We present first-principles based calculations of the tunneling conductance and magnetoconductance of epitaxial Fe(100)͉MgO(100)͉Fe(100) sandwiches. Our results indicate that tunneling is much more interesting and complicated than the simple barrier model used previously. We obtain the following general results: ͑1͒ Tunneling conductance depends strongly on the symmetry of the Bloch states in the electrodes and of the evanescent states in the barrier layer. ͑2͒ Bloch states of different symmetry decay at different rates within the barrier. The decay rate is determined by the complex energy bands of the same symmetry in the barrier. ͑3͒ There may be quantum interference between the decaying states in the barrier. This leads to an oscillatory dependence of the tunneling current on k ʈ and a damped oscillatory dependence on barrier thickness. ͑4͒ Interfacial resonance states can allow particular Bloch states to tunnel efficiently through the barrier. For Fe(100)͉MgO(100)͉Fe(100) our calculations indicate that quite different tunneling mechanisms dominate the conductance in the two spin channels. In the majority channel the conductance is primarily via Bloch electrons with small transverse momentum. One particular state with ⌬ 1 symmetry is able to effectively couple from the Fe into the MgO. In the minority channel the conductance is primarily through interface resonance states especially for thinner layers. We predict a large magnetoresistance that increases with barrier thickness.

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, T k , 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. DOI: 10.1103/PhysRevLett.97.237205 PACS numbers: 85.75.ÿd, 72.10.ÿd, 72.25.ÿb, 73.40.Gk Theoretical calculations predict that when a spinpolarized current passes through a magnetic multilayer structure, whether spin valve [1] or magnetic tunnel junction, (MTJ) [2,3], it can transfer spin angular momentum from one ferromagnetic electrode to another, and hence exert a torque on the magnetic moments of the electrodes. At sufficiently high current densities, this spin transfer can stimulate spin-wave excitations [4,5] and even reverse the magnetization of an individual domain [6]. Currentinduced magnetic switching (CIMS) has now been confirmed in numerous experiments both in spin valves [6,7] and more recently, in MTJs [8,9]. Thus, CIMS provides a powerful new tool for the study of spin transport in magnetic nanostructures. In addition, it offers the intriguing possibility of manipulating high-density nonvolatile magnetic-device elements, such as magnetoresistive random access memory (MRAM), without applying cumbersome magnetic fields [10].While the fundamental physics underlying the spin transfer torque (STT) in spin valves has been extensively studied theoretically [1,[11][12][13], its role in MTJs remains an unexplored area thus far, except for the pioneering work of Slonczewski [2,3], who employed the free-electron model in the low bias regime. One of the most pressing needs is a comprehensive understanding of the bias dependence of the STT in MTJs, which will be important for the development of MRAM that uses CIMS for writing the magnetic memory cell.In this Letter, we present for the first time a comprehensive study of the effect of bias on the spin torques, parallel (T k ) and perpendicular (T ? ) to the interface, in MTJs, using tight-binding (TB) calculations and the nonequilibrium Keldysh formalism. We predict an anomalous bias dependence of the spin torque, contrary to the general consensus. We demonstrate first that depending on the exchange splitting, T k may exhibit an unusual nonmonotonic bias dependence: it may change sign without a sign reversal in bias or current, and it may even have a quadratic bias dependence. Second, by generalizing the equivalent circuit in Ref.[3] using angular-dependent resistances, we show that T k satisfies an expression involving the difference in spin currents between the ferromagnetic (FM) and antiferromagnetic (AF) configurat...

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