Dielectric Transition-Instigated Extraordinary Capacitance Change with Successive Switching in Loss Tangent at the Spin Transition Temperatures of a Spin-Crossover Nanomaterial

The magnetic transition in spin-crossover (SCO) materials with temperature is a dynamic and reversible process, in which the transition metal ions change from a low spin (LS) state to a high spin (HS) state. This work reports extraordinary dielectric transitions at 342 and 382 K reflected in a capacitance fabricated with a Fe-based SCO material. These transitions are predominantly instigated in its nanomaterial phase in thin film-based devices. However, similar devices fabricated with bulk material did not show any significant changes. The associated dielectric transitions occur at the magnetic transition temperature without following their transition sequence. We observed an extraordinary shift in capacitance (ΔC) at the transition temperature of about 1100 nF/cm2, which is remarkable. Additionally, over a 105-fold enhancement in capacitance per unit volume is observed in devices fabricated with nanomaterial compared to those made with bulk powder. Such noteworthy changes could be due to the interface-induced agglomeration of the SCO nanomaterial at a macroscopic length scale within the 200 nm thick film and the polarization of counterions within the two-dimensional confined structures. More significantly, successive switching in the loss tangent from high to low and vice versa is observed, widening the applications of SCO materials in emerging directions.