Spin Torque Writing for Next Generation MRAM - Challenges and Prospects

Abstract

Summary form only given. The initial predictions and subsequent definitive demonstrations of the ability of a spin-polarized current that impinges onto a thin film nanomagnet to reversibly switch the orientation of its magnetic moment, and/or to excite it into microwave precession, by the torque exerted through the transfer of spin angular momentum from the incident conduction electrons, have catalyzed what is now a quite broad and very active area of spin torque research. Since those initial works, there has been remarkable progress in advancing the fundamental understanding of this new spintronics phenomenon, and in successfully moving it towards technological implementations, particularly spin-torque magnetic random access memory (ST-MRAM) and spin-torque microwave oscillator applications, that have the potential for broad impact. In this presentation I will discuss the basics of the spin torque effect and briefly survey the various approaches that can be employed to demonstrate and study it in both all-metallic, spin-valve-type nanostructures and, most importantly for applications, in nanoscale magnetic tunnel junctions (MTJs). I will discuss some recent work that has sought to contribute to the rapidly advancing spin-torque research effort, by helping both to better understand quantitatively the details of the phenomenon and to understand and enhance the efficiency of the effect I will summarize results from studies of spin-transfer switching and microwave excitation in magnetic tunnel junctions (MTJs), which include the use of the spin transfer phenomenon to quantitatively determine, through spin-torque- excited ferromagnetic resonance (ST-FMR), both the bias dependent efficiency of the spin torque in high quality MTJs and the magnetic damping of individual free layer nanomagnets. I will also briefly describe some recent work that has been examining spin torque effects in nanoscale structures with distinctly non-uniform magnetization which is giving us some new information regarding magnetic dynamics in nanoscale structures and are pointing to ways by which it may be possible to significantly reduce the spin polarized current needed for the short- pulse switching of a thermally stable nanomagnet for MRAM applications. I will conclude by briefly discussing some of challenges that remain to be overcome before spin-torque based technologies, particularly ST-MRAM, can be successfully implemented and of the pathways that might be effectively taken to overcome those challenges.