Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study

We report an inelastic neutron scattering study of antiferromagnetic spin dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations in NENP are dominated by a long-lived triplet mode with a gap which is split by orthorhombic crystalline anisotropy into a lower doublet centered at ∆⊥ ≈ 1.2meV and a singlet at ∆‖ ≈ 2.5meV. With pressure we observe appreciable shifts in these levels, which move to ∆⊥(2.5GPa) ≈ 1.45 meV and ∆‖(2.5GPa) ≈ 2.2meV. The dispersion of these modes in the crystalline ĉ-direction perpendicular to the chain was measured here for the first time, and can be accounted for by an interchain exchange J ′ c ≈ 3×10−4J which changes only slightly with pressure. Since the average gap value ∆H ≈ 1.64 meV remains almost unchanged with P , we conclude that in NENP the application of external pressure does not affect the intrachain coupling J appreciably, but does produce a significant decrease of the single-ion anisotropy constant from D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa. PACS numbers: 75.10Jm, 75.40Gb, 75.50Ee. Typeset using REVTEX