Record-High Toroidal Blocking in a {Cu₆Dy₃} Single Molecular Toroid

Single-molecule toroids (SMTs) are promising for quantum technologies because their vortex-like spin arrangement generates magnetically silent ground states with minimal stray fields and enhanced protection against decoherence. However, their technological potential requires toroidal states that remain stable under strong external magnetic fields, since weak intramolecular interactions can induce spin flips and destroy toroidal order. In this study, we report heterometallic {3d-4f} complexes, [Ln₃Cu₆L₆(µ₃-OH)₆(µ₁-H₂O)₆]³⁺ (Ln = Dy (1), Y (2); L^2- = (S,E)-N-(2-(λ¹-oxidaneyl)-1-phenylethyl)-1-(3-methoxy-2-(λ¹-oxidaneyl)phenyl)ethan-1-imine)). Remarkably, the Dy(III) analogue retains its magnetically silent toroidal ground state up to 1.3 T, establishing an unprecedented benchmark for toroidal magnetization blocking and extending the field stability of molecular toroidal states beyond previous limits. Sub-kelvin µ-SQUID measurements, high-frequency EPR spectroscopy, and theoretical modelling show that this high blocking field originates from an unexpectedly strong Dy-Dy exchange interaction, suggested to arise from neighboring Cu(II) ions, whereas direct ferromagnetic Cu-Dy exchange weakly destabilizes the toroidal state. These results reveal a new strategy for stabilizing toroidal states at high fields through targeted control of intramolecular exchange interactions, opening opportunities for electrically addressable molecular nanomagnets and quantum technologies.