A New Circuit Model for Spin-Torque Oscillator Including Perpendicular Torque of Magnetic Tunnel Junction

Spin-torque oscillator (STO) is a promising new technology for the future RF oscillators, which is based on the spin-transfer torque (STT) effect in magnetic multilayered nanostructure. It is expected to provide a larger tunability, smaller size, lower power consumption, and higher level of integration than the semiconductor-based oscillators. In our previous work, a circuit-level model of the giant magnetoresistance (GMR) STO was proposed. In this paper, we present a physics-based circuit-level model of the magnetic tunnel junction (MTJ)-based STO. MTJ-STO model includes the effect of perpendicular torque that has been ignored in the GMR-STO model. The variations of three major characteristics, generation frequency, mean oscillation power, and generation linewidth of an MTJ-STO with respect to the amount of perpendicular torque, are investigated, and the results are applied to our model. The operation of the model was verified by HSPICE simulation, and the results show an excellent agreement with the experimental data. The results also prove that a full circuit-level simulation with MJT-STO devices can be made with our proposed model. 1. Introduction Spin-torque oscillator (STO) is a promising new device which is based on the spin-transfer torque (STT) effect in magnetic multilayered nanostructure. According to the theory presented by Slonczewski [1] and Berger [2], spin-polarized current may produce precessing magnetization in the ferromagnetic material. Many experiments and researches have been carried out to support the theory about STO. Recently, interest in the magnetic tunnel junction (MTJ) which uses a thin dielectric spacer (e.g., MgO) as is shown in Figure 1(a) is rapidly growing because it may overcome the limitation of small power in giant magnetoresistance (GMR) devices. Figure 1: Simple schematic diagrams of (a) MTJ-STO device, (b) three axis components of the external and internal magnetic fields. The perpendicular torque acts like a magnetic field and is added to the direction of the fixed layer magnetization. The effect of STT in MTJ-STO can be explained by Landau-Lifshitz-Gilbert (LLG) equation that represents the movement of the magnetization. LLG equation has been proved very useful in describing the precessional motion of magnetization in the ferromagnetic material. In this equation, spin torque consists of two components: in-plane torque ( ) and perpendicular torque ( ). In a metallic GMR device, the perpendicular torque is very small compared with the in-plane torque such that it can be safely ignored. In MTJ devices, however, the