S = 1/2 Degrees of Freedom in Haldane System Y₂BaNiO₅ Observed by Submillimeter Wave ESR

We have performed submillimeter wave ESR measurement of Haldane system Y₂BaNiO₅ in the frequency region from 50 to 383 GHz using the pulsed magnetic field up to 16 T. Due to the excess oxygens, a few percent of Ni²⁺ ions of Y₂BaNiO₅, which we used, are substituted by the Ni³⁺ ion of which spin is S = 1/2. The ESR signals coming from the S = 1/2 degrees of freedom originated by the Ni³⁺ ion at the end of the finite chain, are observed at 4.2 K. The frequency-field relations of these ESR signals are analyzed by the model based on the VBS model, which is used in the case of Cu²⁺ doped NENP. The signs of the single ion anisotropies D and E are determined from the analysis.