MAGNETIC EXCITATIONS OF CSFECL3 IN AN EXTERNAL MAGNETIC-FIELD PARALLEL TO THE HEXAGONAL PLANE

In CsFeCl3 the Fe-2+-ion with effective spin one has locally a singlet ground state (m(s) = 0). The ferromagnetic interactions along the c-d irection and the anti-ferromagnetic interactions perpendicular to it a re too week as compared to the anisotropy to introduce longrange order in the absence of an external field. By inelastic neutron scattering we have investigated the excitation spectrum in an external magnetic f ield up to 6 T applied perpendicular to the c-axis. Preliminary measur ements in zero field show that dipolar effects lead to a shift of the minimum of the dispersion curve away from the K-point. We show that wi th increasing magnetic field the whole dispersion sheet is shifted tow ards higher frequencies and a splitting appears. With the help of a lo cal Hamiltonian which neglects exchange interactions between all neigh bours we give a qualitative explanation of the intensity of the magnet ic excitations in dependence of the magnetic field. A more sophisticat ed theory (RPA-diagonalisation of the Heisenberg-Hamiltonian that is u sed to describe the magnetic behaviour of CsFeCl3) yields - apart from a mismatch for the modes at high energy transfers - a satisfactory de scription of the magnetic excitations of CsFeCl3 in dependence of the magnetic field.