Origin of Spectral Purity and Tuning Sensitivity in a Spin Transfer Vortex Nano-Oscillator

Hamadeh, A.; Locatelli, Nicolas; Naletov, V. V.; Lebrun, Romain; Loubens, G. de; Grollier, Julie; Klein, O.; Cros, Vincent

doi:10.1103/physrevlett.112.257201

Cited by 47 publications

(45 citation statements)

References 47 publications

(47 reference statements)

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“…We note that a similar interaction of steady state and damped modes via their harmonics has been reported in Ref. 18 for vortex excitations.…”

Section: Ivb4 Discussionmentioning

confidence: 71%

“…However, important questions remain open and many features observed experimentally cannot be explained in the frame of this 'independent free layer' picture. Recent investigations have addressed the coupling between the different magnetic layers of an STO and demonstrated that it strongly impacts the excitations spectra [6][7][8][9][10][11][12][13][14][15][16][17][18] , and that it can be exploited in the interest of the device performances [7][8][9][10] .…”

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confidence: 99%

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Influence of interlayer coupling on the spin-torque-driven excitations in a spin-torque oscillator

et al. 2017

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The influence of dynamic interlayer interactions on the spin torque driven and damped excitations are illustrated for a three layer macrospin model system that corresponds to a standard spin-torque oscillator. The free layer and a synthetic antiferromagnetic (SyF) pinned layer of the spin-torque oscillator are in-plane magnetized. In order to understand experimental results, numerical simulations have been performed considering three types of interlayer interactions: exchange interaction between the two magnetic layers of the SyF, mutual spin torque between the top layer of the SyF and the free layer and dipolar interaction between all three magnetic layers. It will be shown that the dynamic dipolar coupling plays a predominant role. First, it leads to a hybridization of the free layer and the SyF linear modes and through this gives rise to a strong field dependence of the critical current. In particular, there is a field range of enhanced damping in which much higher current is required to drive the modes into steady state. This results in a gap in the excitation spectrum. Second, the dynamic dipolar interaction is also responsible for the non-linear interaction between the current driven steady state mode and the damped modes of the system. Here one can distinguish: (i) a resonant interaction that leads to a kink in the frequency-field and frequency-current dispersions accompanied by a small hysteresis and a reduction of the linewidth of the steady state mode and (ii) a nonresonant interaction that leads to a strong frequency redshift of the damped mode. The results underline the strong impact of interlayer coupling on the excitation spectra of spin-torque oscillators and illustrate in a simple three mode model system how in the non-linear regime the steady state and damped modes influence each other.2

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“…We note that a similar interaction of steady state and damped modes via their harmonics has been reported in Ref. 18 for vortex excitations.…”

Section: Ivb4 Discussionmentioning

confidence: 71%

mentioning

confidence: 99%

Influence of interlayer coupling on the spin-torque-driven excitations in a spin-torque oscillator

et al. 2017

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“…For I dc 10 mA, a narrow microwave emission peak corresponding to the spin transfer driven dynamics of the two coupled vortices is detected on the spectrum analyzer. At fixed I dc , the microwave characteristics of this auto-oscillation peak (frequency and linewidth) can be tuned by varying H [34]. In this study, all measurements are carried out at room temperature.…”

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confidence: 99%

Perfect and robust phase-locking of a spin transfer vortex nano-oscillator to an external microwave source

Hamadeh¹,

Locatelli²,

Naletov³

et al. 2014

Applied Physics Letters

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We study the synchronization of the auto-oscillation signal generated by the spin transfer driven dynamics of two coupled vortices in a spin-valve nanopillar to an external source. Phase-locking to the microwave field h rf occurs in a range larger than 10% of the oscillator frequency for drive amplitudes of only a few Oersteds. Using synchronization at the double frequency, the generation linewidth is found to decrease by more than five orders of magnitude in the phase-locked regime (down to 1 Hz, limited by the resolution bandwidth of the spectrum analyzer) in comparison to the free running regime (140 kHz). This perfect phase-locking holds for frequency detuning as large as 2 MHz, which proves its robustness. We also analyze how the free running spectral linewidth impacts the main characteristics of the synchronization regime.Spin transfer nano-oscillators (STNOs) are nanoscale microwave generators [1,2] which have become very attractive due to their wide range of potential applications (frequency generation [3,4] and detection [5,6], signal processing [7,8], dynamic recording [9,10]). The transfer of angular momentum from a spin-polarized current to a ferromagnetic layer can excite the gyrotropic mode of a magnetic vortex [11,12] having typical frequency between 20 MHz and 2 GHz [13]. Vortex-based STNOs are very promising due to their narrow generation linewidth (about 1 MHz) and potentially high output power [14]. Recently, we have proposed a way to minimize even more the auto-oscillation linewidth by operating a STNO based on two coupled vortices in a spin-valve nanopillar, which can yield highly coherent signals (Q > 15000) with linewidths under 50 kHz at room temperature and near zero magnetic field [15].Synchronization to an external periodic signal and mutual phase-locking of several STNOs have been proposed as means to increase the emitted power and reduce the phase noise of STNOs [16]. It has also been suggested that synchronized arrays of STNOs could be operated as associative memories [17]. So far, mutual phase-locking has been achieved using spin wave coupling between nanocontacts [18-20] and 2D arrays of vortices and antivortices [21]. It is also predicted to occur using the common microwave current emitted [22,23] or the dipolar interaction between adjacent STNOs [24,25]. To demonstrate the efficiency of these two types of coupling, synchronization to an external microwave current passing through the device [26][27][28][29] or to a microwave field produced by an external antenna [30,31] have been studied.Two key characteristics to analyze the quality of the synchronization are the locking range and the generation linewidth in the phase-locked regime, which are respectively related to the coupling efficiency and the response to noise of the oscillator. In a single vortex-based tunneling magnetoresistance (TMR) device, it was shown that using an external microwave current, the locking range could reach up to one third of the oscillator frequency, and the linewidth be reduced by 3 orders of magnitu...

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“…The magnetization configuration of each NiFe layers is at remanence a magnetic vortex state. We previously demonstrated that the excitation by mutual spin transfer torque of coupled vortex modes inside the NiFe/Cu/NiFe trilayers is detected at zero applied magnetic field [7,15,16]. Interestingly, this excited coupled mode displays very narrow linewidth (~100 kHz) [7,15,16].…”

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confidence: 99%

“…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].…”

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confidence: 99%

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

et al. 2015

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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...

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Origin of Spectral Purity and Tuning Sensitivity in a Spin Transfer Vortex Nano-Oscillator

Cited by 47 publications

References 47 publications

Influence of interlayer coupling on the spin-torque-driven excitations in a spin-torque oscillator

Influence of interlayer coupling on the spin-torque-driven excitations in a spin-torque oscillator

Perfect and robust phase-locking of a spin transfer vortex nano-oscillator to an external microwave source

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

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