Observation of field-dependent magnetic parameters in the magnetic molecule { Ni4 Mo12 }

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.