Spin-dependent Seebeck effect, spin-dependent Seebeck diode, thermal spin filtering and figure of merit of nitrophenyl diazonium functionalized graphene

Abstract

Spin caloritronic devices are multifunctional devices that combine spintronics with caloritronics and are set to play significant roles in low-power-consumption systems. In this work, the magnetic properties of nitrophenyl diazonium (NPD)-functionalized graphene (NPDG) sheets with NPD coverage ratios of 1:8, 1:18 and 1:32 are presented. The stable ground states of these sheets have ferromagnetic (FM) configurations and their Curie temperature (TC) values are 540 K, 444 K and 157 K, respectively. The thermal spin transport properties of the NPDG sheets (with two transport directions: zigzag edge and armchair edge) with the NPD coverage ratio of 1:8 are presented. For the zigzag edge, when a temperature gradient was produced between two electrodes, the spin-up and spin-down currents were driven in opposite directions, which indicated the appearance of the spin-dependent Seebeck effect (SDSE). In addition, a spin caloritronic device could be obtained in the form of a spin-dependent Seebeck diode (SDSD). For the armchair edge, when the temperature of the left contact increased beyond a critical value, the thermal spin-up current then increased remarkably from zero, while the thermal spin-down current remained approximately equal to zero over the entire temperature region, thus indicating the formation of a thermal spin filter. Finally, in terms of its behavior with respect to the charge current (Ich), the spin-up thermopower (S↑) was found to be nearly equal to the spin-down thermopower (S↓); consequently, the charge thermopower must be nearly zero, thus further demonstrating that the SDSE has been generated. Our results indicate that NPDG sheets can be designed to produce high-efficiency spin caloritronic devices for room temperature operation.