Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators

Keatley, P. S.; Sani, Sohrab R.; Hrkac, G.; Mohseni, Seyed Majid; Dürrenfeld, Philipp; Loughran, T. H. J.; Åkerman, Johan; Hicken, R. J.

doi:10.1103/physrevb.94.060402

Cited by 20 publications

(23 citation statements)

References 37 publications

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“…The magnetisation dynamics of nano-pillar STOs are physically confined to the patterned magnetic layers, while in nano-contact STOs the dynamics are free to extend beyond the footprint of the nano-contact and are the subject of this article. These extended dynamics have been used as a mechanism to phase-lock multiple nano-contact STOs that share the same magnetic layers [18], [19], [20], [21], [22], [23]. Such phase-locking is anticipated to overcome the challenges of poor output power and phase stability exhibited by individual devices, which currently prevent their use in technological applications [24], [25].…”

Section: Introductionmentioning

confidence: 99%

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting

Keatley¹,

Sani²,

Hrkac³

et al. 2017

J. Phys. D: Appl. Phys.

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Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-todevice variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by injecting an RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

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Section: Introductionmentioning

confidence: 99%

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting

Keatley¹,

Sani²,

Hrkac³

et al. 2017

J. Phys. D: Appl. Phys.

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“…The nanocontact vortex oscillator [23][24][25][26][27][28][29][30] represents an intriguing example, where different commensurate and incommensurate states appear due to competing selfoscillations [27]. The primary oscillation is driven by spin torques and involves self-sustained vortex gyration around the nanocontact [24], which is accompanied by relaxation oscillations in the form of periodic core reversal above a threshold current.…”

mentioning

confidence: 99%

Chaos in Magnetic Nanocontact Vortex Oscillators

et al. 2019

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We present an experimental study of spin-torque driven vortex self-oscillations in magnetic nanocontacts. We find that above a certain threshold in applied currents, the vortex gyration around the nanocontact is modulated by relaxation oscillations, which involve periodic reversals of the vortex core. This modulation leads to the appearance of commensurate but also more interestingly here, incommensurate states, which are characterized by devil's staircases in the modulation frequency. We use frequency-and time-domain measurements together with advanced time-series analyses to provide experimental evidence of chaos in incommensurate states of vortex oscillations, in agreement with theoretical predictions.

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“…In the last few years, many efforts have been made to improve the spectral purity of spintorque oscillators, a crucial step towards most applications. From these studies, a promising approach consists in the synchronization of the oscillators to an external microwave source [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (injection locking), or to other oscillators [21][22][23][24][25][26][27] (mutual synchronization). In both cases, a crucial parameter to optimize for applications is the locking range, which should be as large as possible.…”

mentioning

confidence: 99%

Enhancing the injection locking range of spin torque oscillators through mutual coupling

Romera

Talatchian

Lebrun

et al. 2016

Applied Physics Letters

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We investigate how the ability of the vortex oscillation mode of a spin-torque nanooscillator to lock to an external microwave signal is modified when it is coupled to another oscillator. We show experimentally that mutual electrical coupling can lead to locking range enhancements of a factor 1. 64. Furthermore, we analyze the evolution of the locking range as a function of the coupling strength through experiments and numerical simulations. By uncovering the mechanisms at stake in the locking range enhancement, our results will be useful for designing spin-torque nano-oscillators arrays with high sensitivities to external microwave stimuli.Spin-torque nano-oscillators (STNOs) are promising candidates for the next generation of multifunctional spintronic nano-devices [1] such as efficient integrated microwave generators [2] and detectors [3,4]. One of their characteristic features compared to other auto-oscillators is their high non-linearity [5]. On one hand, this high nonlinearity translates into one of their most attractive properties: their ability to easily adapt their frequencies to external stimuli. On the other hand, by increasing their sensitivity to noise, it has the undesirable effect of broadening the spectral linewidth. In the last few years, many efforts have been made to improve the spectral purity of spintorque oscillators, a crucial step towards most applications. From these studies, a promising approach consists in the synchronization of the oscillators to an external microwave source [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (injection locking), or to other oscillators [21][22][23][24][25][26][27] (mutual synchronization). In both cases, a crucial parameter to optimize for applications is the locking range, which should be as large as possible. A first way to increase the locking range is to work in regimes where the oscillator's nonlinearity is the largest. However, as mentioned earlier, this comes at the detriment of spectral purity. In the case of injection locking, another possibility is to increase the power of the external signal, but this is limited by the breakdown voltage of the tunnel barrier of the device and results in an increase of power consumption. Therefore, finding alternative paths to enhance the locking range is an important step towards designing next generation's magnetic microwave devices.In this manuscript we combine experimental results with numerical simulations to show that the injection locking range of a spin-torque oscillator can be enhanced by coupling it to another spintorque oscillator. Furthermore, it is shown that the locking range can be tuned by changing the mutual coupling strength between oscillators. (7) is the free layer and numbers represent thickness in nanometers. Samples were grown by sputterdeposition and patterned down to the bottom electrode into circular nanopillars with a diameter of 200 nm. The nano-pillars exhibit a TMR of 64% and a resistance-area product of RA~1 Ω.μm 2 . With this combination of materials and geometry, th...

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Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators

Cited by 20 publications

References 37 publications

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting

Chaos in Magnetic Nanocontact Vortex Oscillators

Enhancing the injection locking range of spin torque oscillators through mutual coupling

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