Charge transport properties of spin-crossover material Fe(phen)₂(NCS)₂ in polymer

Spin-crossover materials are a paradigmatic example of bi-stability at molecular level which are able to switch their spin states between high-spin (HS) and low-spin (LS) states upon various external stimuli like temperature, pressure, light irradiation, electric field, magnetic field etc. In this work we report the charge transport properties of the promising SCO material Fe(phen)₂(NCS)₂ in organic polymer (polypyrrole). Scanning electron microscopy, X-ray diffraction and optical absorption measurements confirms the structure and growth of the SCO compounds in polymer. The thermal dependence of ac conductivity measurement shows the formation of a hysteresis loop in its cooling and heating cycle. This is due to the difference in conductivity in high spin and low spin state of the material. The size, shape and width of the hysteresis loop are found to be critically dependent on the applied frequency and the SCO: polymer ratio. The ac conductivity of the SCO/pplypyrrole composite samples follows Jonscher' law: σ(ω) ∝ ωⁿ below a certain frequency, called critical frequency, above which it decreases with increasing frequency. The thermal dependence of the exponent n shows there is a decrease of value of n with the increasing of temperature. The variation of critical frequency with temperature also decreases with the increase in temperature. The addition of polymer can play an important role to tune the conductivity of SCO compounds and its spin state dependence characteristics which may be quite helpful for fabricating future spin-based devices. (author)