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

TL;DR

This neutron scattering study investigates how external pressure affects the spin dynamics and anisotropy in the Haldane chain compound NENP, revealing pressure-induced shifts in energy levels and a reduction in anisotropy without changing intrachain coupling.

Contribution

The study provides the first measurement of mode dispersion perpendicular to the chain and shows pressure reduces anisotropy while leaving intrachain exchange nearly unchanged.

Findings

Pressure shifts magnetic excitation gaps.

Interchain exchange remains nearly constant under pressure.

Single-ion anisotropy decreases significantly with pressure.

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.