Ferromagnetic Zigzag Graphene Nanoribbons Research Articles

Spin Seebeck effect and thermal properties of zigzag graphene nanoribbons with edge magnetism

Zigzag graphene nanoribbons (GNRs), demonstrating edge magnetism, are fascinating materials for electronic and spintronic applications. In this paper, we investigate with explicit consideration of electron-phonon scattering the Seebeck effect and thermal conductivity at various carrier densities in zigzag GNRs, which undergo antiferromagnetic, ferromagnetic, and paramagnetic transitions. Seebeck coefficients are spin dependent in ferromagnetic zigzag GNRs and can have opposite signs for the majority and the minority spin carriers, which enables the separation of spin carriers spatially under a thermal gradient. The electronic thermal conductivity is small compared to the lattice thermal conductivity, except for GNRs in the ferromagnetic state. A 100% increase in thermal conductivity is expected at antiferromagnetic to ferromagnetic transition. These results demonstrate that zigzag GNRs are also materials of high interest for spin caloritronics.

H-BN as a perfect spin splitter in ferromagnetic zigzag graphene nanoribbons

The manipulation of spin transport is always a central task in spintronics and two very well-known findings along this line thus far are the observations of half-metallicity and pure spin current, both very important in practical applications. In this paper, we propose another control over spin, termed as a spin splitter, with an aim to split the two spin channels from the spatially overlapped paths into two isolated branches. Based on a density functional study, we implement this idea in ferromagnetic zigzag graphene nanoribbons (ZGNRs) and find that BN nanoribbon embedded in the ZGNR acts as a perfect spin splitter. It originates from the different interfacial dipoles developed at the N–C and B–C interfaces and the subsequent different built-in transverse electric fields, which drives the two narrow side ZGNRs into half-metallicity with different spin polarity. The findings will be of great significance in spintronics.

Vacuum barrier induced large spin polarization, giant magnetoresistance, and pure spin photocurrent in ferromagnetic zigzag graphene nanoribbons

Based on first-principle calculations combined with non-equilibrium Green’s function method, we investigate the spin-polarized transport properties of a two-probe device which consists of a vacuum barrier sandwiched between two semi-infinite ferromagnetic zigzag graphene nanoribbons (ZGNRs). It is well known that any vacuum barrier itself is not spin-polarized, and meanwhile, the infinite pristine ferromagnetic ZGNR presents no spin polarization in the transmission function around the Fermi level. However, our computational results indicate that robust large spin polarization is found in the proposed device. More interestingly, giant magnetoresistance up to can be obtained at low bias voltage. Furthermore, the photogalvanic effect-induced fully spin-polarized photocurrent or pure spin photocurrent can be generated in such device by illumination with polarized light, depending on the symmetry of structure. Consequently, our results indicate that vacuum barrier may show interesting spin-polarized transport properties in certain systems and should be taken into consideration in the construction of molecular junctions and spintronic devices.

Thermal gradient driven spin current in BN co-doped ferromagnetic zigzag graphene nanoribbons

Abstract Based on first principles calculations, we investigate thermal spin transport in a zigzag graphene nanoribbon (ZGNR) device with B and N atoms doped at the opposite edges. It is found that by applying a thermal gradient without a bias voltage across the BN co-doped ZGNR device, spin-up and spin-down charge currents flowing in opposite directions are induced, leading to spin current. Specifically, a giant Seebeck thermopower with opposite signs for the two spin channels is obtained in the linear response regime and the magnitude is dependent on the doping concentration. More importantly, by slightly tuning the chemical potential, pure spin current can be achieved. These findings suggest a feasible way for producing thermal spin current in ZGNRs and will be greatly instructive in the design of graphene based spintronic devices.

Generating pure spin current with spin-dependent Seebeck effect in ferromagnetic zigzag graphene nanoribbons

Pure spin current is of great importance in spintronics and may be achieved by spin dependent Seebeck effect (SDSE) in magnetic systems. Zigzag-edged graphene nanoribbons (ZGNRs) are very well-known 2D magnetic nanostructures. However, perfect and pristine ZGNRs either in the anti-ferromagnetic ground state or in the ferromagnetic (FM) state are not capable of producing pure spin current by SDSE at low temperature. In this work, by density functional theory calculations, we propose a scheme for generating pure spin current using SDSE in FM-ZGNRs by introducing antidots. Specifically, by creating a hexagonal antidot with either armchair edges or zigzag edges in the scattering region, we can get finite Seebeck thermopower for both spin channels with opposite signs, leading to the opposite flow directions of the two spin channels. The mechanism is well explained by the cooperation of the varying localization features of states around the Fermi level and the antidot induced scattering potential. By slightly tuning the chemical potential, pure spin current can be achieved. The size and edge shape effects have also been systematically studied. The findings indicate a novel scheme for thermally generating pure spin current in zigzag graphene nanoribbons and may find important application in graphene based spintronics.

