Quantum Monte Carlo simulations of a giant {Ni₂₁Gd₂₀} cage with a S = 91 spin ground state

The detailed analysis of magnetic interactions in a giant molecule is difficult both because the synthesis of such compounds is challenging and the number of energy levels increases exponentially with the magnitude and number of spins. Here, we isolated a {Ni₂₁Gd₂₀} nanocage with a large number of energy levels (≈5 × 10³⁰) and used quantum Monte Carlo (QMC) simulations to perform a detailed analysis of magnetic interactions. Based on magnetization measurements above 2 K, the QMC simulations predicted very weak ferromagnetic interactions that would give a record S = 91 spin ground state. Low-temperature measurements confirm the spin ground state but suggest a more complex picture due to the single ion anisotropy; this has also been modeled using the QMC approach. The high spin and large number of low-lying states lead to a large low-field magnetic entropy (14.1 J kg^-1 K^-1 for ΔH = 1 T at 1.1 K) for this material.