Emerging Three-Terminal Magnetic Memory Devices

Spin-transfer torques can switch magnetizations via a current passing through a magnetic tunnel junction, an effect that is being pursued as the switching mechanism in spin-transfer torque magnetic random access memory. Three-terminal devices are also possible. One mechanism is to have a free layer that contains a domain wall that can be manipulated by spin-transfer torques and moved between two configurations that can be read by a separate connection. An alternate approach uses the recent development of spin-orbit torques, which offer an efficient way of manipulating the magnetization of a tunnel junction by current passing through an adjacent layer. These torques allow for the separation of reading and writing currents through three-terminal devices structures. This paper presents the basic principles of spin-orbit torques, the distinguishing features of spin-orbit-torque-induced magnetization dynamics as compared to magnetization dynamics driven by conventional spin-transfer torques. From the application point of view, it presents the pros and cons of spin-orbit-torque-based three-terminal devices including magnetic random access memories. Then, it discusses domain-wall-based three-terminal devices and the advantages and disadvantages of each.