  We investigate the bulk magnetic, electron paramagnetic resonance, and
magneto-optical properties of {Ni4Mo12}, a magnetic molecule with
antiferromagnetically coupled tetrahedral {Ni4Mo12} 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.
