Kaihua Cao

Failure Analysis in Magnetic Tunnel Junction Nanopillar with Interfacial Perpendicular Magnetic Anisotropy

Magnetic tunnel junction nanopillar with interfacial perpendicular magnetic anisotropy (PMA-MTJ) becomes a promising candidate to build up spin transfer torque magnetic random access memory (STT-MRAM) for the next generation of non-volatile memory as it features low spin transfer switching current, fast speed, high scalability, and easy integration into conventional complementary metal oxide semiconductor (CMOS) circuits. However, this device suffers from a number of failure issues, such as large process variation and tunneling barrier breakdown. The large process variation is an intrinsic issue for PMA-MTJ as it is based on the interfacial effects between ultra-thin films with few layers of atoms; the tunneling barrier breakdown is due to the requirement of an ultra-thin tunneling barrier (e.g., <1 nm) to reduce the resistance area for the spin transfer torque switching in the nanopillar. These failure issues limit the research and development of STT-MRAM to widely achieve commercial products. In this paper, we give a full analysis of failure mechanisms for PMA-MTJ and present some eventual solutions from device fabrication to system level integration to optimize the failure issues.

All Spin Artificial Neural Networks Based on Compound Spintronic Synapse and Neuron

Artificial synaptic devices implemented by emerging post-CMOS non-volatile memory technologies such as Resistive RAM (RRAM) have made great progress recently. However, it is still a big challenge to fabricate stable and controllable multilevel RRAM. Benefitting from the control of electron spin instead of electron charge, spintronic devices, e.g., magnetic tunnel junction (MTJ) as a binary device, have been explored for neuromorphic computing with low power dissipation. In this paper, a compound spintronic device consisting of multiple vertically stacked MTJs is proposed to jointly behave as a synaptic device, termed as compound spintronic synapse (CSS). Based on our theoretical and experimental work, it has been demonstrated that the proposed compound spintronic device can achieve designable and stable multiple resistance states by interfacial and materials engineering of its components. Additionally, a compound spintronic neuron (CSN) circuit based on the proposed compound spintronic device is presented, enabling a multi-step transfer function. Then, an All Spin Artificial Neural Network (ASANN) is constructed with the CSS and CSN circuit. By conducting system-level simulations on the MNIST database for handwritten digital recognition, the performance of such ASANN has been investigated. Moreover, the impact of the resolution of both the CSS and CSN and device variation on the system performance are discussed in this work.

Interfacial Perpendicular Magnetic Anisotropy in Sub-20 nm Tunnel Junctions for Large-Capacity Spin-Transfer Torque Magnetic Random-Access Memory

Magnetic tunnel junctions (MTJs) with interfacial perpendicular magnetic anisotropy (PMA) attract much attention due to their utilization in spin-transfer torque magnetic random-access memory (STT-MRAM). Large interfacial PMA provides high thermal stability, which is critical for large-capacity MTJ arrays. We investigate the thermal stability and interfacial PMA needed for STT-MRAM applications. A thermal stability factor of 75 is required for data retention time of 10 years, which implies an interfacial PMA value of 4.7 mJ/m2 as device sizes scale down to 10 nm. Even though a small retention time (e.g., 1 ms) is sufficient in some applications, such as cache memory, an interfacial PMA greater than 3.1 mJ/m2 would be necessary for 10 nm MTJ pillars. When read disturbance is taken into consideration, the PMA should be larger. These findings provide guidelines for the design of sub-20 nm MTJ devices for large-capacity STT-MRAM.

Skyrmions in Magnetic Tunnel Junctions

In this work, we demonstrate that skyrmions can be nucleated in the free layer of a magnetic tunnel junction (MTJ) with Dzyaloshinskii-Moriya interactions (DMIs) by a spin-polarized current with the assistance of stray fields from the pinned layer. The size, stability, and number of created skyrmions can be tuned by either the DMI strength or the stray field distribution. The interaction between the stray field and the DMI effective field is discussed. A device with multilevel tunneling magnetoresistance is proposed, which could pave the ways for skyrmion-MTJ-based multibit storage and artificial neural network computation. Our results may facilitate the efficient nucleation and electrical detection of skyrmions.

Optically Tunable Magnetoresistance Effect: From Mechanism to Novel Device Application

The magnetoresistance effect in sandwiched structure describes the appreciable magnetoresistance effect of a device with a stacking of two ferromagnetic layers separated by a non-magnetic layer (i.e., a sandwiched structure). The development of this effect has led to the revolution of memory applications during the past decades. In this review, we revisited the magnetoresistance effect and the interlayer exchange coupling (IEC) effect in magnetic sandwiched structures with a spacer layer of non-magnetic metal, semiconductor or organic thin film. We then discussed the optical modulation of this effect via different methods. Finally, we discuss various applications of these effects and present a perspective to realize ultralow-power, high-speed data writing and inter-chip connection based on this tunable magnetoresistance effect.

Size dependence of the spin-orbit torque induced magnetic reversal in W/CoFeB/MgO nanostructures

The spin-orbit torque (SOT) induced magnetic switching in structures such as Hall bars cannot be well explained with the macrospin model. The switching process is affected by the domain wall (DW) dynamics. In previous studies, some observed phenomena, such as intermediate states appearing during the magnetic switching of the Hall bar structure and asymmetric switching currents in two directions, were not well explained. In this letter, by studying the SOT induced magnetic switching in W/CoFeB/MgO nanostructures with different size, these phenomena are demonstrated to be governed by the DW propagations in nanowires and asymmetric DW pinnings at the Hall cross. The size dependence of the switching current is observed and explained with the DW depinning model. These studies provide an approach to detect the properties of the structure, such as the quantification of the spin Hall angle in the heavy metal layer.The spin-orbit torque (SOT) induced magnetic switching in structures such as Hall bars cannot be well explained with the macrospin model. The switching process is affected by the domain wall (DW) dynamics. In previous studies, some observed phenomena, such as intermediate states appearing during the magnetic switching of the Hall bar structure and asymmetric switching currents in two directions, were not well explained. In this letter, by studying the SOT induced magnetic switching in W/CoFeB/MgO nanostructures with different size, these phenomena are demonstrated to be governed by the DW propagations in nanowires and asymmetric DW pinnings at the Hall cross. The size dependence of the switching current is observed and explained with the DW depinning model. These studies provide an approach to detect the properties of the structure, such as the quantification of the spin Hall angle in the heavy metal layer.

Influence of heavy metal materials on magnetic properties of Pt/Co/heavy metal tri-layered structures

Pt/Co/heavy metal (HM) tri-layered structures with interfacial perpendicular magnetic anisotropy (PMA) are currently under intensive research for several emerging spintronic effects, such as spin-orbit torque (SOT) [1, 2], domain wall motion [3, 4] and room temperature skyrmions [5, 6].

Spin orbit torques for ultra-low power computing

In this paper, we review recent progress made in ultra-low power computing system with non-volatile magnetic random access memory (MRAM). Compared with the traditional Spin Transfer Torque (STT) based memories, Spin Orbit Torque (SOT) mechanism offers higher write speed, lower power consumption, and potentially infinite endurance. In particular, Spin-Hall assisted STT achieves a purely electrical operation in absence of an external magnetic field, and opens the door to real normally-off/instant-on computing with non-volatility at all levels of the memory hierarchy.