon-phonon interaction is shown to be appreciable only within a radius 0.2*2 pi /a of the magnetic zone centre. The magnetic modes of higher frequency show a market decrease in intensity with increasing temperature but little change in frequency in accordance with expectations for a transition that depends strongly on the spin-orbit coupling but only weakly on the exchange.