Phase-locking of multiple magnetic droplets by a microwave magnetic field

Chengjie Wang,Yaowen Liu,Yan Zhou,J Åkerman,Dun Xiao

doi:10.1063/1.4975660

Abstract

Manipulating dissipative magnetic droplet is of great interest for both the fundamental and technological reasons due to its potential applications in the high frequency spin-torque nano-oscillators. In this paper, a magnetic droplet pair localized in two identical or non-identical nano-contacts in a magnetic thin film with perpendicular anisotropy can phase-lock into a single resonance state by using an oscillating microwave magnetic field. This resonance state is a little away from the intrinsic precession frequency of the magnetic droplets. We found that the phase-locking frequency range increases with the increase of the microwave field strength. Furthermore, multiple droplets with a random initial phase can also be synchronized by a microwave field.

Highlights

Spin-torque oscillators (STOs)[1] have attracted considerable attentions with the potential of enabling novel spintronic devices for telecommunication and logic applications.[2,3,4,5,6,7] The STOs are typically fabricated in two different architectures: Nano-pillar[1] or nanoscale electrical contacts (NC)[8] to ferromagnetic thin films with a free magnetic layer and a fixed spin polarizer layer
By employing microwave (MW) magnetic fields, we show that two magnetic droplets formed at identical or non-identical NCs can phase-lock into a single resonance state over a frequency range close to the MW driving frequency
A droplet pair is generated at two identical NCs (r1 = r2 = 15 nm) by applying a current of 8 mA flowing in each NC

Summary

INTRODUCTION

Spin-torque oscillators (STOs)[1] have attracted considerable attentions with the potential of enabling novel spintronic devices for telecommunication and logic applications.[2,3,4,5,6,7] The STOs are typically fabricated in two different architectures: Nano-pillar[1] or nanoscale electrical contacts (NC)[8] to ferromagnetic thin films with a free magnetic layer and a fixed spin polarizer layer. The spin-transfer torque (STT) in such contacts can compensate the damping torque and excite steady state spin precession in the free layer at a threshold d.c. current. Micromagnetic modeling of the free layer was performed using the open-source simulation software MuMax[3,33] which is based on the Landau-Lifshitz-Gilbert equation including the STT term:[16,34] dm dt. The following material parameters measured on similar Co/Ni multilayers are used for the free layer:[11,14] Ms = 716 kA/m (saturation magnetization), Ku = 447 kJ/m3 (magnetic anisotropy), A = 30 pJ/m (exchange stiffness), α = 0.05 (Gilbert damping), P = 0.5 (spin polarization).

Synchronization of two droplets at identical NCs

Synchronization of a droplet pair at non-identical NCs

CONCLUSION