Giant Resistance Change across the Phase Transition in Spin-Crossover Molecules

The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green’s function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

Figure 1

Electronic properties of $[\mathrm{Fe}{L}_2{]}^{+2}$, $L=2,2^{\prime }:6,2^{\prime \prime }$-terpyridine, along the reaction coordinates $X$. In (a) and (b) we show, respectively, the $2,2^{\prime }:6,2^{\prime \prime }$-terpyridine group and the cell used for the transport calculations. Color code: C, orange; N, dark blue; H, white; S, green; and Au, yellow. In panel (c) we present the total energy $E$ (full symbols to read on the left-hand side scale) and the HOMO-LUMO gap $\Delta E_g$ (open symbols to read on the right-hand side scale) for both LS and HS along $X$. Calculations are performed with the B3LYP functional.

Figure 2

Zero-bias transmission coefficient as a function energy, $T(E;V=0)$, for both the LS (red dashed line) and the HS (solid black line) configuration of the $\mathrm{Fe}{L}_2$-based junction and the molecular orbitals associated to their HOMO- and LUMO-like orbital. Calculations are performed with the GGA functional.

Figure 3

$I\mathrm{\text{−}}V$ curve for a $\mathrm{Fe}{L}_2$ molecule attached to gold electrodes in either the HS (black solid line) or the LS (red dashed line) state. The current is plotted on a logarithmic scale only for positive currents. The inset shows the SCMR ratio $R_{\mathrm{SCMR}}$. Calculations are performed with the GGA functional.

Figure 4

Transmission coefficients as a function of energy for different values of the gate potential $V_g$ (molecule in the LS state). Here $T(E;V=0)$ is plotted as a color code on a logarithmic scale with red corresponding to low transmission (${10}^{-11}$) and blue to high (${10}^{-1}$). In the inset, we show a schematic figure of the device investigated. The purple atoms represent the region in space where the gate is applied. The color code for the atoms is the same as in Fig. 1b.