David M. Bromberg

mLogic: ultra-low voltage non-volatile logic circuits using STT-MTJ devices

This paper introduces the design of logic circuits based exclusively on novel magnetoelectronic devices. Current signals are steered by 2× resistance change switching while operating with sub-100 mV voltage pulses for power and synchronization. The inherent memory of the devices results in fully pipelined nonvolatile logic. We demonstrate that co-optimization of the devices, circuits and logic can achieve ultra-low energy-per-operation for design examples.

Novel STT-MTJ Device Enabling All-Metallic Logic Circuits

Spintronics is an emerging platform for logic circuit design. Though many approaches have been proposed, and several have met with some success, a number of challenges remain. Here, we introduce a new style of magnetic logic-mLogic-enabled by a novel four-terminal device. An input current pulse to the device switches the magnetic state by spin transfer torque-driven domain wall motion, which programs the resistance state of a magnetic tunnel junction in an electrically-insulated but magnetically-coupled path. Despite the low switching ratios of the devices, limited by the tunnel magnetoresistance ratio, logic circuits independent of CMOS may be configured using current-based signaling. Micromagnetic analysis of device performance and the concepts of mLogic circuit design are introduced, with SPICE simulation of logic gates demonstrating correct logic operation at supplies below 100 mV.

Increased Perpendicular TMR in FeCoB/MgO/FeCoB Magnetic Tunnel Junctions by Seedlayer Modifications

By modifying the seedlayer in perpendicular FeCoB/MgO/FeCoB magnetic tunnel junctions (MTJs), we observe an increase in maximum tunneling magnetoresistance (TMR) from 65% up to 138%. Its found that decreasing the Ta deposition rate in Ta/Ru/Ta underlayers allows for greater annealing temperatures (up to 350 ) while still maintaining a perpendicular easy axis. An improvement is also seen at a lower temperature where both seedlayers maintain a perpendicular FeCoB easy axis indicating that the increase in TMR is not solely related to annealing at a higher temperature.

An equivalent circuit formulation of the power flow problem with current and voltage state variables

Steady state analysis of power grids is typically performed using power flow analysis, where nonlinear balance equations of real and reactive power are solved to calculate the voltage magnitude, phase, and power at every bus. Transient analysis of the same power grids are performed using circuit simulation methods. We propose a novel approach to modeling the nonlinear steady state behavior of power grids in terms of equivalent circuits with currents and voltages as the state variables that is a step toward unifying transient and steady state models. A graph theoretic formulation approach is used to solve the circuits that enables incorporation of switch models for contingency analyses. Superior nonlinear steady state convergence is demonstrated by use of current as a state variable and application of circuit simulation methods. Furthermore, current and voltage state variables will offer greater compatibility with future smart grid components and monitors.

Naturally Oxidized FeCo as a Magnetic Coupling Layer for Electrically Isolated Read/Write Paths in mLogic

Recently, a nonvolatile, low power circuit scheme based on current-induced domain wall motion and perpendicular magnetic tunnel junctions known as mLogic has been proposed that requires electrically isolated, magnetically coupled read and write paths. Here, we suggest naturally oxidized FeCo for the magnetic coupling layer. Relevant properties of the FeCo-oxide were evaluated by a preliminary investigation of [FeCo/FeCo-Oxide]N artificial superlattices. It is found that FeCo thin films form an insulating 11 Å passivating oxide layer with a magnetization of 500 emu/cc. Experimental measurements show that FeCo-oxide can couple two perpendicular FeCoB layers with a coupling strength greater than 0.35 ergs/cm2.

Experimental demonstration of four-terminal magnetic logic device with separate read- and write-paths

Magnetic logic has recently become an attractive candidate for future electronics. This paper describes the demonstration of a four-terminal spintronic device with distinct read- and write-paths (“mCell”). The mCell enables a non-volatile circuit technology (mLogic) with gain sufficient to drive fanout independent of CMOS [1]. Measured material properties and prototype device results are presented.

All-magnetic magnetoresistive random access memory based on four terminal mCell device

Magnetoresistive random access memory (MRAM) is a promising candidate to enable fast, non-volatile storage on chip. In this paper, we present an MRAM design where each bitcell is comprised entirely of four-terminal magnetic devices (“mCells”) with no CMOS access transistors. We show that this design can achieve significant energy and area savings compared to the standard one transistor-one magnetic tunnel junction (1T1MTJ) bitcell based design. We estimate a write energy of ≈5 fJ/bit based on bitline and wordline voltages that operate at less than 100 mV with projected area smaller than that possible with aggressively scaled 10 nm node FinFETs in the 1T1MTJ design.

An equivalent circuit formulation for three-phase power flow analysis of distribution systems

In this paper, we describe a power flow formulation for 3-phase distribution systems that is based on an equivalent circuit model. It is shown that this physical model based solution is able to accommodate a wide range of complex and unbalanced loads without loss of generality. The approach is an extension of the single phase formulation in [1] that uses current and voltage as the state variables. This formulation is shown to provide excellent modeling efficiency for distribution system components, such as induction motors that can be modeled as linear circuit elements. The formulation is further capable of incorporating complex nonlinear models to capture more details or represent future bus models. A challenging IEEE 4-bus test case is used as a proof of concept to demonstrate the efficacy of this approach.

Annealing effect and under/capping layer study on Co/Ni multilayer thin films for domain wall motion

Co/Ni multilayer structure with Perpendicular Magnetic Anisotropy (PMA) is considered to be one of the most promising film structures for current-driven domain wall motion. In this work, the field annealing effect on Co/Ni multilayer films with different underlayers is studied. The annealing temperature ranges from 250 °C to 375 °C. The effect of Pt capping layer is also investigated. It was found that the annealing process influences magnetic properties of Co/Ni multilayers with different underlayers differently. For Co/Ni multilayers with Ni underlayer, no PMA is observed in the as-deposited state, but they become perpendicular after annealing, and the effective perpendicular magnetic anisotropy (Keff) increases linearly with annealing temperature. The origin of the Keff increase is discussed. For Co/Ni multilayers with Pt underlayers, large PMA is observed for as-deposited films; however, Keff decreases after annealing. The effects of interfacial lattice mismatch, roughness, and impurities to surface a...

Improving robustness and modeling generality for power flow analysis

In this paper we present an equivalent circuit model for power system networks that facilitates robust and efficient AC power flow simulation and enables the incorporation of more generalized bus and line models. The circuit equations are formulated in terms of voltages and currents in rectangular coordinates using a graph theoretic algorithm that provides for optimal numerical conditioning. A current-source based generator model is introduced that provides for more robust and efficient convergence as compared to our original approach. We show that the proposed framework supports nonlinear models with insensitivity to the initial guess and converges in few iterations. We illustrate the capabilities of generalized modeling by deriving a model for a grid-connected solar panel system that includes AC, DC and semiconductor components.