- Nano Express
- Open Access
Spintronic Transport in Armchair Graphene Nanoribbon with Ferromagnetic Electrodes: Half-Metallic Properties
© The Author(s). 2016
- Received: 19 July 2016
- Accepted: 5 October 2016
- Published: 13 October 2016
Utilizing first-principles theory, we demonstrate that half-metallicity can be realized in a junction composed of non-magnetic armchair graphene nanoribbon (AGNR) and ferromagnetic Ni electrodes. The half-metallic property originates from the AGNR energy gap of the up spin located at the Fermi energy, while large electronic states are generated for the down spin. By altering the interlayer distance and the contact area, namely, the strength of AGNR-Ni interaction, the efficiency of the spin filter becomes lower, since the energy gap moves away from the Fermi energy with the variation of charge transfer intensity.
- Graphene nanoribbon
- Graphene-nickel contact
- Spin filter
- Electron transport
Spintronic nanodevices, which are nanoscale devices utilizing spin degrees of freedom of electrons, have attracted a significant amount of attention. One of the significant functions of spintronics is the spin filter effect. High spin filter efficiency is expected to be realized in half-metallic materials with one metallic spin component and the other semiconducting or insulating spin channel. Up to now, half-metallic property has been found not only in some ferromagnetic metals, such as manganese perovskites  and Heusler compounds , but also in some metal-free materials, for example, carbon nanomaterials [3, 4] and graphitic carbon nitride . Exploring half-metallic materials is of great interest in future spintronic devices, but it still remains a challenge due to the requirement of unique spin-asymmetric electronic states.
Carbon-based nanomaterials, such as graphene, are expected to be promising materials for the realization of spintronics due to the weak spin-orbit coupling and long spin scattering length . The carbon atoms on two sublattices are spin-polarized as two-dimensional (2D) graphene contacts with ferromagnetic metal (FM) [7–9]. Spin-polarized transport behaviors were predicted theoretically in a 2D graphene-FM interface, for both in-plane [10–13] and out-of-plane electron transport [14, 15]. On the other hand, FM-graphene-FM-based spin valve devices were also fabricated experimentally to measure the spin-polarized transport in the direction perpendicular to the graphene plane [16–19]. More recently, both theoretical  and experimental studies  have found efficient spin injection for the FM-graphene system with the insertion of h-BN layers, though the spin-polarized current is greatly reduced due to the high tunneling barrier of h-BN . However, 2D graphene has conical points located at the Fermi energy (E F) with zero density of states; the gapless property limits its application in half-metallic materials.
It is well known that the energy gap can be engineered by cutting a 2D graphene sheet into a one-dimensional (1D) graphene nanoribbon (GNR), where the edge carbon atoms are passivated by hydrogen. Recently, this nanometer-wide GNR with atomically precise width can be achieved via a surface-assisted bottom-up fabrication [22, 23]. Owing to the quantum confinement, electronic states of GNR are mainly governed by the boundary conditions [24, 25]. Consequently, the function of such a graphene nanoribbon-based device is strongly dependent on the edge structures.
One typical structure of the nanoribbon is the zigzag edge, referred to as a zigzag graphene nanoribbon (ZGNR), which shows flat-band magnetism induced by peculiar localized electronic states at each edge. Recently, the first-principles calculations predicted that anti-ferromagnetic ZGNR shows half-metallicity at a finite external electric field across the ribbon [3, 4]. Motivated by this approach, some alternative methods are also proposed to drive ZGNRs into the half-metallic state, such as edge modification by organic molecules , doping B/N atoms [27, 28], and adsorption ferroelectric polymer , acceptor/donor functional groups , or symmetric- and asymmetric-edge hydrogenations . All these studies indicate that ZGNRs hold promising applications in the spintronic nanodevice.
The other signature shape is the armchair edge, termed as an armchair graphene nanoribbon (AGNR). Although AGNR has a band gap at the Fermi energy depending on the ribbon width, the electronic structure of AGNR is not spin-polarized . Therefore, it is hard to fabricate spintronic devices based on the free-standing AGNR without other modification to control the spin of electrons. On the other hand, when AGNR is adsorbed on metals, such as Au, Cu, and so on, the electronic structure is modified by the interfacial coupling between the metal surface and graphene [23, 33]. Especially contacting with ferromagnetic metal onto the AGNR layer seems to be a feasible way of introducing spin polarization of the AGNR. However, the possibility of the AGNR system as spintronics has not been fully investigated.
