Understanding magnetic interactions in the series A2FeX5•H2O (A = K, Rb; X = Cl, Br).: II. Inelastic neutron scattering and DFT studies

The compounds A(2)FeX(5)center dot H(2)O (A=alkali or NH(4), X=Cl, Br) form a series of easy-axis anti ferromagnets with transition temperatures in the range from 6 to 23 K, temperatures considerably higher than those of other similar hydrated salts of transition-metal ions. A study combining inelastic triple axis neutron spectroscopy (TAS) neutron scattering experiments and theoretical density functional theory (DFT) ab initio calculations of the magnetic superexchange constants has been done. A general spin-wave theory for an anti ferromagnetic system with several magnetic ions in the magnetic unit cell and both uniaxial and rhombic magnetic anisotropies was employed to fit the observed magnon dispersion curves. The results obtained with the two techniques (TAS and DFT) allow us to determine with accuracy the magnetic exchange constants and therefore explain the efficiency of the superexchange pathways containing hydrogen bonds in transmitting the magnetic interactions.