<i><i>S</i>=<b>1/2</b></i>Degrees of Freedom in Haldane System Y_<b>2</b>BaNiO_<b>5</b>Observed by Submillimeter Wave ESR

Abstract

We have performed submillimeter wave ESR measurement of Haldane system Y 2 BaNiO 5 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 Ni 2+ ions of Y 2 BaNiO 5 , which we used, are substituted by the Ni 3+ ion of which spin is S =1/2. The ESR signals coming from the S =1/2 degrees of freedom originated by the Ni 3+ 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 Cu 2+ doped NENP. The signs of the single ion anisotropies D and E are determined from the analysis.