Mohammad Kazemi

Compact Model for Spin–Orbit Magnetic Tunnel Junctions

Electrical control of a magnetic tunnel junction (MTJ) through spin-orbit torques (SOTs) offers opportunities to introduce MTJs into high-performance, low energy applications. SOTs support a high-speed and energy-efficient three terminal MTJ with perpendicular-to-the-plane magnetization (PMTJ). The read path is separated from the write path, enhancing the reliability of the device. SOTs exhibit two coexisting contributions: 1) a damping-like torque and 2) a field-like torque. In this paper, a physics-based compact model for a three terminal PMTJ is presented, which accurately models the magnetic, electrical, and thermal behaviors of a PMTJ controlled through SOTs. The proposed compact model is validated with experimental data, exhibiting reasonable accuracy with an average error of <;5.4%. The integration capability of the proposed compact model with CMOS technology is also demonstrated.

Adaptive Compact Magnetic Tunnel Junction Model

Electrical control of magnetic tunnel junctions (MTJs) provides opportunities to introduce MTJs into high-performance applications requiring low power consumption. The magnetic state of an MTJ can be electrically controlled through: 1) the spin transfer torque (STT) effect; 2) the voltage controlled magnetic anisotropy (VCMA) effect; and 3) the fusion of STT and VCMA. Several compact models have been published for MTJs. All of these models consider an MTJ whose magnetic state is controlled through the STT effect. In this paper, a model of an MTJ comprising a free layer, an analysis layer, and a spin polarizing layer is described. The MTJ compact model, adaptive compact MTJ (ACM) model, includes the effects of asymmetry on the MTJ behavior, and models a device controlled through the STT, VCMA, or a fused STT-VCMA mechanism. The ACM model includes the dynamics of the junction temperature. The proposed model can be adapted to experimental configurations including in-plane MTJ (IMTJ), IMTJ with a perpendicular-to-the-plane polarizer, perpendicular-to-the-plane MTJ (PMTJ), and PMTJ with an additional easy axis. The ACM model is validated with published experimental data, showing reasonably accurate results with an average error of less than 6%.

On the capacity of the state-dependent cognitive interference channel

We derive the capacity region of two classes of the discrete memoryless state-dependent cognitive interference channels (SD-CICs) with noncausal channel state information known to only the cognitive transmitter: semideterministic SD-CIC and deterministic SD-CIC. We also provide new inner and outer bounds on the capacity region of the general SD-CIC. We prove that the new outer bound is the capacity region of the SD-CIC in the better cognitive decoding regime when both the cognitive transmitter and its corresponding receiver are aware of the channel state information in a noncausal manner.

Energy-Efficient Nonvolatile Flip-Flop With Subnanosecond Data Backup Time for Fine-Grain Power Gating

A nonvolatile flip-flop (NVFF) is proposed, where magnetic tunnel junctions (MTJs) are incorporated into a CMOS flip-flop (FF) to enable nonvolatility. The voltage-controlled magnetic anisotropy (VCMA) effect is utilized to back up the latched data into MTJs before the power supply is turned off. Switching an MTJ through the VCMA effect does not require a dedicated write circuit for data backup, resulting in reduced area as compared with NVFFs exploiting the spin transfer torque (STT) switching mechanism. In a VCMA-based NVFF, the MTJs are coherently switched, enabling ultra-energy efficient data backup with subnanosecond backup time. Simulation results exhibit more than a 342× (33.7×) improvement in data backup energy per bit, and more than 35.5× (7.7×) improvement in data backup delay per bit as compared with the most efficient STT-based NVFFs (spin Hall effect-based NVFF). The energy efficiency of the VCMA-based NVFF results in sufficiently short breakeven times, enabling effective fine-grain power gating.

On the capacity region of the partially cooperative relay cognitive interference channel

We derive a new upper bound on the capacity region of the discrete memoryless partially cooperative relay cognitive interference channel (PC-RCIC). We show that our new upper bound is the capacity region of the semideterministic discrete memoryless PC-RCIC, where the channel output observed by the relay is a deterministic function of the channel inputs.

