Article pubs.acs.org/JPCC
Electrical Spin Switch in Hydrogenated Multilayer Graphene Elton J. G. Santos* School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States ABSTRACT: Electric field control of magnetism has become increasingly important for the next generation of graphene-based spintronic devices. We predict that the magnetic moment induced by chemisorbed H atoms on the top layer of a few-layer graphene system is tunable by an external electric field. Through accurate first-principles electronic structure calculations, we show that this magnetoelectric effect is negligible in one-layer graphene, but becomes pronounced in bilayer and trilayer graphene, saturating in magnitude in quadrilayer graphene. The effect is due to shifting of the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, induced by the external field. The calculated magnetoelectric coefficient (α) has values comparable to those found for ferromagnetic films or perovskite interfaces. The value of α was also used to identify a half-metallic state at low gate bias, which suggests a new class of spin-polarized materials based on hydrogenated multilayer graphene. Our results point to an experimentally feasible way to create a magnetoelectric coupling in graphene using the interplay between covalent functionalization and electric fields.
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INTRODUCTION
Graphene functionalization as a means of achieving desired device properties has been the focus of intense interest. For example, functionalization could provide the means of overcoming a major limitation to applications of graphene in electronic devices, that is, the absence of a band gap in graphene. Functionalization often relies on the creation of defects, ranging from vacancies1−4 and metallic substitutional atoms5−7 to light adsorbates8−10 and hydrogen.11−15 Recent experiments have found that hydrogenated graphene can show a band gap of tenths of millielectronvolts,16,17 which is also tunable with an applied electric field.18 Upon hydrogenation, graphene appears to be an n-type material with the possibility to convert the majority carrier type from holes to electrons as a function of hydrogen concentration and bias voltage.18 The functionalization of graphene by H also induces magnetic behavior.2,2,15,19,20 This is a topic of increasing interest since there is great demand for materials that exhibit spin degrees of freedom and are based strictly on light elements such as C (with s and p valence electrons). In this context, of particular importance is the dependence of magnetic properties of hydrogenated graphene layers on external fields, which may provide a powerful tool for manipulating the behavior of the system toward achieving useful device characteristics. In this work, we show that the magnetic properties of hydrogenated graphene multilayers can be manipulated with an external electric field, and this process has direct implications for the electrostatic properties of the layers. We predict that the magnetic moment in hydrogenated monolayer (ML) graphene is not affected by the external field, while bilayer (BL), trilayer (TL), and quadrilayer (QL) graphene exhibit sensitive dependence of the magnetic moment on the external field, as shown in Figure 1. The magnitude of the effect depends on the © 2013 American Chemical Society
Figure 1. Spin moment as a function of the electric field for hydrogenated ML, BL (in AB stacking), TL (in ABA and ABC stackings), and QL (in ABCA stacking) graphene. The H concentration on the hydrogenated layer is 3.1%. The inset shows hydrogenated few-layer graphene in an experimental setup that could be used to confirm our predictions. It is in the same spirit of experiments of McCreary et al.,2 Dlubak et al.,21 and Young et al.22 The electric field is applied perpendicular to the hydrogenated graphene structure via the bottom gate to tune the magnetic moment. Source and drain electrodes are used to detect the spin-polarized current through the surface.
number of layers, saturating in QL graphene, and on the concentration of H atoms. This behavior is explained on the basis of band structure features and their dependence on the electric field. Specifically, in the zero-field case the system is fully spin polarized with a spin-down band and a spin-up band separated by a small gap. Application of the electric field shifts Received: October 22, 2012 Revised: February 26, 2013 Published: March 15, 2013 6420
dx.doi.org/10.1021/jp310463k | J. Phys. Chem. C 2013, 117, 6420−6425
The Journal of Physical Chemistry C
Article
in TL graphene there is no difference in the spin dependence for the ABC and ABA stackings, which in principle can show different behaviors under the influence of an electric field bias, as was recently reported.28−30 The dependence of the spin moment on the electric field saturates in QL graphene. To test if other variables play a role in this tunable magnetic moment mechanism, we considered different impurity concentrations on a BL system and calculated the variation of the magnetic moment as a function of the external field (similar results were also obtained for TL graphene but are not shown here). Figure 2 shows the dependence of the spin modulation
the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, producing a charge transfer between layers, which, for sufficiently large fields, saturates the net spin. The calculated magnetoelectric coefficient has magnitude α ≈ 6.46 × 10−14 (G cm2)/V, which is comparable to that found in ferromagnetic films and perovskite interfaces, indicating a strong response of hydrogenated graphene to the electric field. At finite field the functionalized graphene has half-metallic character as a result of the field-dependent spin density. To quantity this effect, we have defined and calculated the spin capacitance in graphene, similarly to what is used to characterize spintronic devices. We find values of the spin capacitance in hydrogenated graphene which are ∼1 order of magnitude larger that those calculated for complex oxide heterostructures, which suggests a route to a novel type of spincapacitor device based on functionalized graphene, using hydrogen or other common molecules commonly employed in the functionalization of carbon nanostructures.
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METHODS The calculations reported here are based on density functional theory calculations using the SIESTA code,23 with the generalized gradient approximation (GGA) to the exchangecorrelation functional24 and norm-conserving Troullier− Martins pseudopotentials.25 Calculations using the nonlocal van der Waals density functional26 were also performed and give similar results. We used a double-ζ basis set for atomic relaxations and a double-ζ polarized basis for the calculation of the magnetic and electronic properties. Atomic coordinates were allowed to relax using a conjugate-gradient algorithm until all forces were smaller in magnitude than 0.03 eV/Å. Tests performed at better accuracy (
