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

Abstract

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 ${\Delta }_{\perp }\approx$ 1.2meV and a singlet at ${\Delta }_{\parallel }\approx$ 2.5meV. With pressure we observe appreciable shifts in these levels, which move to ${\Delta }_{\perp }(2.5GPa)\approx$ 1.45 meV and ${\Delta }_{\parallel }(2.5GPa)\approx$ 2.2meV. The dispersion of these modes in the crystalline c-direction perpendicular to the chain was measured here for the first time, and can be accounted for by an interchain exchange J'_c approximately 3e-4*J which changes only slightly with pressure. Since the average gap value ${\Delta }_H\approx$ 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.

LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to Phys. Rev. B