Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H 2 B(pz) 2 } 2 (bipy)] thin films is demonstrated. This isothermal switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric. The adjacent molecular ferroelectric, either polyvinylidene fluoride hexafluoropropylene or croconic acid (C 5 H 2 O 5), appears to lock the spin crossover [Fe{H 2 B(pz) 2 } 2 (bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization. In both a planar two terminal diode structure and a transistor structure, the voltage controlled isothermal reversible spin crossover switching of [Fe{H 2 B(pz) 2 } 2 (bipy)] is accompanied by a resistance change and is seen to be nonvolatile, i.e., retained in the absence of an applied electric field. The result appears general, as the voltage controlled nonvolatile switching can be made to work with two different molecular ferroelectrics: croconic acid and polyvinylidene fluoride hexafluoropropylene.

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H2B(pz)2}2(bipy)] polycrystalline films on Al2O3 substrates have been studied, where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. For [Fe{H2B(pz)2}2(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H2B(pz)2}2(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H2B(pz)2}2(bipy)]/Al2O3 interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperaturedependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H2B(pz)2}2(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.

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.

Using photoemission and inverse photoemission, we have been able to characterize the Cr2O3 oxide surface of CrO2 thin films. The Cr2O3 surface oxide exhibits a band gap of about 3 eV, although the bulk CrO2 is conducting. The thickness of this insulating Cr2O3 layer is twice the photoelectron escape depth which is about 2 nm thick. The effective Cr2O3 surface layer Debye temperature, describing motion normal to the surface, is about 370 K. From a comparison of CrO2 films grown by different techniques, with different Cr2O3 content, evidence is provided that the CrO2 may polarize the Cr2O3.