Enhancement of Spin-Orbit Torque Efficiency using A15 Crystalline W3Ta in W3Ta/CoFeB/MgO Structure

Abstract

As 1-Gb DDR4 spin-transfer torque magnetic random access memory (STT-MRAM) began mass production as embedded memory in 2019, growth and expectations for MRAM are also increasing in the non-volatile memory(NVM) market. [1] However, since the driving method of STT-MRAM uses the MgO tunnel barrier for both read/write, research and development for spin-orbit torque MRAM (SOT-MRAM), which is superior in terms of endurance, is actively progressing. The SOT MRAM device separates the writing method by using channels of heavy metal materials such as Pt, W, and Ta. As a result, it has the advantage that less voltage stress is applied to the MgO tunnel barrier than the STT switching method. One way to increase the write efficiency of SOT MRAM is to increase the charge to spin conversion efficiency by using a heavy metal material with high SOT efficiency. As an example, many studies on SOT devices using beta-phase W heavy metals have continued. [2] However, to be applied as a device, it is necessary to lower the current for the writing method. In this thesis, the beta-phase W3Ta, which is an alloy material of W and Ta, is used as a channel to enhance SOT efficiency. First, by using X-ray Diffraction (XRD), a beta-phase W 4 nm and W3Ta 15 nm were obtained, and a perpendicular magnetic anisotropy(PMA) structure was fabricated by fixing the measured thickness of the channel. To calculate the SOT efficiency, the saturation magnetic moment was measured using a Vibrating Sample Magnetometer (VSM), and then a Hall bar structure was fabricated to determine anomalous hall resistance and torque per unit moment. As a result, W and W3Ta had an average SOT efficiency each 0.195, 0.395 and it was evaluated that W3Ta was a more suitable heavy metal for SOT MRAM devices.