Spintronic Device Physics and Device Applications for Novel and Future Electronic Information Processing and Storage

Abstract

Current complementary metal–oxide–semiconductor (CMOS) technologies have been facing great challenges for decades. Spintronics, a field that addresses transport phenomena, coupled with magnetism, has already found successful solutions for applications in hard disk drives, and recently in embedded memories. It offers a vision of solutions that is still electrical, but also more energy efficient, faster and non-volatile. Among various spintronic devices, the magnetic tunnel junction (MTJ) switched by spin transfer torque (STT) shows immense potential for memory applications. Electric field-induced magnetic anisotropy (EMA) promises more energy-efficient ways to switch MTJ. In this dissertation, EMA is studied by the technique of spin torque-ferromagnetic resonance (ST-FMR) in the in-plane hard-axis transverse direction. Modeling and numerical simulations indicate two distinct FMR spectral line shapes Experimental results confirm the contribution of EMA in FMR process. Spin-orbit torque (SOT) is another promising candidate to switch magnetization. In devices that employ SOT switching, the newly discovered unidirectional spin Hall magnetoresistance (USMR) could sense magnetization change without requiring any additional structures or electrical terminals. To further understand and improve the USMR, ferromagnet (FM)-topological insulator (TI) material systems are studied in this dissertation. The unidirectional spin-Hall and Rashba-Edelstein magnetoresistance (USRMR) is observed in such new material systems and is found to be larger than the USMR in Co/Ta systems. Then the USRMR is studied in magnetic insulator (MI)-TI systems in this dissertation. The USRMR is experimentally observed in this system and its amplitude is further improved. With the large USRMR, a memory prototype of current-induced magnetization switching and USRMR read out is experimentally demonstrated. The MTJ, being a type of more mature spintronic device, has been developed to be integrated into CMOS systems for memory applications. From a different angle, MTJ’s unique properties are being exploited to tackle challenges in specific circuit applications in this dissertation. A true random number generator (TRNG) based on MTJ and utilizing its probabilistic nature of switching is demonstrated experimentally. Two proposals for an analog-to-digital converter (ADC) based on MTJ are also studied by either experiments or simulations. Stochastic computing was proposed decades ago but has not been widely used due to difficulty in employing appropriate devices. Further along this line of effort, a stochastic computing unit based on a single MTJ is proposed and experimentally demonstrated in this dissertation. The unit takes physical conditions, such as field, bias current, pulse amplitude and pulse width, as input and output random bit streams, which in a stochastic manner carry the result of scaled addition and multiplication.