All-Spin-Orbit Switching of Perpendicular Magnetization

Devices with ferromagnetic layers possessing a perpendicular magnetic easy axis are of great interest due to miniaturization capability and thermal stability, retaining deeply scaled magnetic bits over long periods of time. While the tunneling magnetoresistance effect has significantly enhanced electrical reading of magnetic bits, fast and energy efficient writing of magnetic bits remains a challenge. Current-induced spin-orbit torques (SOTs) have been widely considered due to significant potential for fast and energy-efficient writing of magnetic bits. However, to deterministically switch the magnetization of a perpendicularly magnetized device using SOTs, the presence of a magnetic field is required, which offsets possible advantages and hampers applications. In this paper, a perpendicularly magnetized device is presented, which, without the need for a magnetic field, can be deterministically switched in both toggle and nontoggle modes using a damping-like SOT induced by an in-plane current pulse. This capability is realized by shaping the magnetic energy landscape. Present device does not require any materials other than those widely utilized in conventional spin-orbit devices. The device provides two orders of magnitude enhancement in switching energy-time product as compared with state-of-the-art perpendicularly magnetized devices operating on spin-transfer torques.

gMRAM: Gain-cell magnetoresistive random access memory for high density embedded storage and in-situ computing

High density embedded memories have been demanded increasingly to enhance the performance and reduce the power dissipation of advanced systems, such as multicore processors, which have been used in a wide variety of applications from servers to Internet-of-things (IoT) devices. In this paper, a memory cell, referred to as the gain-cell magnetoresistive random access memory (gMRAM), is introduced. The gMRAM significantly reduces the cell area per bit as compared to state-of-the-art embedded memories, provides opportunities for the joint enhancement of performance and reduction of power dissipation, and provides a natural basis for in-situ computing. A gMRAM cell simultaneously retains two bits, a nonvolatile bit using a magnetic tunnel junction (MTJ) and a dynamic bit using the access transistor of the MTJ. The two bits are independently and nondestructively accessible for read and write. This paper focuses on the design and characterization of the gMRAM cell, array architecture, and read/write circuitry. Simulation results from an 8 kb gMRAM array using a 14 nm standard FinFET CMOS technology demonstrate a 750 ps / 475 ps access time for dynamic read/write, while the nonvolatile bit can be read from or written to the same cell with a, respectively, 750 ps and 3.5 ns access time. The gMRAM cell area per bit is 3× (2×) smaller than a SRAM (3T eDRAM) cell in the same technology.

An electrically reconfigurable logic gate intrinsically enabled by spin-orbit materials

The spin degree of freedom in magnetic devices has been discussed widely for computing, since it could significantly reduce energy dissipation, might enable beyond Von Neumann computing, and could have applications in quantum computing. For spin-based computing to become widespread, however, energy efficient logic gates comprising as few devices as possible are required. Considerable recent progress has been reported in this area. However, proposals for spin-based logic either require ancillary charge-based devices and circuits in each individual gate or adopt principals underlying charge-based computing by employing ancillary spin-based devices, which largely negates possible advantages. Here, we show that spin-orbit materials possess an intrinsic basis for the execution of logic operations. We present a spin-orbit logic gate that performs a universal logic operation utilizing the minimum possible number of devices, that is, the essential devices required for representing the logic operands. Also, whereas the previous proposals for spin-based logic require extra devices in each individual gate to provide reconfigurability, the proposed gate is ‘electrically’ reconfigurable at run-time simply by setting the amplitude of the clock pulse applied to the gate. We demonstrate, analytically and numerically with experimentally benchmarked models, that the gate performs logic operations and simultaneously stores the result, realizing the ‘stateful’ spin-based logic scalable to ultralow energy dissipation.

Capacity region and optimum power allocation strategies for fading cognitive relay multiple access channels

We consider a fading Gaussian cognitive relay multiple-access channel (CR-MAC), consisting of K sources and an active cognitive relay that wish to simultaneously communicate with a single destination. We assume that the relay has its own message to communicate and is aware of the messages of the other K sources non-causally. Furthermore, the message of the relay and the messages of the other K sources are independent. Assuming that all the sources, the cognitive relay and the destination are aware of the instantaneous channel state information (CSI) we characterize the ergodic capacity region. For the case where the cognitive relay does not have its own message to transmit, we derive the optimal power allocation strategies that achieve any arbitrary point on the boundary surface of the capacity region.

The influence of container geometry and thermal conductivity on evaporation of water at low pressures.

Evaporation is a ubiquitous phenomenon that occurs ceaselessly in nature to maintain life on earth. Given its importance in many scientific and industrial fields, extensive experimental and theoretical studies have explored evaporation phenomena. The physics of the bulk fluid is generally well understood. However, the near-interface region has many unknowns, including the presence and characteristics of the thin surface-tension-driven interface flow, and the role and relative importance of thermodynamics, fluid mechanics and heat transfer in evaporation at the surface. Herein, we report a theoretical study on water evaporation at reduced pressures from four different geometries using a validated numerical model. This study reveals the profound role of heat transfer, not previously recognized. It also provides new insight into when a thermocapillary flow develops during water evaporation, and how the themocapillary flow interacts with the buoyancy flow. This results in a clearer picture for researchers undertaking fundamental studies on evaporation and developing new applications.