UCLA

Magnetic memory that utilizes spin to store information has become one of the most promising candidates for next-generation non-volatile memory. Electric-field-assisted writing of magnetic tunnel junctions (MTJs) that exploits the voltage-controlled magnetic anisotropy (VCMA) effect offers great potential for high density and low power memory applications. This emerging Magnetoelectric Random Access Memory (MeRAM) based on the VCMA effect has been investigated due to its lower switching current, compared with traditional current-controlled devices utilizing spin transfer torque (STT) or spin-orbit torque (SOT) for magnetization switching. It is of great promise to integrate MeRAM into the advanced CMOS back-end-of-line (BEOL) processes for on-chip embedded applications, and enable non-volatile electronic systems with low static power dissipation and instant-on operation capability. To achieve the full potential of MeRAM, it is critical to design magnetic materials with high voltage-induced writing efficiency, i.e. VCMA coefficient, to allow for low write energy, low write error rate, and high density MeRAM at advanced nodes.

In this dissertation, we will first discuss the advantage of MeRAM over other memory technologies with a focus on array-level memory performance, system-level 3D integration, and scaling at advanced nodes. Then, we will introduce the physics of the VCMA effect, map out the VCMA coefficients requirements and other challenges when MeRAM is scaled down, and discuss the electrical measurement techniques used in later chapters to characterize the VCMA effect. Next, we will discuss three experimental approaches taken to enhance the VCMA coefficient. First, a high dielectric-constant hybrid tunnel barrier is used to increase the VCMA coefficient. Then, by carefully controlling the Mg insertion thickness at the CoFeB/MgO interface, the Fe/O interfacial oxidation condition can be precisely controlled to identify the optimal oxidation condition for large VCMA coefficient. Last, different heavy metal based seed/Mo materials are explored to achieve stable VCMA coefficient, TMR, and perpendicular magnetic anisotropy (PMA) when annealed at temperatures exceeding 400oC, making MeRAM compatible with embedded applications. In addition, we have carefully studied the correlation between atomic elemental distribution and the magnetic properties of these stacks via high resolution transmission electron microscopy (TEM). The insight obtained will provide a critical guidance to future development of both spin-transfer torque and voltage-controlled magnetic memory.