Leveraging 3d-4f Coordination: Molecular Quantum Spring-Magnet Behavior in Axial Ni(2)Ln Complexes.

We report heterotrimetallic 3d-4f complexes, mimicking classical exchange spring magnets at the molecular scale. The complexes feature a linear Ni···Ln···Ni core, where the lanthanide ion is sandwiched between two Ni centers coordinated by NO ligand environments. The complexes are isostructural, while CASSCF calculations reveal collinear anisotropy axes and favorable electronic configurations for magnetic bistability in selected systems. Magnetic characterization via DC, AC, and µSQUID magnetometry down to 30 mK demonstrates slow magnetic relaxation and open hysteresis loops exclusively in NiTb, NiDy, and NiHo. These systems exhibit ferromagnetic 3d-4f coupling, while their isolated or antiferromagnetically coupled analogs (NiY, ZnTb/Dy) and NiEr/Yb counterparts show fast relaxation and closed loops. Analysis suggests that the Ni ions alone, with modest anisotropy, deviate from the expected "hard" magnetic behavior due to a broad zero-field QTM, while the Ln ions alone serve as the "soft" phase with large magnetic moments and sharp zero-field QTM. Nevertheless, when brought together, their coupling and alignment of the anisotropy axis enhances the magnetic performance with exchange-bias features mimicking the macroscopic exchange spring magnets. We highlight an optimal utilization of 3d-4f coordination in designing molecular magnets with tunable relaxation and bistability, advancing prospects for quantum information and nanoscale magnetic devices.