Spin-dependent transport for armchair-edge graphene nanoribbons between ferromagnetic leads

We theoretically investigate the spin-dependent transport for the system of an armchair-edge graphene nanoribbon (AGNR) between two ferromagnetic (FM) leads with arbitrary polarization directions at low temperatures, where a magnetic insulator is deposited on the AGNR to induce an exchange splitting between spin-up and -down carriers. By using the standard nonequilibrium Green's function (NGF) technique, it is demonstrated that, the spin-resolved transport property for the system depends sensitively on both the width of AGNR and the polarization strength of FM leads. The tunneling magnetoresistance (TMR) around zero bias voltage possesses a pronounced plateau structure for system with semiconducting 7-AGNR or metallic 8-AGNR in the absence of exchange splitting, but this plateau structure for 8-AGNR system is remarkably broader than that for 7-AGNR one. Interestingly, the increase of exchange splitting $\Delta$ suppresses the amplitude of the structure for 7-AGNR system. However, the TMR is enhanced much for 8-AGNR system under the bias amplitude comparable to splitting strength. Further, the current-induced spin transfer torque (STT) for 7-AGNR system is systematically larger than that for 8-AGNR one. The findings here suggest the design of GNR-based spintronic devices by using a metallic AGNR, but it is more favorable to fabricate a current-controlled magnetic memory element by using a semiconducting AGNR.Comment: 8 pages, 8 figure

Thermoelectric figure of merit in a quantum wire coupled to a graphene sheet between ferromagnetic leads

We theoretically investigate the figure of merit ZT for a quantum wire side-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Green’s function combining with the tight-binding Hamiltonian, we demonstrate that the ZT for the system develops an oscillating behavior and weakly depends on the wire-graphene coupling strength as well as magnetic configuration of the leads. On the contrary, it is strongly dependent on temperature and the polarization strength of the leads. Importantly, the maximum value of ZT for the system without the polarization strength (p = 0) is about 1.1 at temperature kBT = 0.015Γ0, which is in agreement with the experimental measurements for silicon nanowires

Switching, dual spin-filtering effects, and negative differential resistance in a carbon-based molecular device

We present ab initio calculations for spin-dependent electron transport in a molecular device constructed by two carbon chains capped with a phenyl ring, which is sandwiched between two zig-zag-edged graphene nanoribbon (ZGNR) electrodes, where the ZGNRs are modulated by external magnetic field. The coexistence of switching, dual spin-filtering effects, and negative differential resistance (NDR) in the model device is demonstrated with the theory of carbon π-electrons. Interestingly, a two-state molecular conformational switch can be realized by changing the orientation between the planes of phenyl ring and electrodes, where the majority-spin current modulation (ON/OFF ratio) is 170–479 within the considered bias window. Moreover, the device shows perfect dual spin-filtering effect and can generate and control a full dual spin-polarized current through either the source-drain voltage or magnetic configuration of the electrodes. The selective spin current is due to a dual selection rule, the symmetry match between two ZGNR electrodes spin channel, and the carbon chain’s spin selection in our system. In addition, the obvious NDR behavior has also been observed in our model