Anomalous Bias Dependence of Spin Torque in Magnetic Tunnel Junctions

We predict an anomalous bias dependence of the spin transfer torque parallel to interface, $T_{||}$, in magnetic tunnel junctions (MTJ), which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal {\it without} a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.Comment: 4 pages, 5 figure

Impurity-induced tuning of quantum well states in spin-dependent resonant tunneling

We report exact model calculations of the spin-dependent tunneling in double magnetic tunnel junctions in the presence of impurities in the well. We show that the impurity can tune selectively the spin channels giving rise to a wide variety of interesting and novel transport phenomena. The tunneling magnetoresistance, the spin polarization and the local current can be dramatically enhanced or suppressed by impurities. The underlying mechanism is the impurity-induced shift of the quantum well states (QWS) which depends on the impurity potential, impurity position and the symmetry of the QWS.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Unveiling temperature dependence mechanisms of perpendicular magnetic anisotropy at Fe/MgO interfaces

The perpendicular magnetic anisotropy (PMA) at magnetic transition metal/oxide interfaces is a key element in building out-of-plane magnetized magnetic tunnel junctions for spin-transfer-torque magnetic random access memory (STT-MRAM). Size downscaling renders magnetic properties more sensitive to thermal effects. Thus, understanding temperature dependence of magnetic anisotropy becomes crucial. In this work, we theoretically address the correlation between temperature dependence of PMA and magnetization in typical Fe/MgO-based structures. In particular, the possible mechanisms behind experimentally reported deviations from the Callen and Callen scaling power law are analyzed. First-principles calculations reveal small high-order anisotropy terms ruling out an intrinsic microscopic mechanism underlying those deviations. Neglecting higher-order anisotropy terms in the atomisitic spin Hamiltonian, two possible extrinsic macroscopic mechanisms are unveiled: influence of the dead layer, always present in storage layer of STT-MRAM cells, and spatial inhomogeneities of interfacial magnetic anisotropy. We show that presence of a dead layer simultaneously with scaling the anisotropy constant by the total magnetization of the sample rather than that of the interface itself lead to low scaling powers. In the second mechanism, increasing the percentage of inhomogeneity in the interfacial PMA is revealed to decrease the scaling power. Apart from those different mechanisms, the layer-resolved temperature-dependence of PMA is shown to ideally follow the Callen and Callen scaling power law for each individual Fe layer. These results allow coherently explaining the difference in scaling powers relating anisotropy and magnetization thermal variations reported in earlier experiments. This is crucial for the understanding of the thermal stability of the storage layer magnetization in STT-MRAM applications.Comment: 9 pages, 8 figure

Enhancing spin-transfer torque through the proximity of quantum well states

We predict that the spin-transfer, Ti,k, and fieldlike, Ti,_, components of the local spin torque are dramatically enhanced in double-barrier magnetic tunnel junctions. The spin-mixing enhancement is due to the energetic proximity of majority and minority quantum well states (QWSs) of different quantum numbers within the bias window. The local-spin-torque enhancement is not associated with a corresponding enhancement of the spinpolarized currents. Ti,k exhibits a switch-on and switch-off steplike bias behavior when spin-polarized QWSs enter the bias window or exit the energy band, while Ti,_ changes sign between switch-on biases. The net T_ exhibits an anomalous angular behavior due to the bias interplay of the bilinear and biquadratic effective exchange couplings.Physical Review B 76(22), 224406. (2007)1098-012

Spin-transfer torque in magnetic tunnel junctions

We present a theoretical study of the spin-transfer torque vector and the tunneling magnetoresistance (TMR) for symmetric magnetic tunnel junctions (MTJ) using the single-band tight-binding model and the nonequilibrium Keldysh formalism. We provide a comprehensive analysis of the effect of band filling and exchange splitting of the FM leads on the bias behavior of the spin-transfer component, Tk, in the plane containing the magnetizations of the two magnetic layers, and the fieldlike component, T_, perpendicular to this plane. We demonstrate that both components of the spin torque and the TMR can exhibit a wide range of interesting and unusual bias behavior. We show that Tk(V) satisfies an expression involving the difference in spin currents between the ferromagnetic (FM) and antiferromagnetic (AF) configurations, which is general and independent of the details of the electronic structure. The spin current for the FM (AF) alignment is shown to have a linear (quadratic) bias dependence, whose origin lies in the symmetric (asymmetric) nature of the barrier. On the other hand, the bias dependence of T_ is quadratic with d2T_/dV20, and it can change sign at finite bias. Finally, we show that the exchange splitting and band filling have a large effect on the bias dependence of the TMR.Physical Review B 79(17), 174416. (2009)1098-012

Effect of disorder on spin-transfer torque in magnetic tunnel junctions

We have generalized the nonequilibrium Green's functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, , in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.Journal of Applied Physics 109(7), 07C920. (2011)0021-897

Anomalous bias dependence of spin torque in magnetic tunnel junctions

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, Tk, in magnetic tunnel junctions, which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.Physical Review Letters 97(23), 237205. (2006)0031-900

Voltage Dependence of Spin Transfer Torque In Magnetic Tunnel Junctions

Theoretical investigations of spin transfer torque in magnetic tunnel junctions using the tight-binding model in the framework of nonequilibrium Green functions formalism are presented. We show that the behavior of the spin transfer torque as a function of applied voltage can vary over a wide range depending on the band parameters of the ferromagnetic electrodes and the insulator that comprise the magnetic tunnel junction. The behavior of both the parallel and perpendicular components of the spin torque is addressed. This behavior is explained in terms of the spin and charge current dependence and on the interplay between evanescent states in the insulator and the Fermi surfaces of ferromagnetic electrodes comprising the junction. The origin of the perpendicular (field-like) component of spin transfer torque at zero bias, i.e., exchange coupling through the barrier between ferromagnetic electrodes is discussed.IEEE Transactions on Magnetics 44(11), 2543-2546. (2008)0018-946