Observation of field-dependent magnetic parameters in the magnetic molecule{Ni4Mo12}

Abstract

We investigate the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of {Ni4Mo12}, a magnetic molecule with antiferromagnetically coupled tetrahedral Ni II in a diamagnetic molybdenum matrix. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Allowing the exchange and anisotropy parameters to depend on the magnetic field provides the best fit to our data, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field.

Determination of spin Hamiltonian in the Ni4magnetic molecule

“…Following the previous study, 10 we assume that the spin Hamiltonian of the Ni 4 spin cluster consists of the exchange interaction, the single ion anisotropy, the biquadratic interaction, and the Zeeman term;…”

“…Distances between Ni 2+ ions within a cluster are 6.69, 6.70, 6.62, and 6.60Å, whereas the shortest distance between Ni 2+ ions that belong to different clusters is 7.15Å. Previous bulk property measurements using bulk susceptibility, high field magnetization, electron paramagnetic resonance, optical conductivity, and magneto-optical response [8][9][10] showed that the dominant interaction in the system is antiferromagnetic, which is due to the superexchange interaction through the Ni-O-Mo-ONi bonds. 9 The most interesting property of the Ni 4 nanaomagnet is an adiabatic change with non-equidistant steps observed in the magnetization measurements as a function of external magnetic field.…”

“…9 The most interesting property of the Ni 4 nanaomagnet is an adiabatic change with non-equidistant steps observed in the magnetization measurements as a function of external magnetic field. [9][10][11] This was explained by a model Hamiltonian that consists of a field-dependent exchange parameters, a single-ion anisotropy, and a biquadratic interaction. 10 The spin Hamiltonian should be determined by a more direct tool than the bulk property measurements to check its validity.…”

“…[9][10][11] This was explained by a model Hamiltonian that consists of a field-dependent exchange parameters, a single-ion anisotropy, and a biquadratic interaction. 10 The spin Hamiltonian should be determined by a more direct tool than the bulk property measurements to check its validity.…”

“…In the calculation of χ balk , the geometrical factor was taken to be g = 2.22. [8][9][10][11] The background due to isolated Ni 2+ ions was estimated by the Curie law and added to the calculated χ bulk . The existence of the biquadratic terms in H indicates a spin-lattice coupling in this system.…”