Spin–transfer torque oscillator in magnetic tunneling junction with short–wavelength magnon excitation

Shizhu Qiao,Tie Zhou,Qiang Li,Shandong Li,Shishen Yan,Yinrui Zhao

doi:10.1063/1.5023697

Abstract

Bloch–Bloembergen–Slonczewski (BBS) equation is established by extending Bloch–Bloembergen equation, and it is used to study magnetization oscillation in the free magnetic layer of a magnetic tunneling junction. Since both short–wavelength magnon excitation and spin–transfer torque are taken into account in the BBS equation, it is distinguished from Landau–Lifshitz–Gilbert–Slonczewski equation. The macro–spin BBS model predicts that the transverse relaxation time in free magnetic layer should be long enough, as compared with the longitudinal relaxation time, to achieve stable magnetization oscillation for spin–transfer torque oscillator application. Moreover, field–like torque favors the tolerance of fast transverse relaxation, which makes magnetic tunneling junction a better choice than spin valve for the spin–transfer torque oscillator application.

Highlights

Spin–transfer torque (STT) oscillator is one of the main applications of STT for its simple structure, small size, wide range of frequency and working temperature, ease of integration, and current tuning features.[1,2] Spin valve–based and magnetic tunneling junction (MTJ)–based STT oscillators have been successfully fabricated in laboratories.[3,4,5] The former could afford relatively large current density, but is subjected to relatively low output power
We show that the transverse relaxation time T 2 of free magnetic layer in MTJ should be long enough for STT oscillator application
Stable magnetization oscillation of the free magnetic layer, which is the prerequisite of STT oscillator, could be achieved when T 2 is large enough

Summary

INTRODUCTION

Spin–transfer torque (STT) oscillator is one of the main applications of STT for its simple structure, small size, wide range of frequency and working temperature, ease of integration, and current tuning features.[1,2] Spin valve–based and magnetic tunneling junction (MTJ)–based STT oscillators have been successfully fabricated in laboratories.[3,4,5] The former could afford relatively large current density, but is subjected to relatively low output power By contrast, the latter could output relatively large power, but is subjected to low current density limited by the barrier breakdown.[2,6]. Spin–orbit torque (SOT) caused by spin Hall effect generated pure spin current includes both field–like torque and damping–like torque,[26] and the conclusion of the BBS model in MTJ–based STT oscillator can be extended into SOT in heavy metal/ferromagnet structure. The field–like torque favors tolerating high transverse relaxation rate, which makes MTJ more suitable for STT oscillator application than spin valve

THEORETICAL MODEL

CALCULATION RESULTS AND DISCUSSION

CONCLUSIONS