Spin-orbit-induced torque in a collinear spin valve: A possible route to design fast magnetic memory T. P. Pareek Harish Chandra Research Institute, Chhatnag Road, Jhusi, Allahabad 211019, India Received 7 July 2006; revised manuscript received 25 December 2006; published 9 March 2007 We argue and show that spin-orbit SO interaction can induce a torque for the collinear configuration of spin valve structure F1/2DEG/F2. Further, it is argued that this SO-induced torque can induce magnetization reversal even in collinear spin valves. The theoretical estimates of this effect that are made are well within the reach of present day experimental observations. In addition, it is shown that the same mechanism allows one to increase the purity of the outgoing current and produces entangled spin states essential for quantum com- putation. A scattering theory for the spin-density matrix is developed. Using this theory, the SO-induced torque and the associated von Neumann entropy are studied quantitatively. DOI: 10.1103/PhysRevB.75.115308 PACS numbers: 72.25.Dc, 72.25.Mk Spin transfer magnetization switching 1,2 has been pro- posed as an alternative method for generating high-density magnetic random access memory. Though it has been experi- mentally confirmed in nanopillar devices, 3,4 the application of this method is still limited by the high critical current I c  and switching time required to reverse the magnetization. Switching speed 1/ ,  is switching time in these devices scale as |I - I c |ln , 5 where  is the initial canting between the magnetization of two ferromagnets. The initial canting is important, since for collinear magnetization, the spin transfer torque vanishes and, consequently, switching time becomes infinite. In other words, switching is not possible for collin- ear configuration. To overcome this, many schemes have been proposed to produce initial canting, e.g., application of a field pulse 6 or pulse-shaped microwaves 7 and precharging the device with a dc bias current to excite steady-state pre- cession so that magnetization is very unlikely to be near  =0. 8 In view of the above, we propose an intrinsic mechanism which makes switching possible even for collinear configu- ration. Our mechanism is based on the Rashba spin-orbit RSO interaction, which is a dominant spin-orbit interaction SOI in two-dimensional heterostructures. 9 Consider a col- linear two-dimensional spin valve a two-dimensional elec- tron gas 2DEG sandwiched between two ferromagnetic FU injector and detector contacts FM1/2DEG/FM2. In the absence of SO interaction, the standard spin transfer torque arising due to noncollinearity of magnetization vanishes, 1,2 as discussed above. Therefore, magnetization switching is not possible for collinear spin valve. However, in the presence of SOI, the situation differs dramatically. Since SO interaction causes spin precession and rotation, therefore, the injected spins will develop a component per- pendicular to the initial direction, which is parallel to the magnetization of FM1. Hence, there will be spin torque aris- ing due to SOI even for collinear configuration of an injector and detector ferromagnet. Microscopically, this torque arises because electrons passing through the detector ferromagnet FM2 experiences exchange coupling to the local magneti- zation of FM2. This SOI induced torque will tilt the magne- tization away from the collinear direction, thus producing initial canting intrinsically. As soon as canting is produced, the standard spin transfer torque arising due to noncollinear- ity also starts to act, thus enhancing the total torque acting on the magnetization. Therefore, in our setup, magnetization switching becomes possible even for collinear spin valves. The SOI induced torque achieves two objectives: 1 it makes the switching time finite for collinear spin valves, which is infinite in the absence of SOI torque, and 2 it reduces the magnetization switching time further since it en- hances the total torque acting on the system. Moreover, the SOI induced torque and, consequently, initial canting can be controlled by tuning RSO interaction externally. This is pos- sible in two-dimensional heterostructures, where dominant SO interaction is due to structural asymmetry known as the Rashba spin-orbit interaction. The RSO can be tuned exter- nally by applying a gate voltage. 9 In addition to the above, the SOI has another effect which will enhance the total torque further. Two-dimensional SOI polarizes an incident unpolarized beam perpendicular to the scattering plane. Re- cently, it was shown that this property of SOI can be used to generate pure spin currents. 11 Due to this effect, the polariza- tion of the outgoing current will be higher compared to the incident current, therefore further enhancing the torque act- ing on the detector ferromagnet and, as a consequence, re- ducing the switching time further. Thus, the SO interaction will have a cascade effect on the reduction of the magneti- zation switching time in nanostructures. Further, since the injected electrons are no longer collin- ear with the magnetization of the detector ferromagnet due to SO-induced rotations, they therefore, create entangled spin states essential for solid-state realization of quantum compu- tation. The same mechanism also generates a polarized elec- tron beam from an unpolarized source and, as a consequence, increases the purity of the outgoing current. 12 To quantify these effects, we study von Neumann entropy, which is a measure of entanglement and purity of state. We show that the von Neumann entropy and SOI induced torque are inter- related. Therefore, the seemingly different phenomena of magnetization switching time, spintronics, and quantum computation are related at fundamental level and can be studied within the single framework provided here. To get further insight, we look at the simple example of a quasi-one-dimensional system and give an analytical expres- PHYSICAL REVIEW B 75, 115308 2007 1098-0121/2007/7511/1153086 ©2007 The American Physical Society 115308-1