Field-dependent magnetic parameters in $\{$Ni$_4$Mo$_{12}$$\}$: Magnetostriction at the molecular level?

We present the optical and magneto-optical properties (0 - 32 T) of Mo$_{12}^V$O$_{30}$($\mu$$_2$-OH)$_{10}$H$_2$\{Ni$^{II}$(H$_2$O) $_3$\}$_4$, a magnetic molecule with antiferromagnetically coupled tetrahedral Ni$^{II}$ in a diamagnetic molybdenum matrix. A magnetochromic effect, centered at $\sim$1.9 eV, is observed at 4.2 K, and it is attributed to a change in the Ni $d \rightarrow d$ on-site excitation. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Field-dependent exchange parameter, however, provides the best fit to magnetization, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field. The magneto-optical response of Mo$_{12}^V$O$_{30}$($\mu$$_2$-OH)$_{10}$H$_2$\{Ni$^{II}$(H$_2$O) $_3$\}$_4$ supports a small change in the NiO$_6$ coordination geometry and the associated electronic single-ion properties.