Magnetic memory has attracted substantial interest due to its non-volatility and zero power dissipation in stand-by mode. The key of magnetic memory is deterministic control of magnetic bits, especially those with perpendicular magnetic anisotropy (PMA), which have higher thermal stability and smaller footprint compared to in-plane memory bits. Among all the magnetization control mechanism, the strain-mediated multiferroics has surprisingly high energy efficiency (1-3 orders of magnitude better) compared to the other mechanisms using nanoscale magnetization control. The strain-mediated multiferroic control employs a piezoelectric/magnetoelastic heterostructure. To write the magnetic memory, a voltage pulse is applied to the piezoelectric substrate and the induced mechanical strain is transferred to a magnetic element attached to the piezoelectric substrate causing magnetization rotation due to the magnetoelastic effect. The magnetization change can be read out using a magnetic tunnel junction (MTJ). Using strain-mediated multiferroics to control in-plane magnetization has been successfully demonstrated both numerically and experimentally. However, there is little work on using strain-mediated multiferroics to control perpendicular magnetization.
In this dissertation, we provide a thorough study on strain-mediated perpendicular magnetization control, including modeling, experiments, and several new device concepts that extend beyond the traditional memory applications. In Chapter 1, we briefly introduce the memory hierarchy and present non-volatile memory technologies, including magnetic memory. We compare the strain-mediated multiferroic magnetization control with other popular control mechanisms. We also describe the simulation basics, micro-fabrication processes, and characterization techniques for the strain-mediated multiferroics. In Chapter 2, we focus on the simulation of strain-mediated perpendicular magnetization control. Three systems are investigated: 1) single nanodot with constant voltage actuation, 2) multiple nanodots coupled by dipole interaction, 3) single nanodot with AC voltage actuation. In Chapter 3, we focus on the experimental investigation of strain-mediated perpendicular magnetization control. Micro-scale magnetic devices are fabricated with two kinds of piezoelectric substrate: PMN-PT bulk and PZT thin film. By analyzing the test results, the challenge and limitation of multiferroic control of perpendicular magnetization are identified. Empirical experiences are summarized to help guide future multiferroic device design. In Chapter 4, we examine a hybrid strain and spin-orbit torque control mechanism. Two models are developed to simulate the hybrid system and surprisingly interesting phenomena are observed. Using the simulation capabilities developed, we propose several new devices that are go beyond standard memory applications. Finally the last chapter summarizes the contents of this dissertation.
