Understanding of Phase Noise Squeezing Under Fractional Synchronization of a Nonlinear Spin Transfer Vortex Oscillator

Abstract

We investigate experimentally the synchronization of a vortex based spin transfer oscillator to an external rf current whose frequency is at multiple integers, as well as half integer, of the oscillator frequency. Through a theoretical study of the locking process, we highlight both the crucial role of the symmetries of the spin torques acting on the magnetic vortex and the nonlinear properties of the oscillator on the phase locking process. Through the achievement of a perfect injection locking state, we report a record phase noise reduction down to -90dBc/Hz at 1 kHz offset frequency. The phase noise of these nanoscale oscillators is demonstrating as being low and controllable which is of significant importance for real applications using spin transfer devices.In the last decade, large expectations have been anticipated on how the rich spin transfer physics will give birth to a new generation of multifunctional spintronic devices [1]. The tunable response of spin torque devices has been predicted to play a crucial role in several domains such as radio frequency [2] or magnonic [3] nanoscale and low energy cost devices for ICTs as well as neuroinspired memory devices [4]. For all of these potential applications, and notably for the corresponding microwave applications, it is essential to identify the mechanisms leading to a fine control of the phase of these spin torque devices. Indeed, it has been often emphasized that their nonlinear behavior gives a unique opportunity to tune their radiofrequency properties [5][6][7] but at the cost of large phase noise, not compatible with targeted applications [1,2]. In order to tackle these issues, a solution is to rely either on their synchronization to a reference external signal [8][9][10][11] or to achieve mutual synchronization [12,13] in arrays of spin torque nano-oscillators (STNOs). However, in all the reported studies made on the locking regime of STNOs, the phase noise, often measured through the estimation of the spectral linewidth measured with a spectrum analyzer, remains large, typically in the kHz range. This feature reveals that phase slips associated with the large thermal energy lead to a loss of synchronization [9,10,14] and have a detrimental and non-controllable impact on the expected behavior of STNOs.In this letter, we investigate the mechanism leading to a perfect phase locking of a double vortex based STNO to an external rf current with a F s frequency at f 0 /2, f 0 and 2f 0 , where f 0 is the frequency of our STNO. Indeed, thanks to their large intrinsic coherence compared to other types of STNOs [7,15,16], we succeed to elucidate the strong correlation between the oscillator parameters and the locking process through a thorough experimental study combining time domain measurements and analytical developments. This allows understanding of the locking range characteristics [8,17,18] as well as the high phase coherence in the locked regime [19][20][21]. Our results demonstrate the specific spin transfer locking process of our vortex based STNO, a...

Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction

“…As the RF power is increased and a τ lock of 100% is achieved, we only observe phase slips of π, which, according to Ref. [38], is a signature of phase locking with an injected signal at f RF = 2f 0 .…”

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

“…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.…”

Superharmonic injection locking of nanocontact spin-torque vortex oscillators

“…The CPW was formed using a sputtered Cu(1200)/Au (20) bilayer and a lift off technique, and was specifically designed to only cover approximately half of the spin valve mesa allowing for optical access to the exposed FL (capped with SiO2), Figure 1(c) (c.f. CPW design in Reference 13 ).…”

“…6 At the same time, it is recognised that the linewidth of STOs can be undesirably large, owing to the frequency and phase noise of the auto-oscillations. 15,16,17,18,19,20 This has led to a number of studies of the mutual synchronization of multiple STOs 21,22,23,24,25,26,27,28,13,29,30 and phase-locking of STOs to an injected microwave (RF) current 20,31,32,33,34,35,36 in an attempt to enhance their output power and phase stability.…”

“…20 In a nano-pillar STVO the magnetization dynamics are confined by the physical boundaries of the pillar. In contrast, here we demonstrate SHIL in a nano-contact (NC) geometry in which the applied current was confined by the NC before entering an extended spin valve film.…”