The compound Y 2BaNiO 5: A new example of a haldane gap in A S = 1 magnetic chain

Abstract

The crystal structure of Y 2BaNiO 5 is characterized by the existence of isolated linear magnetic chains. Nickel is located within compressed oxygen octahedra sharing two opposite corners to form (NiO 5) n chains along the a-axis separated by Y and Ba atoms. Within a large temperature range, the magnetic susceptibility can be fitted using a one dimensional S = 1 Heisenberg model. The best fit is obtained for J/k ≈ −285 K (T max ≈ 410 K). The three dimensional long range magnetic ordering has not been observed by neutron diffraction down to 1.8 K, implying an inter-intrachain coupling ratio J'/J ⪡ 10 −2. Inelastic neutron scattering experiments have given evidences for a singlet ground state and two gaps in the excitation spectrum at energies Δ xy ≈ 8.5 meV and Δ z ≈ 16 meV. Our experimental data are interpreted quantitatively within the framework of the Haldane conjecture for the S = 1 antiferromagnetic chain.