Exchange-Biased Dimers of Single-Molecule Magnets in OFF and ON States

摘要

Single-molecule magnets (SMMs) are molecules that possess a significant barrier (vs kT) to reorientation of their magnetization (magnetic moment) vector as a result of the combination of a large ground state spin (S) and Ising (easy axis) magnetoanisotropy (negative axial zero-field splitting parameter (D). As such, they represent a molecular (bottom-up) approach to nanomagnetism. They also straddle the classical/quantum interface by displaying not just classical magnetization hysteresis but also quantum tunneling of magnetization (QTM) and quantum phase interference. For all these reasons, they have been proposed as potential qubits for quantum computation. In this regard, the report of supramolecular hydrogen-bonded pairs of [Mn_4O_3Cl_4(O_2CEt)_3(py)_3] ([Mn_4]_2; py = pyridine) SMMs was an important development. A weak intradimer antiferromagnetic exchange interaction coupled the two halves of [Mn_4]_2 and provided an exchange-bias that shifted the QTM positions away from zero field for the first time. Subsequent EPR and magnetization studies established that the two connected SMMs of [Mn_4]_2 are entangled and in quantum superposition states. The next hurdle to the potential use of dimers as two-qubit systems is how to have the exchange-bias OFF and ON; [Mn_4]_2 is not amenable to removal of the intradimer H-bonds. The present work reveals one way that such a two-state system could be developed by showing how an [Fe_9]_2 supramolecular dimer of SMMs can be in OFF and ON exchange-coupled states mediated by a single H-bond.