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...

We present a theoretical study of the spin-transfer torque vector and the tunneling magnetoresistance ͑TMR͒ for symmetric magnetic tunnel junctions ͑MTJ͒ 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, T ʈ , in the plane containing the magnetizations of the two magnetic layers, and the fieldlike component, T Ќ , 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 T ʈ ͑V͒ satisfies an expression involving the difference in spin currents between the ferromagnetic ͑FM͒ and antiferromagnetic ͑AF͒ configurations, which is general and independent of the details of the electronic structure. The spin current for the FM ͑AF͒ alignment is shown to have a linear ͑quadratic͒ bias dependence, whose origin lies in the symmetric ͑asymmetric͒ nature of the barrier. On the other hand, the bias dependence of T Ќ is quadratic with d 2 T Ќ / dV 2 Ͻ 0, 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.

We predict an oscillatory bias behavior of the fieldlike spin torque, T ? , in magnetic tunnel junctions, which can be selectively controlled via the asymmetry in band filling between the ferromagnetic leads. This can lead to a linear or quadratic low-bias behavior, including tuning the bias-induced reversal of T ? . These findings reconcile the apparently contradictory experimental results recently reported in the literature. The underlying mechanism for the nonequilibrium interlayer exchange coupling (IEC) of noncollinear configurations is the interplay of four independent IEC for the majority-and minority-spin bands of the leads solely in the ferromagnetic configuration. [3][4][5][6][7] and theoretically [8][9][10][11]. The CIMR offers the promise for making nonvolatile magnetoresistive random access memory devices (MRAM), in which information is written using the STT effect rather than the field-induced magnetic switching.The spin torque can be decomposed into a fieldlike, T ? , and a spin transfer, T k , components both orthogonal to the magnetization of the free ferromagnetic (FM) lead, where the first (latter) are perpendicular (parallel) to the plane of the magnetizations of the left and right FM leads, but with different bias behavior. For example, while recent experiments indicate [5][6][7] that T k reverses sign on changing the current direction, we have recently predicted an anomalous bias behavior, where T k can exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence [8]. Unlike spin valves, where T ? ( T k , T ? % T k in MTJ, thus playing also an important role in the CIMR [5][6][7].The bias behavior of T ? , directly related to the nonequilibrium interlayer exchange coupling (IEC), remains unresolved and controversial. On the theoretical side, the pioneering work of Slonczewski [1] showed that for symmetric MTJ at zero bias, T ? can change sign with decreasing potential-barrier height. Recent calculations [8,10,12] have predicted a purely quadratic bias dependence of T ? in symmetric MTJ, with d 2 T ? ðVÞ=dV 2 < 0 for any value of band filling (BF) and exchange interaction. On the experimental side, Sankey et al. The purpose of this Letter is to understand the underlying electronic mechanism that controls the bias behavior of T ? , and reconcile the origin of the experimental controversies, without invoking the recently proposed inelastic effects [13]. We predict that T ? oscillates with bias which can be tuned via the MTJ asymmetry. We find an interesting low-bias behavior of IEC, ranging from linear to quadratic bias dependence, with positive or negative bias curvature, including tuning the bias-induced reversal of T ? . We derive a novel general expression relating the bias behavior of T ? in noncollinear MTJ with that of collinear [FM and antiferromagnetic (AFM)] configurations, independent of the details of the electronic structure. We demonstrate that the wide range of bias behavior of IEC in noncollinear MTJ can be understood by the int...

A femtosecond-laser pulse can induce ultrafast nonthermal melting of various materials along pathways that are inaccessible under thermodynamic conditions, but it is not known whether there is any structural modification at fluences just below the melting threshold. Here, we show for silicon that in this regime the room-temperature phonons become thermally squeezed, which is a process that has not been reported before in this material. We find that the origin of this effect is the sudden femtosecond-laser-induced softening of interatomic bonds, which can also be described in terms of a modification of the potential energy surface. We further find in ab initio molecular-dynamics simulations on laser-excited potential energy surfaces that the atoms move in the same directions during the first stages of nonthermal melting and thermal phonon squeezing. Our results demonstrate how femtosecond-laser-induced coherent fluctuations precurse complete atomic disordering as a function of fluence. The common underlying bond-softening mechanism indicates that this relation between thermal squeezing and nonthermal melting is not material specific.