Dirac tunneling magnetoresistance in a double ferromagnetic graphene barrier structure

The quantum magneto-transport properties of a double ferromagnetic graphene barrier NG/FG/NG'/FG/NG junction are investigated, where NG's are normal graphene layers, NG' is a normal graphene layer of thickness d and FG's are ferromagnetic graphene layers of thickness L. The FG layers with exchange energy H are in contact with the gates of potential energy U. The electrical conductance's (G(q)(P) and G(q)(AP)) for parallel (P) and antiparallel (AP) alignment of the polarization in the two FG layers, respectively, and the tunneling magnetoresistance (TMR) defined as (G(q)(P)-G(q)(AP)/G(q)(P)) x 100% are derived. We find that at zero bias, the oscillatory behavior of the TMR for our multilayer junction has a maximal value of 50% which can be tuned by adjusting the gate voltage and exchange fields in the FG's. The conductance and the TMR's are found to exhibit periodic (oscillatory) depending on the thicknesses of the NG layer and on the value of the ferromagnetic barrier strengths chi(U(H)) defined as U(H)L/hv(F). It is also seen that the amplitudes of oscillation do not decrease as d, U or H increases. This is due to the relativistic nature of the mobile electrons (with the effective speed of light being the Fermi velocity V-F similar to 10(6) m/s) in graphene. For junctions having d = 0, we find that in the AP junction, the quantum conductance modulation due to the combination between spin up and spin down conductance does not appear. The conductance of the AP junction exhibits an oscillatory dependence on chi(H) when d not equal 0. (C) 2009 Elsevier B.V. All rights reserved.