Quantitative Study of the Energy Changes in Voltage-Controlled Spin Crossover Molecular Thin Films

Voltage-controlled nonvolatile isothermal spin state switching of a [Fe{H₂B(pz)₂}₂(bipy)] (pz = tris(pyrazol-1-1y)-borohydride, bipy = 2,2'-bipyridine) film, more than 40 to 50 molecular layers thick, is possible when it is adsorbed onto a molecular ferroelectric substrate. Accompanying this high-spin and low-spin state switching, at room temperature, we observe a remarkable change in conductance, thereby allowing not only nonvolatile voltage control of the spin state ("write") but also current sensing of the molecular spin state ("read"). Monte Carlo Ising model simulations of the high-spin state occupancy, extracted from X-ray absorption spectroscopy, indicate that the energy difference between the low-spin and high-spin state is modified by 110 meV. Transport measurements demonstrate that four terminal voltage-controlled devices can be realized using this system.

Phase Stability of Spin-Crossover Nanoparticles Investigated by Synchrotron Mössbauer Spectroscopy and Small-Angle Neutron Scattering

Spin-crossover nanomaterials have been actively studied in the past decade for their potential technological applications in sensing, actuating, and information processing devices. Unfortunately, an increasing number of the metallic centers become inactive at reduced sizes, presumably due to surface effects, limiting their switching ability and thus the scope of applications. Here we report on the investigation of "frozen" metallic centers in nanoparticles (2-80 nm size) of the spin-crossover compound Fe(pyrazine)[Ni(CN)₄]. Magnetic measurements reveal both high-spin and low-spin residual fractions at atmospheric pressure. A pressure-induced transition of the high-spin residue is observed at around 1.5 GPa by synchrotron Mössbauer spectroscopy. We show that it is equivalent to a downshift of the transition temperature by ca. 400 K due to the size reduction. Unexpectedly, small-angle neutron scattering experiments demonstrate that these high-spin residual centers are not confined to the surface, which contradicts general theoretical considerations.

Pressure and Temperature Sensors Using Two Spin Crossover Materials

The possibility of a new design concept for dual spin crossover based sensors for concomitant detection of both temperature and pressure is presented. It is conjectured from numerical results obtained by mean field approximation applied to a Ising-like model that using two different spin crossover compounds containing switching molecules with weak elastic interactions it is possible to simultaneously measure P and T. When the interaction parameters are optimized, the spin transition is gradual and for each spin crossover compounds, both temperature and pressure values being identified from their optical densities. This concept offers great perspectives for smart sensing devices.

A chiral spin crossover metal–organic framework

A rare chiral spin crossover metal–organic framework, [Fe^II(mptpy)₂]·solv, has been solvothermally synthesized through spontaneous resolution.