In the field of molecular spintronics, the use of magnetic molecules for information technology is a main target and the observation of magnetic hysteresis on individual molecules organized on surfaces is a necessary step to develop molecular memory arrays. Although simple paramagnetic molecules can show surface-induced magnetic ordering and hysteresis when deposited on ferromagnetic surfaces, information storage at the molecular level requires molecules exhibiting an intrinsic remnant magnetization, like the so-called single-molecule magnets (SMMs). These have been intensively investigated for their rich quantum behaviour but no magnetic hysteresis has been so far reported for monolayers of SMMs on various non-magnetic substrates, most probably owing to the chemical instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic circular dichroism synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temperatures, we have now found that robust, tailor-made Fe(4) complexes retain magnetic hysteresis at gold surfaces. Our results demonstrate that isolated SMMs can be used for storing information. The road is now open to address individual molecules wired to a conducting surface in their blocked magnetization state, thereby enabling investigation of the elementary interactions between electron transport and magnetism degrees of freedom at the molecular scale.

A fundamental step towards atomic- or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface, or of a small paramagnetic molecule embedded in a nanojunction, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices. Here we show that a preferential orientation of Fe(4) complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs-their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization-can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

We have measured quantum transport through an individual Fe(4) single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties and, moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition/subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.