1. Laboratoire Louis Néel-CNRS, BP166, 25 Avenue des Martyrs, 38042 Grenoble, Cedex 9, France
2. Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA
3. Department of Chemistry, University of California at San Diego, La Jolla, California 92093-0358, USA

Correspondence to: George Christou² Correspondence and requests for materials should be addressed to G.C. (e-mail: Email: christou@chem.ufl.edu).

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard^{1, 2, 3}. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such 'single-molecule magnets' (SMMs)⁴ straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization^{5, 6} and quantum phase interference⁷. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing⁸. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.