# High Performance MRAM with Spin-Transfer-Torque and Voltage-Controlled Magnetic Anisotropy Effects
## 1 Introduction  
In recent years, non-volatile memories (NVMs) have been developed for power solutions of Internet of Things (IoTs) nodes. NV memories with spintronic devices, e.g., magnetic tunnel junction (MTJ), have demonstrated outstanding performance in terms of zero standby power, switching energy efficiency, infinite endurance and high density [1]. The development of spintronics devices originates from the giant magnetoresistance (GMR) effect [1; 2; 3]. As one of the most important spintronics devices, magnetic tunnel junction (MTJ) could be a promising apparatus to replace conventional memories, e.g., static random-access memory (SRAM). As illustrated in Figure 1, MTJ consists of one nonmagnetic layer sandwiched by two ferromagnetic layers, in which the tunnel magnetoresistance (TMR) effect was discovered for the first time by [PERSON] [4]. The device resistance depends on the orientation of MTJ magnetization (m\({}_{\mathrm{z}}\)) with ferromagnetic layers (R\({}_{\mathrm{P}}\) at parallel state and R\({}_{\mathrm{ap}}\) at antiparallel state). As the MTJ resistance can be configured in a way comparable with Complementary Metal-Oxide-Semiconductor (CMOS) transistors, it can be integrated in the memories and logic circuits to represent logic '0' or '1'. Its characteristic is quantified by TMR ratio (R\({}_{\mathrm{ap}}-\)R\({}_{\mathrm{P}}\))/R\({}_{\mathrm{P}}\). As a promising memory candidate, the switching approaches of MTJ are always with intensive research. Field-induced magnetic switching (FIMS) was firstly employed in the early realizations of MTJ based magnetoresistive random access memory (MRAM) [1; 5]. A relatively high current (\(\sim\)10 mA) is required by the FIMS method to generate magnetic fields, which leads to a critical constraint for FIMS to realize high density and low power memory due to high power consumption, large die area and high disturbance. In order to decrease the threshold of switching currents, thermally assisted switching (TAS) MTJ was developed in 2003 [6]. In this method, a current flows into MTJ to heat this magnetic device and facilitates its switching. TAS has effectively decreased the power consumption of the writing operation (switching current \(\sim\)1 mA), but the scalability issue still remains unsolved and the switching speed is lower due to the necessary cooling down after the heating. To address the power and scalability, a novel switching approach named spin transfer torque (STT) was firstly presented in [7; 8]. This method uses a lower current (\(\sim\)100 uA) flowing through the MTJ to switch its state. Without the need of magnetic fields, STT makes it possible to achieve high density and low-power MRAM. [PERSON] et al. used the interfacial perpendicular magnetic anisotropy (PMA) MTJ to realize low switching current (49 uA), and high thermal stability [9]. Furthermore, the spin hall effect (SHE)-assisted STT switching method has been experimentally demonstrated to overcome the incubation delay generated by STT switching [10; 11]. Compared with the traditional STT approach, the SHE plus STT method could remove the undesirable incubation. In this approach, the writing and sensing operations are completely separated by the three terminal configuration. Therefore, low resistance can be realized for easier writing and high resistance can be realized for sensing. The switching current can be reduced by nearly one order of magnitude compared with STT switching mechanism by optimizing the thickness of heavy metal layer. With these advantages, SHE assisted STT features lower power, faster speed and better reliability. Table 1 demonstrates the universal memories which drive most of research and development [12; 13]. F represents feature size of the lithography. Energy consumption is evaluated at the cell level. The endurance is signified by the writing cycles. As IoT node memory requires specific features such as ultra-low power consumption, small size and being always connected, STT-MTJ is well fit for this purpose.