Magnetic dynamics of Fe4 molecular clusters in crystals and on graphene

Abstract

Graphene, a two-dimensional carbon layer, is expected to contribute to a new revolution in electronics. However, the fact that graphene is not magnetic, just as any other carbon derivative, hinders its application to spintronics, i.e. to magnetic data storage and processing. A way to circumvent this difficulty is by doping graphene with either point-like defects or with molecules having a localized spin, such as free-radical systems. The nature of the ensuing magnetism and the existence of a coupling to graphene’s Dirac electrons remains, however, a subject of controversy. Besides, very little is known about the dynamics of spins in graphene. In this work, we have studied a new hybrid material formed by anchoring Fe4 molecular clusters, with a net spin S = 4, to graphene layers. In crystalline form, these clusters behave as single-molecule magnets, i.e. they show magnetic memory effects and slow relaxation at very low temperatures. Below 1 K, the dominant relaxation process is mediated by pure spin tunneling events. Because of the compact structure of its magnetic core, Fe4 clusters retain their electronic and magnetic properties when they are deposited onto solid substrates. Therefore, they provide an close to ideal situation to study how the spin dynamics is affected by the interaction with graphene. We have performed ac susceptibility experiments, with frequencies ranging from 0.1 Hz up to 200 kHz, using a high-sensitivity micro-SQUID susceptometer installed in a 3He-4He dilution refrigerator that gives access to the close neighbourhood of absolute zero (T > 11 mK). Experiments have been performed on the Ge4@graphene hybrid and on single crystals of Fe4 molecular magnets having different ligands and concentrations. The results confirm that properties such as the net molecular spin and magnetic anisotropy are preserved in the former material. However, the spin dynamics is dramatically affected by the presence of the graphene layer. In particular, the tunneling rate, measured at very low temperature, is enhanced by six orders of magnitude. We argue that the effect is due to a very effective shielding of dipole-dipole interactions between the molecules and to the crystal field generated by the graphene layer, which introduces new terms in the molecular spin Hamiltonian that break spin tunneling selection rules. As a result, the molecular spins enter a new dynamical regime, in which tunneling proceeds coherently. These results show that graphene can provide a useful platform for the coherent control of quantum spins with electric fields.