Determination of spin Hamiltonian in the Ni ₄ magnetic molecule

Magnetic excitations in a Ni ₄ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility (χ _bulk) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments exhibits three modes at energy transfers of ω=0.5, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependencies of the inelastic neutron scattering data and χ _bulk can be well reproduced by an effective spin Hamiltonian consisting of intramolecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and a Zeeman term. Under hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system.