Spin-dependent transport properties of a graphene electrode-single quintuple bond [PhCrCrPh] molecule junction

ABSTRACTWe apply a first-principle computational approach to study the spin-dependent electron transport properties of a single-quintuple bond [PhCrCrPh] molecule with the -bent and linear configurations sandwiched between two ferromagnetic zigzag graphene nanoribbon electrodes. Theoretical results suggest that the current through the -bent configuration is significantly larger than the corresponding linear one. The device shows a perfect two-state molecular conformational switch, and a maximum predicted ON/OFF ratio of currents can be up to around for majority-spin (α)/minority-spin (β) state. Interestingly, the device shows an obvious spin-polarisation behaviour and negative differential resistance effect. The spin filtering efficiency can reach up to 100, and the maximum value of the peak-to-valley ratio of NDR is up to about 46. These results suggest that our proposed device has attractive potential in molecular switch as well as spin-filtering.

Azulene-like molecular devices with high spin filtering, strong spin rectifying, and giant magnetoresistance effects

Abstract By using the nonequilibrium Green's function method combined with density functional theory, we investigate the spin transport properties of an azulene-like molecule sandwiched between two ferromagnetic zigzag graphene nanoribbon (ZGNR) electrodes. The results show that the width of electrodes and the magnetization configurations of electrodes are critical to spin transport. By tuning the spin configurations of ZGNR electrodes, excellent spin filtering, spin rectifying, and giant magnetoresistance behaviors can be obtained simultaneously in devices with 4-ZGNR electrodes. Analysis of symmetric of Bloch wave functions, spin resolved band structures of the electrodes with applied bias, transmission spectra, as well as the projected density of states reveals the mechanism of these intriguing properties. These results might be useful in the design of high performance multifunctional spintronic devices.

Spin Filter and Negative Differential Resistance in Carbon-Based Device with Defects

In this work, we report the electronic transport properties of an atomic carbon chain sandwiched between two ferromagnetic zigzag graphene nanoribbon electrodes with symmetrical nitrogen-vacancy defects using the density functional theory combining with the non-equilibrium Green’s function method. Our results show that a perfect spin filter is observed with almost 100% spin polarization. Moreover, we also see the negative differential resistance effect from the spin-up current under a low positive voltage bias. These results may promise potential applications in spintronic devices with multi-function in the future.

Huge inelastic current at low temperature in graphene nanoribbons

The nonequilibrium Green’s function and the generalized lowest-order expansion method with consideration of electron–phonon interactions (EPIs) are used to investigate the spin-dependent electronic transport properties of ferromagnetic zigzag graphene nanoribbons. Results show that the consideration of EPIs will lead to a 4–5 orders of magnitude increase of the current in some bias regions when the spin polarizations of two electrodes are antiparallel. This results in the vanishing of the dual spin filtration effect and a narrowing of the effective bias region of giant magnetoresistance. The increases of the current mainly from the first Born scattering process, and can be described by the Fermi’s golden rule, and may be a result of the breaking of the structural symmetry by the introduction of phonons.

Enhancement of thermospin effect in ZGNRs via p-n co-doping on edge

Electronic and thermoelectric properties are studied for ferromagnetic zigzag graphene nanoribbons (ZGNRs) co-doped by a pair of boron and nitrogen atoms on one edge. In case of the 1st nearest neighbor co-doping, the two dopants are partly neutralized by each other and the corresponding impurity states approach to the Fermi energy symmetrically from high and low energies. As a result, the electronic properties are effectively manipulated and the conductances of different spin show opposite behaviours. Over a large range from zero temperature to higher-than-room temperature, the system shows a strong spin thermopower with a spin Seebeck coefficient much bigger than the charge one. These findings suggest that doped ZGNRs can be promising materials for spintronic and thermospintronic devices.

Graphene nanoribbon intercalated with hexagonal boron nitride: Electronic transport properties from ab initio calculations

A magnetic tunnel junction consisting of a boron nitride nanoribbon contacted by two semi-infinite electrodes composed of (3,0) ferromagnetic zig-zag graphene nanoribbons was investigated. Performing spin-polarized ab initio transport calculations based on a scheme that combines non-equilibrium Green's function with density functional theory (NEGF+DFT) we predict that such system could act as spin-filter (the efficiency reaches 50%) and present a magnetoristance of 105%, depending on the length of the boron nitride region.

Gate voltage induced spin-filtering effect in a junction based on zigzag graphene nanoribbons

The spin polarized electronic transport in a junction based on ferromagnetic (FM) zigzag graphene nanoribbons (ZGNRs) is investigated by means of the non-equilibrium Green function technique within the mean field Hubbard model. By applying a step-like potential, the spin polarized current can be got, since the band structures of two spin states in FM ZGNRs are non-degenerate and the coherent electron transport obeys the band selective rule. The junction works as a spin-filter and the spin-filtering efficiency can reach the value 100%.