Elastic and inelastic neutron scattering studies on Sr$_{2}$FeSi$_{2}$O$_{7}$

Evolution of static and dynamic spin correlations in a new multiferroics material Sr$_{2}$FeSi$_{2}$O$_{7}$ under an external magnetic field was investigated by elastic and inelastic neutron scattering techniques. An external magnetic field up to $B = $ 14 Tesla induces four different magnetic and ferroelectric phases in Sr$_{2}$FeSi$_{2}$O$_{7}$. The static magneto-electric coupling can be understood as the p-d hybridization proposed for a related material Ba$_{2}$CoGe$_{2}$O$_{7}$. By analyzing the neutron scattering data obtained from a single crystal of Sr$_{2}$FeSi$_{2}$O$_{7}$ under magnetic field, we have determined the spin structure and the effective spin Hamiltonian in this material. The spin structure and spin wave excitations show interesting changes as upon ramping up the system enters the field-induced phases for $B$ \textgreater\ 6.5 Tesla, which will also be discussed.