Chapter 7 - Magnetic Tunnel Junctions and Spin Torques

Abstract

Publisher Summary This chapter briefly examines the concept magnetic RAM (“MRAM”) and how it is expected to compete with the current high speed data transfer techniques. Magnetic RAM (MRAM) is expected to handle various shortcomings of data transfers, providing instant boot-up capability, with high speed access, low power consumption and high density. Early MRAM devices used magnetic hysteresis to store information. A basic storage mechanism is tied to the magnetic state of a tunnel junction. MRAM does not require any refreshing to maintain the magnetic state of a given bit, once it is magnetized, and hence the memory is non-volatile. Not only does this mean that it retains its memory with the power turned off, but also that there is no need for a constant power supply. A magnetic tunnel junction (MTJ) is different from the metallic sandwich structures discussed with regard to the GMR effect. A MTJ consists of two metallic layers, sometimes called electrodes, separated by an insulating layer thin enough to allow some tunneling current. The interest in such devices is at least partly due their potential applications in magnetoresistive random access memory (MRAM) devices and sensors. The chapter discusses the theological aspects of tunneling magnetoresistance (TMR), devices with large TMR values, double barriers, vortex domain structures, spin transfer torques in metallic multilayers, ultra-fast reversal of magnetization, and transistors based on spin orientation. The chapter illustrates two key features of the scattering process. The first is that the spin current along the direction of the magnetization is con-served. The second point is that the reflected and transmitted currents have no transverse components. When combined, these two results show that the spin transfer torque is approximately given by the absorption of the transverse component of the incident spin current.