Development of an integrated magneto optical Kerr microscope and anomalous Hall effect measurement system for investigating spin-orbit torque effect

With the commercialization of spin-transfer torque magnetic random-access memory (STT-MRAM), spin-orbit torque MRAM (SOT-MRAM) has emerged as a promising next-generation technology due to its faster write speeds and nearly unlimited endurance. Understanding current-induced SOT switching is crucial for MRAM development, requiring specialized instrumentation. Traditional wide-field magneto-optical Kerr effect (MOKE) microscope detects the magnetic domain, while electrical measurements need to be performed separately. This work presents an integrated system combining MOKE imaging with anomalous Hall effect (AHE) measurements, enabling efficient and accurate characterization of SOT devices.
This work mainly explores the advancement of MOKE microscopy, aiming to design, optimize and implement a wide-field MOKE microscope equipped with an electrical measurement system based on the Keithley 6221 current source and the 2182A nanovoltmeter. This setup facilitates the simultaneous measurement of AHE voltages and MOKE imaging, enabling the observation of magnetic domain dynamics and electrical characteristics under various conditions. The current-induced SOT effect in layered thin films was investigated. In Pt/Co/Cu/Ta samples, AHE voltage measurements were found to be more sensitive than MOKE measurements. The behaviour of this structure under a tilted magnetic field was also studied. In Ta/CoFeB/MgO samples, differences were discovered between AHE voltage measurements taken at the peak of the excitation pulse and those taken during direct current (DC) measurements. The latter was influenced by domain relaxation, introducing a degree of randomness to the DC measurement results. Extensive studies were conducted to investigate the potential for achieving controllable field-free SOT switching through a composition gradient in CoPt films, as well as enhancing SOT efficiency by incorporating an α-Fe2O3 underlayer. The experimental results demonstrate significant reductions in the critical switching current and applied external magnetic field. This thesis contributes to the field of spintronics by providing innovative approaches to integrate MOKE and electrical measurements, improving the understanding of current-induced SOT effects.