Magnetic Structure of the Single-Layer Graphene/Nickel Interface

Spintronics is a new field of electronics that may lead to novel devices taking advantage of both the charge and spin of electrons. Graphene-based spintronics is a particularly active area of research because of the great potential of graphene for spin transport as represented by the long spin-diffusion length and extremely high carrier mobility. Efficient injection of spin-polarized currents into graphene is one of the most significant issues in developing graphene-based spintronics. Despite the theoretical prediction of the spin filtering effect of graphene/ferromagnetic metal (FM) interfaces [1], a satisfactory high efficiency of spin injection has not been reported yet [2]. Elucidation of the magnetic structures of graphene/FM interfaces may clarify the underlying reasons for the difficulty of efficient spin injection in graphene-based devices. In the present study, the electronic and magnetic structures of graphene/FM interfaces are being investigated for the bilayer structure of single-layer graphene (SLG) and Ni(111) thin film by applying X-ray magnetic circular dichroism (XMCD) with atomic-layer level depth-resolution [3]. Figures 1(a) and (b) show the Ni L-edge XMCD spectra measured at two different incident angles of the circularly-polarized X-ray beam. The X-ray incident angles of 30° and 60° are sensitive to the magnetic moments aligned in the in-plane and out-of-plane directions, respectively. The XMCD spectra were obtained by collecting the Auger electrons at different values of detection angle θ using a 2D electron detector. Since the effective escape depth of the emitted electrons changes depending on the detection angle due to the scattering attenuation in the sample, the relative fraction of the signals from the Ni atomic layer near the interface increases with decreasing detection angle θ. In Fig. 1(a), the XMCD intensity at θ = 30° (red line) is larger than that at θ = 3° (blue line), and this relationship is inverted in Fig. 1(b). This contrasting behavior indicates that the state of the magnetic moment in the Ni(111) thin film changes depending on the distance from the interface. To gain insight into the magnetic structure of the interface, the total magnetic moment (Mtot) and its angle from the inplane direction were estimated for the Ni atoms near and far from the interface (Niinterface and Nibulk) by applying the magnetic sum rules [4, 5]. The obtained values were Mtot = 0.55μB and = 87° for Niinterface, and Mtot = 0.66μB and = 3° for Nibulk. Mtot of Nibulk agrees well with that of the Ni(111) thin film [6]. This indicates that the Ni mag netic moment shows perpendicular magnetic anisotropy (PMA) at the interface, different from the in-plane orientation in the Ni(111) thin film.