Nonlinear Spin Current and Magnetoresistance of Molecular Tunnel Junctions

We report on a theoretical study of spin-polarized quantum transport through a Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction (MTJ). Our study is based on carrying out density functional theory within the Keldysh nonequilibrium Green’s function formalism, so that microscopic details of the molecular MTJ are taken into account from first principles. A magnetoresistance ratio of $\sim 27\%$ is found for the Ni-BDT-Ni MTJ which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states and magnetic leads.

Figure 1

(a) Schematic plot of the Ni-BDT-Ni device. The Ni leads have infinite extent in the ($x,y$) direction and extends to $z=\pm \infty$ in the (100) orientation. Each unit cell in the leads has a linear size of 7.04 Å in the ($x,y$) direction and includes 32 Ni atoms. Three layers of Ni atoms in each lead are included in the center region of the device. (b) TMR $R_{\mathrm{TMR}}\equiv (I_{\mathrm{APC}}-I_{\mathrm{PC}})/I_{\mathrm{APC}}$ vs bias voltage for a device where BDT is bond at the hollow site of the Ni surface. Inset: TMR for bridge site bonding. (c) $I\mathrm{\text{−}}V$ curves for PC setup of the leads’ magnetization. Solid line: total current; dashed line: $I_{\uparrow }$; dotted line: $I_{\downarrow }$. (d) $I\mathrm{\text{−}}V$ curves for APC (antiparallel configuration) setup. Inset: spin-injection coefficient $\eta$ vs bias voltage $V_b$, solid line: APC; dashed line: PC (parallel configuration).

Figure 2

(a) Transmission coefficient $T_{\sigma }$ vs energy $E$ for $V_b=0$. Data are collected with energy spacing of 0.05 eV. $E=0$ is the Fermi energy of the leads. Solid: $T_{\uparrow }$ for PC setup; dashed: $T_{\downarrow }$ for PC; dotted: $T_{\uparrow }=T_{\downarrow }$ for APC. Insets: transmission coefficients at energies around 2 eV and $-3\mathrm{eV}$. (b) DOS for the scattering region of the device in PC. (c), (d) Transverse momemtum resolved transmission coefficient $T_{\uparrow }^{k_x,k_y}$ and $T_{\downarrow }^{k_x,k_y}$ vs $k_x,k_y$ at $E_F$ for PC setup, respectively. The vertical color coding bar is for the values of $T_{\sigma }$.