Shouzhong Peng

Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures

Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts extensive attentions due to its non-volatility, high density and low power consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well as a large value of perpendicular magnetic anisotropy (PMA). It has been experimentally proven that a capping layer coating on CoFeB layer is essential to obtain a strong PMA. However, the physical mechanism of such effect remains unclear. In this paper, we investigate the origin of the PMA in MgO/CoFe/metallic capping layer structures by using a first-principles computation scheme. The trend of PMA variation with different capping materials agrees well with experimental results. We find that interfacial PMA in the three-layer structures comes from both the MgO/CoFe and CoFe/capping layer interfaces, which can be analyzed separately. Furthermore, the PMAs in the CoFe/capping layer interfaces are analyzed through resolving the magnetic anisotropy energy by layer and orbital. The variation of PMA with different capping materials is attributed to the different hybridizations of both d and p orbitals via spin-orbit coupling. This work can significantly benefit the research and development of nanoscale STT-MRAM.

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.

Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures

Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of great interest due to its application in the spin-transfer-torque magnetic random access memory (STT-MRAM). Large interfacial perpendicular magnetic anisotropy (PMA) is required to achieve high thermal stability. Here we use first-principles calculations to investigate the magnetic anisotropy energy (MAE) of MgO/CoFe/capping layer structures, where the capping materials include 5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate that it is feasible to enhance PMA by using proper capping materials. Relatively large PMA is found in the structures with capping materials of Hf, Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to giant PMA (6.09 mJ/m2), which is about three times larger than that of the MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the contributions to MAE from each atomic layer and orbital. These findings provide a comprehensive understanding of the PMA and point towards the possibility to achieve advanced-node STT-MRAM with high thermal stability.

Large influence of capping layers on tunnel magnetoresistance in magnetic tunnel junctions

It has been reported in experiments that capping layers, which enhance the perpendicular magnetic anisotropy (PMA) of magnetic tunnel junctions (MTJs), induce a great impact on the tunnel magnetoresistance (TMR). To explore the essential influence caused by the capping layers, we carry out ab initio calculations on TMR in the X(001)|CoFe(001)|MgO(001)|CoFe(001)|X(001) MTJ, where X represents the capping layer material, which can be tungsten, tantalum, or hafnium. We report TMR in different MTJs and demonstrate that tungsten is an ideal candidate for a giant TMR ratio. The transmission spectrum in Brillouin zone is presented. It can be seen that in the parallel condition of MTJ, sharp transmission peaks appear in the minority-spin channel. This phenomenon is attributed to the resonant tunnel transmission effect, and we explained it by the layer-resolved density of states. In order to explore transport properties in MTJs, the density of scattering states was studied from the point of band symmetry. It has b...

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.

High Tunnel Magnetoresistance in Mo/CoFe/MgO Magnetic Tunnel Junction: A First-Principles Study

The tunnel magnetoresistance (TMR) ratio in a magnetic tunnel junction (MTJ) is influenced by heavy metal capping layer due to the interfacial effect. We report a systematic first-principles study on MTJ based on CoFe/MgO with capping layer, demonstrate that TMR ratios are sensitive to capping layer material, and show that TMR in Mo-capped MTJ is three times as high as that in Ta-capped MTJ. Besides, TMR in Mo-capped MTJ remains high at finite voltage bias. By analyzing the transmission spectrum and density of scattering states, we found that coherent transmission of

Novel Magnetic Tunneling Junction Memory Cell With Negative Capacitance-Amplified Voltage-Controlled Magnetic Anisotropy Effect

\Delta _{1}

Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance

state dominates the majority-spin conductance in Mo-capped MTJ, while the resonant tunneling contributes significantly in Ta-capped MTJ. The evolution of TMR for varying MgO and CoFe thickness in Mo-capped MTJ is presented. TMR oscillates as a function of CoFe thickness because of the confined wave function in ferromagnetic layer, while TMR rises with MgO thickness increasing due to the enhanced filtering effect of MgO. This work clarifies the physical mechanism on high TMR in Mo-capped MTJ, which is promising to benefit the design of spintronics device.

Tuning the Dzyaloshinskii-Moriya interaction in Pt/Co/MgO heterostructures through the MgO thickness

The high current density required by magnetic tunneling junction (MTJ) switching driven by the spin transfer torque (STT) effect leads to large power consumption and severe reliability issues, hindering the timetable for STT magnetic random access memory to mass market. By utilizing the voltage-controlled magnetic anisotropy (VCMA) effect, the MTJ can be switched by the voltage effect and is postulated to achieve ultralow power (fJ). However, the VCMA coefficient measured in experiments cannot meet the requirement for MTJ with dimensions below 100 nm. And an external in-plane magnetic field usually is demanded for precessional VCMA switching. Here, in this paper, a novel approach for the amplification of the VCMA effect, which borrows ideas from negative capacitance, is proposed. The feasibility of the proposal is proved by physical simulation and in-depth analysis. Since the amplified VCMA effect, the external magnetic field can be eliminated. A three-terminal novel MTJ memory cell is designed with which both low power and high speed can be achieved.