Nonlocal magnetoresistance measurements of the organic zero-gap conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$

We prepared nonlocal spin valves based on single crystals of an organic multilayered zero-gap conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ and succeeded in evaluating the spin-diffusion length (1.1 μm) and relaxation time (3 ns) at 2.5 K under a static pressure of 1.6 GPa using a polyethylene naphthalate as a substrate. Although $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ includes heavy atoms, such as iodine, it exhibited a rather long spin-relaxation time comparable to that of graphene. The spin-orbit interaction (SOI) estimated on the basis of the experimental values of the spin-relaxation time and carrier lifetime (1.2 ps) was 90 mK. The long spin-relaxation time and small SOI evaluated for $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ are considered to originate from its layered structure in which spin scattering induced by surface defects is suppressed. In addition, the inversion asymmetry, which generates an extra term in the equation for the SOI, might be reduced in layered structures. These findings provide guiding principles for materials design in organic spintronics.

Figure 1

(a) Schematic of crystal structure of $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ and nonlocal spin-valve devices. The structural formula of a BEDT-TTF molecule is also shown. (b) Optical microscope image of a lateral spin valve based on $\alpha \text{−}{\mathrm{I}}_3$ fabricated on a flexible PEN substrate. The solid line in the figure represents the edge of the crystal.

Figure 2

Hall effect of an $\alpha \text{−}{\mathrm{I}}_3$ single crystal on a flexible PEN substrate. Dependence of effective carrier density $\left(n_{\mathrm{eff}}\right)$ and mobility $\left({\mu }_{\mathrm{eff}}\right)$ on temperature $\left(T\right)$ derived from the Hall effect in a thin $\alpha \text{−}{\mathrm{I}}_3$ single crystal on PEN under a pressure of 1.6 GPa.

Figure 3

Spin-valve characteristics of local and nonlocal MR. (a) Dependence of the local 2T and nonlocal 4T resistance on a magnetic field at 2.5 K. (b) Dependence of the voltage drop on the bias current at the switching field in the nonlocal 4T configuration. (c) MR ratios in the local 4T and nonlocal 4T measurements. (d) Dependence of change in the nonlocal resistance $\mathrm{\Delta }R$ on gap length.

Figure 4

Dependence of the nonlocal 4T MR on temperature $\left(T\right)$ for a 1300-nm gap spacing. $\mathrm{\Delta }R$ monotonically decreases with increasing $T$.