Jonathan Harms

A Scaling Roadmap and Performance Evaluation of In-Plane and Perpendicular MTJ Based STT-MRAMs for High-Density Cache Memory

This paper explores the scalability of in-plane and perpendicular MTJ based STT-MRAMs from 65 nm to 8 nm while taking into consideration realistic variability effects. We focus on the read and write performances of a STT-MRAM based cache rather than the obvious advantages such as the denser bit-cell and zero static power. An accurate MTJ macromodel capturing key MTJ properties was adopted for efficient Monte Carlo simulations. For the simulation of access devices and peripheral circuitries, ITRS projected transistor parameters were utilized and calibrated using the MASTAR tool that has been widely used in industry. 6T SRAM and STT-MRAM arrays were implemented with aggressive assist schemes to mimic industrial memory designs. A constant JC0·RA/VDD scaling scenario was used which to the first order gives the optimal balance between read and write margins of STT-MRAMs. The thermal stability factor ensuring a 10 year retention time was obtained by adjusting the free layer thickness as well as assuming improvement in the crystalline anisotropy. Our studies based on the proposed scaling methodology show that in-plane STT-MRAM will outperform SRAM from 15 nm node, while its perpendicular counterpart requires further innovations in MTJ material in order to overcome the poor write performance scaling from 22 nm node onwards.

SPICE Macromodel of Spin-Torque-Transfer-Operated Magnetic Tunnel Junctions

The electrical behavior of a magnetic tunnel junction (MTJ) using spin-torque-transfer (STT) switching was modeled using a SPICE subcircuit. The subcircuit is a two-terminal device that exhibits the electrical characteristics of an STT-MTJ. These characteristics include all the major transient characteristics of an MTJ, including the hysteresis, bias voltage dependence of the resistance, and the critical switching current versus the critical switching time. The model was designed to work over a wide range of operating conditions. Simulation and analysis of an MTJ-based D flip-flop are presented to demonstrate possible applications of the model.

Magnetic Tunnel Junction-Based Spintronic Logic Units Operated by Spin Transfer Torque

Magnetic tunneling junction (MTJ)-based programmable logic devices have been proposed and studied for future reconfigurable and nonvolatile computation devices and systems. Spin transfer torque (STT)-based switching has advantages in device scaling compared to the field-switching mechanism. However, the previously proposed MTJ logic devices have operated independently and, therefore, are limited to only basic logic operations. Consequently, the MTJ device has only been used as an ancillary device, rather than the main computation device. As a result, the full benefits of MTJ-based computation have not been explored. New designs are needed to accelerate the development of the MTJ-based logic devices. Specifically the realization of direct communication between the MTJ devices is crucial to fully utilize the MTJ devices in the circuits to implement more advanced logic functions. In this paper, new MTJ-based spintronic logic units (building blocks) for spintronic circuits using the STT switching mechanism have been proposed and investigated, which includes the designs of a basic STT-MTJ logic cell, a direct communication between the MTJ logic cells, a three-MTJ logic unit and a spintronic logic circuit acting as an arithmetic logic unit.

Direct communication between magnetic tunnel junctions for nonvolatile logic fan-out architecture

We experimentally demonstrated a magnetic tunnel junction (MTJ) based circuit that allows direct communication between elements without intermediate sensing amplifiers. The input of the circuit consists of three MTJs connected in parallel. The direct communication is realized by connecting the output in series with the input and applying voltage across the series connections. Combining the circuit with complementary metal oxide semiconductor current mirrors allows for fan-out to multiple outputs. The change in resistance at the input resulted in a voltage swing across the output of 150–200 mV for the closest input states which is sufficient to realize all of the Boolean primitives.

Reduction of switching current density in perpendicular magnetic tunnel junctions by tuning the anisotropy of the CoFeB free layer

The spin torque switching behavior of perpendicular magnetic tunnel junctions consisting of a CoFeB free layer and a CoFeB/Ru/(Co/Pd)n exchanged coupled fixed layer is investigated. At first, the Ru and CoFeB layer thickness is tuned in the CoFeB/Ru/(Co/Pd)n structure to form a ferromagnetically exchange coupled structure with a strong PMA at an annealing treatment of 325 °C for 1 h. Then it is shown that that the CoFeB free layer thickness plays an important role in the switching current density. The switching current density decreases with the increase of the CoFeB free layer thickness. A minimum switching current density of 1.87 MA/cm2 is achieved for a device with 60 nm diameter. The mechanism involved in the switching current reduction with the decrease of CoFeB free layer thickness is also studied.

