We are currently investigating the electronic properties of spin crossover (SCO) compounds at a single molecule level. Spin crossover molecules are a class of molecule that undergo a change in spin state in response to an external stimuli. This stimulus could be a change in temperature, application of pressure or light.
By trapping spin crossover molecules in this nanogap we aim to probe the electronic transport as a function of temperature and spin state. We expect to see switching effects in the transport properties of a single molecule if the switching properties observed in the bulk film are maintained when the molecule is confined within the nanogap.
Figure 2. a) Resistance of a Ni81Fe19 junction during electromigration, b) dI/dV showing electron tunnelling after a nanogap formation.
To facilitate this study we have developed a feedback controlled electromigration technique to produce nanogaps between two ferromagnetic metal electrodes. In this technique a nanosized metal channel (with dimensions below 200 nm x 100 nm) is broken in a highly controlled manner due to the passage of a large electrical current. A combination of localized heating and momentum exchange between electrons and ions causes a movement of material and thinning of the nanojunction. By using a feedback technique we can control the process so that a 2 nm gap forms. We fabricate these devices using bilayer nanoimprint lithography and standard photolithography.