First Principles Modeling of Tunnel Magnetoresistance ofFe/MgO/FeTrilayers

Abstract

By carrying out density functional theory analysis within the Keldysh non-equilibrium Green's functional formalism, we have calculated the nonlinear and non-equilibrium quantum transport properties of Fe/MgO/Fe trilayer structures as a function of external bias voltage. For well relaxed atomic structures of the trilayer, the equilibrium tunnel magnetoresistance ratio (TMR) is found to be very large and also fairly stable against small variations in the atomic structure. As a function of external bias voltage, the TMR reduces monotonically to zero with a voltage scale of about 1V, in agreement with experimental observations. We present understanding of the nonequilibrium transport properties by investigating microscopic details of the scattering states and the Bloch bands of the Fe leads.PACS numbers: 85.35.-p, 72.25.-b, 85.65.+h Since the prediction and elegant physics explanation [1,2] that magnetic tunnel junction (MTJ) of Fe/MgO/Fe trilayer structure may have very high tunnel magnetoresistance (TMR), MgO based MTJ has progressed at a rapid pace in recent years and produced the highest measured TMR at room temperature: several groups [3,4] reported TMR ratio in the range of 180% to 250%. TMR effect presents an excellent opportunity for spintronics, it is the key to magnetoresistive random-access-memory . There are, however, a number of important issues remain to be understood from atomic first principles. Most existing work predicted[1, 2] TMR to be greater than 1000%, experimental data are still lower. More seriously is that experimental data on MgO based MTJs show a monotonically decreasing TMR as a function of applied bias voltage [3,4] and it reduces to zero when bias is about one volt. To the best of our knowledge, there have been two atomistic calculations of bias dependence of TMR for MgO barriers [7,8], both used the Korringa-Kohn-Rostoker numerical technique. Ref.7 predicted a substantial increase of TMR versus bias for the asymmetric system analyzed there, while Ref.8 found a roughly constant TMR, a decaying TMR, or an entirely negative TMR versus bias depending on atomic structures of the interface. The origin of these differences were not clear. Earlier theory[9] on Al 2 O 3 based MTJs has attributed small bias dependence of magneto-resistance to magnon scattering. Given the extreme importance of MgO based MTJ in near future spintronics and the accumulated experimental data, further quantitative understanding on quantum transport in Fe/MgO/Fe at finite bias is urgently needed.Here we present a first principles atomistic analysis of nonlinear and non-equilibrium quantum transport in Fe/MgO/Fe MTJ. We use a state-of-the-art quantum transport technique [10,11] which is based on real-space, Keldysh nonequilibrium Green's function (NEGF) formalism combined with density functional theory (DFT). The basic idea of the NEGF-DFT formalism [10] is to calculate device Hamiltonian and electronic structure by DFT, populate this electronic structure using NEGF which properly takes into account nonequilibrium q...

I−Vcurves of Fe/MgO (001) single- and double-barrier tunnel junctions

“…The competition between the rates of increase as a function of bias of the P majority and AP currents produces a rapid decay of the TMR, and we calculate V 1/2 being at around 0.7 V. The TMR peaks at zero bias where it reaches up to 290% but, because of its almost exponential decay, it approaches the mean value over the bias range investigated of 160% within only 0.2 V. All of these features are in good agreement with experimental data 15,28,29 and early theoretical results. 30,31 It is important to note also that, at very low bias voltages, the P minority current is slightly larger than the AP current. As it was shown by Rungger et al, 9 this large P minority current near the zero bias is due to the resonance of a Fe minority surface state through the MgO barrier and it is washed out as soon as a voltage is applied across the device.…”

Nonequilibrium spin injection in monolayer black phosphorus

“…It was known that in the operational bias range the TMR ratio monotonically diminishes to zero for MgO based MTJs. 17 For MBP, we found that the spin injection and TMR ratio maintains a relatively large value and independent of a significant range of bias. We investigate physical properties of the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device contact structures where MBP is contacted by Ni(111) and by Ni(100).…”

“…An important device merit is the TMR ratio and much theoretical and experimental efforts have been devoted to create MTJs with different ferromagnetic metals and insulating materials in order to generate a large ratio. While materials such as MgO and Al 2 O 3 are the most popular barrier materials in practical MTJs, [16][17][18][19][20] 2D materials graphene 21,22 and transition-metal dichalcogenides 23,24 have also been investigated in this context.…”

First-principles investigation of transient current of molecular devices by using complex absorbing potential

“…[36,37] Specifically, a linear combination of atomic orbitals (LCAO) is employed to solve KS equations. The exchange-correlation is treated at the LDA level and the nonlocal norm-conserving pseudopotential [38] is used to define the atomic core.…”