THE COMPOUND Y2BANIO5 - A NEW EXAMPLE OF A HALDANE-GAP IN A S = 1 MAGNETIC CHAIN

The crystal structure of Y2BaNiO5 is characterized by the existence of isolated linear magnetic chains. Nickel is located within compressed oxygen octahedra sharing two opposite comers to form (NiO5)n chains al ong the a-axis separated by Y and Ba atoms. Within a large temperature range, die magnetic susceptibility can be fitted using a one dimensio nal S = 1 Heisenberg model. The best fit is obtained for J/k almost-eq ual-to - 285 K (T(max) almost-equal-to 410 K). The three dimensional l ong range magnetic ordering has not been observed by neutron diffracti on down to 1.8 K, implying an inter-intrachain coupling ratio J'/J muc h less than 10(-2). Inelastic neutron scattering experiments have give n evidences for a singlet ground state and two gaps in the excitation spectrum at energies DELTA(xy) almost-equal-to 8.5 meV and DELTA(z) al most-equal-to 16 meV. Our experimental data are interpreted quantitati vely within the framework of die Haldane conjecture for the S = 1 anti ferromagnetic chain.