TUNING THE SPIN-HAMILTONIAN OF NI(C2H8N2)(2)NO2CLO4 BY EXTERNAL-PRESSURE - A NEUTRON-SCATTERING STUDY

We report an inelastic neutron-scattering study of antiferromagnetic s pin dynamics in the Haldane chain compound Ni(C2H8N2)(2)NO2ClO4 (NENP) under external hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations in NENP are dominated by a long-lived triple t mode with a gap that is split by orthorhombic crystalline anisotropy into a lower doublet centered at Delta(perpendicular to)approximate t o 1.2 meV and a singlet at Delta(parallel to)approximate to 2.5 meV. W ith pressure we observe appreciable shifts in these levels, which move to Delta(perpendicular to)(2.5 GPa) approximate to 1.45 meV and Delta (parallel to)(2.5 GPa)approximate to 2.2 meV. The dispersion of these modes in the crystalline (c) over cap direction perpendicular to the c hain was measured here and can be accounted for by an interchain excha nge J(c)'approximate to 3x10(-4)J that changes only slightly with pres sure. Since the average gap value Delta(H) approximate to 1.64 meV rem ains almost unchanged with P, we conclude that in NENP the application of external pressure does not affect the intrachain coupling J apprec iably, but does produce a significant decrease of the single-ion aniso tropy constant from D/J = 0.16(2) at ambient pressure to D/J = 0.09(1) at P = 2.5 GPa.