High Speed MRAM with Voltage Control of Magnetic Anisotropy (VCMA) Effect
Magnetic random access memory (MRAM) is gaining intensive interest for embedded and stand-alone memory applications. Its inherent non-volatility is believed to address the large stand-by energy consumption issues in the present memory hierarchy. In recent years, the spin-transfer torque (STT)-MRAM has gradually matured and started to appear in the market. Typically, STT writing of perpendicular magnetic tunnel junction (pMTJ) is limited to a few nanoseconds. Because STT is collinear with intrinsic damping of the free-layer (FL), the FL needs to wait for the thermal fluctuation to tilt its magnetization to gain a finite torque. Further, it requires a large enough STT current to overcome the damping torque, which consumes a great amount of writing energy. One can overcome these issues by using different writing mechanisms that do not directly compete with the intrinsic damping. Among several proposals, the most attractive ones are the voltage control of magnetic anisotropy (VCMA) and the spin-orbit torques (SOT). In particular, VCMA promises low power operation of MRAM as it allows modification of the FL perpendicular magnetic anisotropy (PMA), a parameter directly linked to the writing current/energy. This PhD study will focus on the understanding of VCMA both as the writing mechanism and as the assisting mechanism to SOT.