Spintronic logic gates for spintronic data using magnetic tunnel junctions

The emerging field of spintronics is undergoing exciting developments with the advances recently seen in spintronic devices, such as magnetic tunnel junctions (MTJs). While they make excellent memory devices, recently they have also been used to accomplish logic functions. The properties of MTJs are greatly different from those of electronic devices like CMOS semiconductors. This makes it challenging to design circuits that can efficiently leverage the spintronic capabilities. The current approaches to achieving logic functionality with MTJs include designing an integrated CMOS and MTJ circuit, where CMOS devices are used for implementing the required intermediate read and write circuitry. The problem with this approach is that such intermediate circuitry adds overheads of area, delay and power consumption to the logic circuit. In this paper, we present a circuit to accomplish logic operations using MTJs on data that is stored in other MTJs, without an intermediate electronic circuitry. This thus reduces the performance overheads of the spintronic circuit while also simplifying fabrication. With this circuit, we discuss the notion of performing logic operations with a non-volatile memory device and compare it with the traditional method of computation with separate logic and memory units. We find that the MTJ-based logic unit has the potential to offer a higher energy-delay efficiency than that of a CMOS-based logic operation on data stored in a separate memory module.

Probing dipole coupled nanomagnets using magnetoresistance read

We experimentally demonstrated magnetoresistance (MR) read of dipole coupled nanomagnets using magnetic tunnel junctions. The MR allowed the magnetic state of individual nanomagnets to be electrically measured. The sensitivity of the read scheme enabled a systematic study regarding the nanomagnet spacing and revealed a transition in behavior. Below a spacing of 15 nm the dipole field overcomes the individual shape anisotropy and redefines the individual element easy axis along the direction transmission line. The demonstration of MR electrical read marks a significant step forward for applications such as magnetic quantum cellular automata logic devices.

Magnetic Tunnel Junction Logic Architecture for Realization of Simultaneous Computation and Communication

We investigated magnetic tunnel junction (MTJ)-based circuit that allows direct communication between elements without intermediate sensing amplifiers. Two- and three-input circuits that consist of two and three MTJs connected in parallel, respectively, were fabricated and are compared. The direct communication is realized by connecting the output in series with the input and applying voltage across the series connections. The logic circuit relies on the fact that a change in resistance at the input modulates the voltage that is needed to supply the critical current for spin-transfer torque switching the output. The change in the resistance at the input resulted in a voltage swing of 50-200 mV and 250-300 mV for the closest input states for the three and two input designs, respectively. The two input logic gate realizes the AND, NAND, NOR, and OR logic functions. The three-input logic function realizes the majority, AND, NAND, NOR, and OR logic operations.

Communication Between Magnetic Tunnel Junctions Using Spin-Polarized Current for Logic Applications

We investigated spin-polarized current controlled magnetic tunnel junctions (MTJs) connected with a nano-magnetic channel using micromagnetic simulations. A spin polarized current is used to switch the MTJ and form a domain wall in the nanochannel that is captured at a notch in the channel. A current can then be applied along the nanochannel to drive the domain wall to the other MTJ. The nanochannel design was optimized by varying MTJ pillar size, nanochannel width, material, and notch sizes. Both in-plane and perpendicular anisotropy were simulated. Perpendicular anisotropy was found to be the most beneficial by alleviating the dominate effect of shape anisotropy. Four different nanochannels were connected to a single MTJ with perpendicular anisotropy and a domain wall formed in each channel. This allows fan out to be realized and consequently more complex logic functions. The current densities to switch the MTJ elements and drive the domain wall were found to be on the order of 1 × 108 A/cm2 and 6 × 109 A/cm2, respectively.

Spin transfer torque programming dipole coupled nanomagnet arrays

We experimentally demonstrated spin transfer torque (STT) programming of dipole coupled nanomagnets using magnetic tunnel junctions. The STT write operations were performed in conjunction with a clock field used in magnetic quantum cellular automata (MQCA) operations. The spacing and number of nanomagnets in the transmission line strongly affected the STT programming of the individual pillars. These MQCA transmission lines ranged in length from 2 elements to 20 elements, while device sizes ranged between 50 nm × 80 nm and 70 nm × 100 nm with spacing between 10 nm and 15 nm. With the application of the clock field, currents of 100-200 μA are sufficient to STT program the device. The demonstration of STT programming of individual nanomagnets in a dipole coupled array marks a significant step forward for applications such as MQCA logic device.