Multilevel Resistance Switching and Enhanced Spin Transition Temperature in Single- and Double-Molecule Spin Crossover Nanogap Devices

Spin crossover (SCO) molecules are promising bistable magnetic switches with applications in molecular spintronics. However, little is known about the switching effects of a single SCO molecule when it is confined between two metal electrodes. Here, we examine the switching properties of a [Fe(III)(EtOSalPet)(NCS)] SCO molecule that is specifically tailored for surface deposition and binding to only one gold electrode in a nanogap device. Temperature-dependent conductivity measurements on an SCO molecule containing electromigrated gold break junctions show voltage-independent telegraphic-like switching between two resistance states at a temperature below 200 K. The transition temperature is very different from the transition temperature of 83 K that occurs in a bulk film of the same material. This indicates that the bulk, cooperative SCO phenomenon is no longer preserved for a single molecule and that the surface interaction drastically increases the temperature of the SCO phenomenon. Another key finding of this work is that some devices show switching between multiple resistance levels. We propose that in this case, two SCO molecules are present within the nanogap, with both participating in the electronic transport and switching.