The dynamic characteristics of a spin transfer torque magnetoresistive random access memory crossbar array during write operations were investigated. A spin transfer torque magnetic tunnel junction was combined with a two-terminal selector device instead of a three-terminal CMOS transistor in the crossbar array architecture. The characteristics of the crossbar array architecture were investigated under different bias schemes and transient simulations of write operations were performed under various operating conditions. The variance of the switching time and problematic behaviors was investigated. The floating bias scheme was compared with the 1/2 bias scheme, and simulation results revealed that write errors may be induced in the floating bias scheme by abnormal glitches occurring when the parasitic capacitance becomes large.
Power consumption of large-scale crossbar array architecture is investigated by the comprehensive crossbar array matrix model. The power dissipation is examined as functions of array size, leakage current of selector, and various bias schemes. The power consumption increases as the array size and the leakage current of selector increases. In addition, 1/3 bias scheme shows power consumption about 1~2 orders of magnitude larger than other bias schemes. This phenomenon is induced from the unselected cells which is delivered with voltage about V dd /3, whereas the voltage of unselected cells are almost 0 V for 1/2 bias and floating bias schemes.
Spin-transfer torque-based magnetoresistive random access memory (STT-MRAM) is a promising candidate for universal memory that may replace traditional memory forms. It is expected to provide high-speed operation, scalability, low-power dissipation, and high endurance. MRAM switching technology has evolved from the field-induced magnetic switching (FIMS) technique to the spin-transfer torque (STT) switching technique. Additionally, material technology that induces perpendicular magnetic anisotropy (PMA) facilitates low-power operation through the reduction of the switching current density. In this paper, the modeling of magnetic tunnel junctions (MTJs) is reviewed. Modeling methods and models of MTJ characteristics are classified into two groups, macromodels and behavioral models, and the most important characteristics of MTJs, the voltage-dependent MTJ resistance and the switching behavior, are compared. To represent the voltage dependency of MTJ resistance, some models are based on physical mechanisms, such as Landau-Lifshitz-Gilbert (LLG) equation or voltage-dependent conductance. Some behavioral models are constructed by adding fitting parameters or introducing new physical parameters to represent the complex switching behavior of an MTJ over a wide range of input current conditions. Other models that are not based on physical mechanisms are implemented by simply fitting to experimental data.
Spin-torque oscillators (STO) is a new device that can be used as a tunable microwave source in various wireless devices. Spin-transfer torque effect in magnetic multilayered nanostructure can induce precession of magnetization when bias current and external magnetic field are properly applied, and a microwave signal is generated from that precession. We proposed a semi-empirical circuit-level model of an STO in previous work. In this paper, we present a refined STO model which gives more accuracy by considering physical phenomena in the calculation of effective field. Characteristics of the STO are expressed as functions of external magnetic field and bias current in Verilog-A HDL such that they can be simulated with circuit-level simulators such as Hspice. The simulation results are in good agreement with the experimental data.
The spin-torque oscillator (STO) is a new compact device operating as a tunable RF oscillator in the tens of gigahertz range whose characteristics are determined by the applied current and magnetic field. In this paper, we present a physics-based empirical circuit-level model of an STO that is compatible with circuit-level simulators such as SPICE. The characteristics of an STO are modeled as physics-based analytic functions of the applied current and external magnetic field. The validity of our model was verified by the HSPICE simulation of a current mirror circuit that contains an STO element. The simulation results are in good agreement with the experimental data in the normal operation range. High-order nonlinear effects at large currents are not included in our model because there is no theoretical equation available yet that can precisely explain these effects.
The spin-torque oscillator (STO) is a new compact device operating as a tunable RF oscillator in the tens of gigahertz range whose characteristics are determined by the applied current and magnetic field. In this paper, we present a physics-based empirical circuit-level model of an STO that is compatible with circuit-level simulators such as SPICE. The characteristics of an STO are modeled as physics-based analytic functions of the applied current and external magnetic field. The validity of our model was verified by the HSPICE simulation of a current mirror circuit that contains an STO element. The simulation results are in good agreement with the experimental data in the normal operation range. High-order nonlinear effects at large currents are not included in our model because there is no theoretical equation available yet that can precisely explain these effects.
The spin-torque oscillator (STO) is a new compact device operating as a tunable RF oscillator in the tens of gigahertz range whose characteristics are determined by the applied current and magnetic field. In this paper, we present a physics-based empirical circuit-level model of an STO that is compatible with circuit-level simulators such as SPICE. The characteristics of an STO are modeled as physics-based analytic functions of the applied current and external magnetic field. The validity of our model was verified by the HSPICE simulation of a current mirror circuit that contains an STO element. The simulation results are in good agreement with the experimental data in the normal operation range. High-order nonlinear effects at large currents are not included in our model because there is no theoretical equation available yet that can precisely explain these effects.
Spin-torque oscillators (STOs) are new oscillating devices based on spintronics technology with many advantageous features, i.e., nanoscale size, high tunability, and compatibility with standard silicon processing. Recent research has shown that two electrically connected STOs may operate as a single device when specific conditions are met. To overcome the limitation of the small output power of STOs, the phase-locking behavior of multiple STOs is hereby extensively investigated. In this paper, we present a circuit-level model of two coupled STOs considering the interaction between them such that it can represent the phase-locking behavior of multiple STOs. In our model, the characteristics of each STO are defined first as functions of applied DC current and external magnetic field. Then, the phase-locking condition is examined to determine the properties of the two coupled STOs on the basis of a theoretical model. The analytic model of two coupled STOs is written in Verilog-A hardware description language. The behavior of the proposed model is verified by circuit-level simulation using HSPICE with CMOS circuits including a current-mirror circuit and differential amplifiers. Simulation results with various CMOS circuits have confirmed the effectiveness of our model.
The read margin and power consumption for various selector characteristics and bias schemes are analyzed during read operation. The 1/2 and 1/3 bias schemes exhibit different read operation properties. There is a trade-off between the read margin and power consumption that depends on the bias scheme and characteristics of the selector. The read margin of the 1/2 bias scheme is decreased at low on/off ratios because of the output voltage drop across the LRS cell. This drop is due to a rapid increase in the leakage current when a selector with low on/off ratio is used in the 1/2 bias scheme. Therefore, the bias scheme and selector should be selected appropriately according to the purpose of the application.
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