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Graphene for Magnetoresistive Junctions

J. Inoue1, T. Hiraiwa1, R. Sato1, A. Yamamura1, S. Honda2, and H. Itoh3 1

Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan

2

ORDIST, Kansai University, Suita 564-8680, Japan

3

Department of Pure and Applied Physics, Kansai University, Suita 564-8680, Japan

ABSTRACT Influence of the linear energy-momentum relationship in graphene on conductance and magnetoresistance (MR) in ferromagnetic metal (FM)/graphene/FM lateral junctions is studied in a numerical simulation formulated using the Kubo formula and recursive Green's function method in a tight-binding model. It is shown that the contribution of electron tunneling through graphene should be considered in the electronic transport in metal/graphene/metal junctions, and that the Dirac point (DP) is effectively shifted by the band mixing between graphene and metal electrodes. It is shown that MR appears due to spin-dependent shift of DP or spin-dependent change in the electronic states at DPs. It is shown that the MR ratio caused by the latter mechanism can be very high when certain transition metal alloys are used for electrodes. These results do not essentially depend on the shape of the junction structure. However, to obtain high MR ratios, the effects of roughness should be small. INTRODUCTION Magnetoresistance (MR) is a key phenomenon in the field of spintronics. The giant MR (GMR) [1,2] in ferromagnetic metal (FM)/non-magnetic metal/FM junctions and the tunnel MR (TMR) [3,4] in FM/insulator/FM junctions have been widely used for technological applications such as in magnetic sensors and magnetoresistive memories. In GMR, spin-dependent scattering at the interfaces of junctions plays an essential role [5], whereas in TMR the crucial phenomena are spin polarization in FMs and spin-dependent decay rate in insulating spacers [6,7]. Currently, the search for novel combinations of FM and non-magnetic spacers is required to develop advanced spintronics devices. Among various spacer materials, graphene is one of the most attractive for device applications because of its high mobility, long spin-diffusion length, and a planar lattice structure [8-10]. Graphene, which is composed of a two-dimensional honeycomb lattice of carbon atoms, has a characteristic electronic structure near the Fermi level: its conduction and valence bands meet at only two states called Dirac points (DPs) in the Brillouin zone, and its energy momentum dispersion is linear near the DPs. In other words, graphene is a gapless semiconductor in which electrons behave as massless electrons. The electronic structure of graphene is somewhat different from that of conventional non-magnetic metals, semiconductors, and insulators, and therefore, may produce novel features of conductance in junctions when used as a spacer material. It is also expected that the graphene spacers may lead to a novel mechanism of MR in FM/graphene/FM lateral junctions, distinct from those in GMR and TMR junctions. So far, a few

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experiments have been conduct