ESR study of the Haldane gap system Y₂BaNiO₅ in high magnetic fields

We performed the ESR experiments in Y₂BaNiO₅, which has been proposed as a good candidate for the Haldane gap system, over a wide magnetic field range up to 100T. We observed a relatively broad EPR signal at temperatures above the gap energy and another resonance peak which predominates the spectra below 100K. We also found some more resonant peaks, which exhibit an infinite energy when extrapolated to zero magnetic field. The gap energy is estimated to be 5.22 meV. This resonance seems to correspond to the transition between the singlet state and the triplet state, because the intensity of this peak becomes stronger as the temperature is lowered. However, the gap energy obtained in the present experiment is smaller than the Haldane gap obtained from the neutron inelastic scattering.