Determination of spin Hamiltonian in the Ni$_4$ magnetic molecule

TL;DR

This study investigates the magnetic excitations of a Ni4 molecule using neutron scattering and susceptibility measurements, revealing a detailed spin Hamiltonian and evidence of spin-lattice coupling under pressure.

Contribution

It presents a comprehensive determination of the spin Hamiltonian for Ni4, including exchange, anisotropy, biquadratic interactions, and pressure effects, advancing understanding of molecular magnetism.

Findings

Identified three magnetic excitation modes at specific energies.

Successfully modeled data with an effective spin Hamiltonian.

Observed pressure-induced decrease in magnetization indicating spin-lattice coupling.

Abstract

Magnetic excitations in a Ni$_4$ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility ($\chi_\text{bulk}$) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments exhibits three modes at energy transfers of $\hbar\omega=0.5$, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependences of the inelastic neutron scattering data and $\chi_\text{bulk}$ can be well reproduced by an effective spin Hamiltonian consisted of intra-molecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and Zeeman term. Under a hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system.