Figure 1 illustrates the studied model of a Ni/AGNR/Ni junction, constructed from three layers of Ni atoms with AGNR adsorbed on the surface. The unit cell used to model the junction is shown by the dotted rectangle in Fig. 1a. In the contact region, one carbon atom locates above a surface Ni atom and the other carbon is above a third-layer Ni atom. The contact area contains three repeating units at each side of the AGNR, and the junction is labeled as M3. The geometric structure of the contact region was firstly optimized with a 1 × 1 unit cell of graphene on three layers of Ni (111) surface (red diamond in Fig. 1a) using the density functional theory (DFT) code PHASE . We used the PW91 functional parametrized by Perdew and Wang for the exchange-correlation term  and the Troullier-Martins-type atomic pseudopotentials [36, 37]. A plane-wave basis set with cutoff energy of 25 Ry was employed. A k-mesh of 20 × 20 × 1 was adopted to sample the Brillouin zone (BZ) for structural relaxations. During structural relaxation, the atoms are relaxed until the total energy change is less than 10−9 hartree and the force on each atom is smaller than 10−3 hartree/bohr.
In this structure optimization, we fixed the lattice constant of graphene to be the optimized value, a = 2.458 Å, adopting the same value for lattice constant of the Ni electrodes [38, 39]. With this orientation, the lattice mismatch between graphene and Ni(111) surfaces is about 1.3 %. Both C atoms and Ni atoms (except the Ni atoms in bottom layer) were relaxed to release the strain induced by lattice mismatch. The interlayer distance between the graphene and the Ni(111) surface is optimized to be 1.995 Å, as noted by d eq in Fig. 1(b). This predicted distance of Ni-graphene is close to the experimental value (2.11 ± 0.07 Å) , previous LDA reports (2.05 Å) [7, 41] and the DFT calculation with van der Waals (vdW) correction (2.07 Å) , in which vdW density functional together with the C09 exchange functional was used.
Cutoff radius r c of the pseudoatomic orbitals and number of primitive orbitals for s-, p-, and d-orbitals (n s, n p, n d)
r c (a.u.)
We used k-point N k = 201 in the periodic x direction, and 1 in the y and z directions. The energy cutoff and k-point convergences were performed for all calculations.
As mentioned above, to realize this half-metallicity, it is necessary to drive the gap of the up spin to reside at E F. The gap location is determined by the strength of interfacial interaction which directly correlates with, for example, the interlayer distance between AGNR and the metal surface and the contact area, because the interaction strength can alter the amount of charge transfer which relates to the gap location. This indicates that the efficiency of the spin filter might be strongly dependent on the interaction strength.
At last, it is emphasized that the present system utilizes the non-magnetic carbon nanomaterial in contact with magnetic substrates, distinct from previously predicted half-metals with intrinsic magnetism, such as manganese perovskites , Heusler compounds , and recently discovered metal-free half-metals (for example, graphitic carbon nitride) . These results may bring us the possibility of fabricating spintronic nanodevices based on non-magnetic 1D graphene nanoribbon by interfacial manipulation.
In summary, we observe the half-metallic property in an armchair graphene nanoribbon in contact with Ni electrodes. The junction exhibits a spin injection value of −0.98, indicating a nearly insulating behavior for up spin and a metallic behavior for down spin. This spin-filtering effect originates from the following mechanisms: Owing to the interaction between AGNR and Ni states, the AGNR energy gap of up spin is located at E F, suppressing the up-spin transmission, while a large transmission of down spin is observed at E F. The efficiency of the spin filter varies with the AGNR gap location, which correlates with the charge transfer intensity determined by the interaction strength between graphene and the Ni surface, that is, the interlayer distance and contact area. This device design suggests a potential application of AGNR-based materials in spintronic nanodevices.
This work was supported by the RISS Project of the IT program of MEXT and the grant for “Strategic Programs for Innovative Research” from MEXT of the Japanese Government. H. Liu acknowledges the support of the National Natural Science Foundation of China (Nos. 11204120, 11274151, 51431004). We thank the Numerical Materials Simulator in NIMS for providing the computing resources.
HML carried out the first-principles calculations, analyzed the data, and wrote the manuscript. HK participated in the data analysis and manuscript preparation. TO conceived the idea and supervised the project. All authors discussed, read, and approved the final manuscript.
The authors declare that they have no competing interests.
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