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The precessional switching mechanism has governed the magnetic switching in magnetic tunnel junctions (MTJs) in the sub-nanosecond range, which exponentially increases the switching current density of magnetic random access memory (MRAM). Thus, there needs to be an alternative switching mechanism with much higher energy efficiency to bring down the switching current density significantly and make the MRAM compatible with high-speed L1/2—static random access memory (SRAM) at sub-nanosecond range. Using the recent discovered external electric field ( $E$ -field) tunable Ruderman–Kittel–Kasuya–Yosida (RKKY) phenomena in a synthetic ferrimagnet (E-SFi), we propose a totally different Chrysanthemum -like switching mechanism to realize a low-energy picosecond writing MRAM design, which breaks the precessional switching mechanism at picosecond region. And our results show that the critical switching current density can be significantly reduced by one order of magnitude compared to that of a conventional MTJ design down to 100 ps. In addition, we study the robustness of the asynchronous conditions between the charge current pulse and the $E$ -field pulse for its practical applications.

Spin-transfer torque magnetic random-access memory (STT-MRAM) is one of the next-generation nonvolatile memories, which has the most promising industrial prospect and has generated lots of new ideas. One of the continuous challenges in STT-MRAM development is to reduce the critical write current. Recently, one published paper reported that under a small electric bias voltage, the synthetic anti-ferromagnetic (SAF) multilayer system could be switched between an anti-ferromagnetic coupling state and a ferromagnetic coupling state. Based on this phenomenon, we propose a new type of STT-MRAM of which the critical write current can be reduced by an assisting electric-field (E-field). Micromagnetic simulation has been performed to study the dynamic switching behavior of the magnetization of the SAF free layer with tunable Ruderman–Kittel–Kasuya–Yosida interaction under the impact of an E-field.

The effect of four different seed layers-Ru, Hf/Ru, Ru/Hf, and Hf on the magnetic properties of [Co 5/Pt 3 A] multilayer (ML) stack with perpendicular magnetic anisotropy was investigated. The structural quality of the ML stacks was studied and correlated with the magnetic properties. Among the abovementioned seed layers, Ru promotes stronger fcc (111) texture in the ML due to less lattice mismatch (8% between Ru and Co) in Co/Pt MLs. As a result, higher anisotropy field (H K ~ 17 kOe) and larger effective perpendicular anisotropy (K eff ~ 1.2 × 10 7 erg/cm 3 ) were achieved for the MLs on Ru seed layer compared with other seed layers after annealing at 300 °C for 30 min. The perpendicular magnetic anisotropy of the MLs is improved at higher annealing temperature up to 400 °C for Ru and Hf/Ru seed layer. For the Co/Pt MLs on Ru, 1.3 × 10 7 erg/cm 3 perpendicular magnetic anisotropy was obtained even after annealing at 400 °C. Detail investigations of postannealing stability of the ML films on different seed layers were also discussed. The magnetic properties and structural properties of the films on four different seed layers were characterized after rapid thermal annealing process at 300 °C, 350 °C, and 400 °C for 30 min annealing duration. The high temperature endurance limit up to 400 °C for 30 min of the Co/Pt MLs on Ru and Hf/Ru seed layers makes them compatible with the back-end-of-line process temperature.

We analyze the opportunity of inserting a Ta layer between the polarizing section and the high anisotropy section in the reference subsystem of CoFeB-based tunnel junctions with perpendicular magnetic anisotropy. Using vector network analyzer ferromagnetic resonance, polar Kerr magnetometry, and magnetization dynamics modeling, we deduce the strength of the ferromagnetic interlayer exchange coupling energy J through various thickness of tantalum that impacts on the overall performance of the tunnel junctions. J culminates at 0.44 mJ/m 2 through 3 A of Ta, but the Co/Pt properties are then suboptimal. Our methodology can be used to rationally find the performance optimum in the reference layers of perpendicularly magnetized tunnel junctions.

As a promising nonvolatile memory technology, magnetic tunnel junction (MTJ) based spin-torque transfer RAM (STT-RAM) has recently attracted much attention. However, recent device research suggested that, in order to maintain sufficient MTJ write margin to prevent device breakdown, MTJs may be subject to unconventionally high random write error rates (e.g., 10-3 and above) as memory cell size is being scaled down. In this paper, we aim to develop a STT-RAM cache design solutions that can effectively tolerate high MTJ write error rates at small performance and implementation cost, which makes it much easier to maintain sufficient MTJ write margin and hence push the STT-RAM scalability envelope. Using the full system simulator PTLsim and a variety of benchmarks, we show that the proposed architecture design can readily accommodate MTJ write error rate upto 2% at the penalty of less than 3% processor performance degradation, less than 10% silicon area overhead, and negligible energy consumption overhead.

Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below 10-9. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching.

Using experimental and micromagnetic modeling, the intermediate magnetization states in deep submicrometer MRAM cells, their effects on the switching behavior and designs to eliminate them, are presented. The elliptical MRAM cell with varied aspect ratio and minor axis length is built using synthetic pinned layer-AlO/sub x/-free layer structure. At-field and remnant R-H curves are collected for various design and process parameters: free layer thickness, cell aspect ratio, different free layer material NiFe, CoFe, CoFeB and combinations of them. For not well optimized designs, the at-field R-H curves are not desirable squares with significant amount of irregularities or kinks that can be separated into two types: type V and type H. From micro-magnetic modeling, the magnetization of the type V kink state is identified as vortex configuration. For the type H, the magnetization state has a horseshoe configuration.

The cross track performance of permanent magnet biased MR/SAL heads for high density magnetic recording is investigated. Two designs using different methods to define the read width (with and without inactive MR wing regions under the conductors) are fabricated and compared. The design with the wings is easier to manufacture than the other design, but it has some shortfalls that limit its cross track performance. The cross track characteristics such as servo capability and side reading range can be affected significantly by the design and sensor stripe height. A 3-dimensional micromagnetic model and servo calculations are utilized to understand the cross track behavior of the two designs and the impact on system performance. The influence of top pole curvature due to the underlying conductor topography in the read head on the cross track profile is also studied.