Spin-torque-induced dynamics at fine-split frequencies in nano-oscillators with two stacked vortices.

V Sluka,C M Schneider,A Kákay,A M Deac,R Hertel,D E Bürgler

doi:10.1038/ncomms7409

Abstract

The gyrotropic rotation around the equilibrium position constitutes the fundamental excitation of magnetic vortices in nanostructures. The frequency of this mode varies with material and sample geometry, but is independent of the vortex handedness and its core direction. Here, we demonstrate that this degeneracy is lifted in a spin-torque oscillator containing two vortices stacked on top of each other. When driven by spin-polarized currents, such devices exhibit a set of dynamic modes with discretely split frequencies, each corresponding to a specific combination of vorticities and relative core polarities. The fine splitting occurs even in the absence of external fields, demonstrating that such devices can function as zero-field, multi-channel, nano-oscillators for communication technologies. It also facilitates the detection of the relative core polarization and allows for the eight non-degenerate configurations to be distinguished electrically, which may enable the design of multi-state memory devices based on double-vortex nanopillars.

Highlights

The gyrotropic rotation around the equilibrium position constitutes the fundamental excitation of magnetic vortices in nanostructures
Magnetic vortices in thin-film elements have been intensively studied in the past few years, as their simple structure combined with their highly non-trivial dynamic properties make them fascinating objects of research
The tiny core is surprisingly important for the fundamental excitation of a vortex, the gyrotropic mode[4,5,6], as it determines the sense of rotation

Summary

Introduction

The gyrotropic rotation around the equilibrium position constitutes the fundamental excitation of magnetic vortices in nanostructures The frequency of this mode varies with material and sample geometry, but is independent of the vortex handedness and its core direction. The tiny core is surprisingly important for the fundamental excitation of a vortex, the gyrotropic mode[4,5,6], as it determines the sense of rotation This mode is the lowest-frequency excitation known in magnetism, with frequencies far below the ferromagnetic resonance of the corresponding material. STNOs exploiting the gyrotropic mode of vortices can overcome most of these difficulties, since they operate without applied field and display extremely narrow linewidths (in the order of MHz) Their tuneability is limited, as the gyrotropic frequency is specific to the individual sample design[17]. While spin-transfer switching has been identified as a potential write method early-on[23], the lack of suitable methods of reading the core orientation has been the main cause hampering the development of vortex-based memory devices to-date

Methods

Results

Conclusion