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Resonant Spin Transfer Torque in Double Barrier Magnetic Tunnel Junctions
Ioannis Theodonis 1* , Alan Kalitsov 2 , Nicholas Kioussis 3
1 Department of Physics, National Technical University Athens, Zografou Campus 15780, Greece
2 Theoretische Physik, Universität Kassel, Heinrich-Plett-Strasse 40, 34132 Kassel, Germany
3 Department of Physics, California State University Northridge, CA 91330-8268, USA
* ytheod@mail.ntua.gr
Current-induced magnetization switching (CIMS) [1] in single-barrier magnetic tunnel junctions (MTJ) has attracted much
scientific interest due to its potential applications in future spintronics (magnetoelectronics) devices [2], such as magnetic
random access memories (MRAM) [3] and high frequency oscillators [4]. The central idea in CIMS is of a spin transfer
torque that is exerted on the magnetization of a
nanometer-scale free ferromagnet (FM) by a spinpolarized
current originating from a preceding noncollinear
FM [1]. A limiting factor in single-barrier
MTJ is the rather high value of critical current
required to induce the magnetization switching, due to
the low efficiency of the spin transfer torque. On the
other hand in double barrier magnetic tunnel junctions
(DBMTJ) both theoretical [5,6] and experimental
[7,8] studies of spin-dependent transport in collinear
configurations, have shown that resonant tunneling
Figure 1: Schematic of the DBMTJ (FM/I/FM/I/FM) consisting of controls and can enhance the tunneling
a FM central wire of N C atomic sites (AS), connected to left and magnetoresistance (TMR) [9]. In this work, we
right FM leads through the tunneling barriers I. The spin-transfer propose the use of resonant effect to drastically
(parallel) T i,|| and the field-like(perpendicular) T i,┴ , components of enhance the spin torque efficiency [10]. The
the spin torque lie in the x and y directions respectively.
calculations are based on the tight-binding method
and the non-equilibrium Keldysh formalism.
The DBMTJ systems consist of a central FM nano-wire (FMC) sandwiched between two thin non-magnetic tunnel barriers
(I), themselves sandwiched between two semi-infinite ferromagnetic leads (FML,FMR), as shown in Figure 1. The
magnetization of the central FM, M C , is along the z axis of the coordinate while the magnetization of the FM leads M L(R) lies
in the x−z plane, i.e. it is rotated by angle θ around the wire axis y. In this geometry, the central free FM layer forms a spin–
polarized quantum well. The majority-(full triangles) and minority-(open triangles) quantum well states (QWS) energies E σ n,
relative to the Fermi energy, as a function of the thickness N C in atomic sites (AS), of the central FM wire are shown in
Figure 2 for QWS between -0.5 eV and 0.5 eV. The numbers next to each series of data points, indicate the quantum number,
n σ = 1 σ ,2 σ ...,N σ C, of the spin-polarized QWS.
Figure 2: Spin-polarized QWS energy positions, E σ n as a function of the number of atomic sites, N C , in the central FM region
for zero bias and θ=0. The bottom of the majority and minority conduction bands of the leads are denoted by dotted lines. At
finite bias V, the chemical potentials of the L,R leads are shifted by eV=µ R -µ L around the Fermi energy E F =0.
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