Micro and Nanopatterning of Spin Transition Compounds into Logical Structures

The demand for more efficient information storage is one of the pivotal requirements of knowledge-based development and a prime challenge for current scientific and technological research.[1] Currently, computer hard disks store data by defining the magnetic anisotropy orientation of small regions of a spinning disk. In one of the most successful technological developments of the last decades, scientists and engineers have boosted the capacities of storage devices by shrinking the size of the effective magnetic storage regions.[2] This trend is predicted to continue until the so-called superparamagnetic limit is reached.[3] Close to that frontier, ambient heat can trigger de-orientation of magnetic domains as a result of their reduced size. Among the proposals to overcome this obstacle (e.g. perpendicular recording,[4] antiferromagnetic coupled media,[5] added heaters[6]), nanopatterning of the storage domains seems to be very promising.[7] In this context, a series of different molecular switching units have been tested based on, for example, change in conformation,[8] redox states,[9] spin states,[10] and shape,[11] which can be influenced by external stimuli, such as pressure,[ 12] temperature,[13] magnetic fields,[14] irradiation,[15] and mechanical perturbation.[11] Among the investigated materials, molecular spin-transition (ST) compounds of 3d4 to 3d7 transition-metal ions have been proven to be excellent candidates for application in information technology.[16] Particular interest, both from a fundamental and applicative point of view, was attracted by the ST of FeII ions in an octahedral ligand field, since, by populating the respective t2g and eg sets of d orbitals, the 3d6 valence shell may exist in either the diamagnetic (S=0) low-spin (LS) or the paramagnetic (S=2) high-spin (HS) state. The LS$HS transition can be triggered by external stimuli (temperature, pressure, electromagnetic radiation) giving rise to variations in color, spin state, metal-ligand distance, and dielectric constant.[17] Furthermore, it was shown that steep switching and hysteretic behavior could be realized at ambient conditions.[18] Based on these properties, ST compounds have been proposed to act as active switching units in molecular devices for applications in molecular memory, sensors, and displays.[