Discriminating ferrotoroidic from antiferrotoroidic ground states using a 3d quantum spin sensor

Molecular toroidal states have come to the forefront as candidates for next-generation quantum information devices owing to their bistability and protection from weak, short-range magnetic interactions. The protection offered by these non-magnetic vortex spin states proves to be a double-edged sword as inferring their existence in a molecular system has yet to be achieved through experimental means alone. Here, we investigate the anomalous, sickle-shaped, single-crystal magnetisation profile arising in μ-SQUID measurements of a novel CrDy₃ molecule. Theoretical modelling supported by ab initio calculations demonstrates that the weak field CrDy₃ spin dynamics is resultant from quantum superposition of the Cr^III spin states determined by three competing interactions: (i) the alignment of the Cr^III magnetic moment to the external magnetic field, (ii) the zero-field splitting of the Cr^III ground quartet, and (iii) coupling to the remnant magnetisation of the toroidal ground state in the Dy₃ triangle. If zero-field splitting of the central transition metal ion is quenched, it operates as a quantum spin sensor, which can be exploited to experimentally discriminate between ferrotoroidic and antiferrotoroidic ground states in MDy₆ double triangle complexes through electron paramagnetic resonance experiments and single-crystal magnetisation measurements with a restricted field sweeping domain.