Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars

Mistral, Q.; Kim, Joo-Von; Devolder, T.; Crozat, P.; Chappert, C.; Katine, J. A.; Carey, M. J.; Ito, K.

doi:10.1063/1.2201897

Cited by 124 publications

(86 citation statements)

References 23 publications

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“…On the other hand, the linewidth above I c monotonically decreases as the current increases, whereas that below I c depends nonmonotonically on the current. Such complex current dependence of the linewidth was found in previous experimental studies [7,26]. Above I c , the length of the precession trajectory becomes long as the current, as well as the cone angle, increases.…”

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confidence: 79%

Linewidth of power spectrum originated from thermal noise in spin torque oscillator

Taniguchi

2014

Appl. Phys. Express

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A theoretical formula of the linewidth caused by the thermal activation in a spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was developed by solving the stochastic Landau-Lifshitz-Gilbert equation in the energy-phase representation. It is shown that the linewidth can be suppressed down to 0.1 MHz by applying a large current (10 mA for typical material parameters). A quality factor larger than 10 4 is predicted in the large current limit, which is two orders of magnitude larger than the recently observed experimental value.The spin torque oscillator (STO) [1-6] is a promising candidate for a future nanocommunication device because of its small size, high emission power, and frequency tunability. Recently, it was found [7,8] that an STO with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer [9-14] can achieve a large emission power of close to 1 µW. Therefore, this type of STO will be the model structure for practical STO applications.Another important quantity characterizing the STO's properties is the linewidth ∆f of the power spectrum. For example, a narrow linewidth is necessary to obtain a high quality factor (Q-factor) Q = f 0 /∆f , where f 0 is the peak frequency of the power spectrum. The physical origin of the linewidth is the nonuniform magnetization dynamics due to thermal activation. Therefore, theoretical evaluation of the linewidth due to thermal activation and of the Q-factor is desirable to clarify the theoretically possible values of these parameters.In the self-oscillation state of an STO, the energy supplied by the spin torque balances the energy dissipation due to damping; therefore, the magnetization steadily precesses almost on a constant energy curve. This situation is very similar to magnetization switching in the thermally activated region, where the magnetization precesses on a constant energy curve many times during switching. Recent studies [15][16][17][18][19][20] have shown that the energy-phase representation of the Landau-LifshitzGilbert (LLG) equation is useful for theoretical investigations of the magnetization switching properties, such as the switching probability, in the thermally activated region. Accordingly, we were motivated to develop a theory of the linewidth of an STO based on the LLG equation in the energy-phase representation.In this letter, the theoretical formula for the linewidth of an STO is derived on the basis of the LLG equation in the energy-phase representation. It is shown that the linewidth can be suppressed to 0.1 MHz by applying a large current (∼ 10 mA). The Q-factor can reach more than 10 4 in this type of STO, which is two orders of magnitude larger than the previously reported value [7]. netization directions of the free and pinned layers are denoted as m and p, respectively. The z-axis is normal to the film-plane, and the x-axis is parallel to p. The external field H appl is applied along the z-axis. The current I flows uniformly along the z-axis, where positive...

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confidence: 79%

Linewidth of power spectrum originated from thermal noise in spin torque oscillator

Taniguchi

2014

Appl. Phys. Express

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“…[29][30][31] Three scenarios are usually suggested to account for this phenomenon: process-induced damages, 15,32 current-induced heating, 33,34 or a nonlinear change of the frequency with high mode amplitude. 35 As we work with low bias current, current induced heating can be excluded in our case. Similarly, since spin-torque induced auto-oscillations in our samples occur typically for currents above 1.6 mA for size L, 19 high amplitude nonlinear effects as possible cause can be rejected, too.…”

Section: A Materials and Geometry Parametersmentioning

confidence: 99%

Quantized spin-wave modes in magnetic tunnel junction nanopillars

et al. 2010

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We present an experimental and theoretical study of the magnetic field dependence of the mode frequency of thermally excited spin waves in rectangular shaped nanopillars of lateral sizes 60 × 100, 75 × 150, and 105 × 190 nm 2 , patterned from MgO-based magnetic tunnel junctions. The spin wave frequencies were measured using spectrally resolved electrical noise measurements. In all spectra, several independent quantized spin wave modes have been observed and could be identified as eigenexcitations of the free layer and of the synthetic antiferromagnet of the junction. Using a theoretical approach based on the diagonalization of the dynamical matrix of a system of three coupled, spatially confined magnetic layers, we have modeled the spectra for the smallest pillar and have extracted its material parameters. The magnetization and exchange stiffness constant of the CoFeB free layer are thereby found to be substantially reduced compared to the corresponding thin film values. Moreover, we could infer that the pinning of the magnetization at the lateral boundaries must be weak. Finally, the interlayer dipolar coupling between the free layer and the synthetic antiferromagnet causes mode anticrossings with gap openings up to 2 GHz. At low fields and in the larger pillars, there is clear evidence for strong non-uniformities of the layer magnetizations. In particular, at zero field the lowest mode is not the fundamental mode, but a mode most likely localized near the layer edges.

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“…6,8 The nonlinearity inherent to STNOs is both the boon and bane of these devices: non-linearities couple the frequency and amplitude of the oscillator, allowing for the large frequency tunability, but also providing the main source of linewidth broadening. [9][10][11] Experimentally, linewidth reduction has been recently achieved through strategies aimed at controlling the oscillator's phase 12 such as injection locking to an external signal, 13 self-synchronization of several oscillators [14][15][16] and phase-locked loop techniques. 17 In this study, we calculate the effect of delayed self-injection on the critical current, frequency response, and linewidth of STNOs.…”

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Critical current and linewidth reduction in spin-torque nano-oscillators by delayed self-injection

Khalsa

Stiles

Grollier

2015

Applied Physics Letters

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ABSTRACT:Based on theoretical models, the dynamics of spin-torque nano-oscillators can be substantially modified by re-injecting the emitted signal to the input of the oscillator after some delay. Numerical simulations for vortex magnetic tunnel junctions show that with reasonable parameters this approach can decrease critical currents as much as 25 % and linewidths by a factor of 4. Analytical calculations, which agree well with simulations, demonstrate that these results can be generalized to any kind of spin-torque oscillator.Spin-torque nano-oscillators (STNOs) based on magnetic tunnel junctions (MTJs) provide the framework for current driven and tunable frequency sources with enormous range 1 (from megahertz to gigahertz) that are compatible with existing semiconductor processes. With a direct electrical current applied to the devices, spin-transfer torques transfer angular momentum from a fixed polarizing magnetic layer to a free magnetic layer and induce oscillatory magnetization dynamics. 2,3 The oscillation of the magnetization causes an oscillatory electrical response through the magnetoresistance effect. Due to their small scale, frequency range, and technological compatibility, STNOs may have applications in the telecommunications industry. 4,5 Hurdles to their use come from the large critical current needed to sustain magnetization oscillations with sufficient spectral purity for industrial adoption, as well as low power output. Research has therefore focused on reducing the critical current, 6 decreasing the linewidth, 7 and increasing the power output of STNOs. 6,8 The nonlinearity inherent to STNOs is both the boon and bane of these devices: non-linearities couple the frequency and amplitude of the oscillator, allowing for the large frequency tunability, but also providing the main source of linewidth broadening. 9-11 Experimentally, linewidth reduction has been recently achieved through strategies aimed at controlling the oscillator's phase 12 such as injection locking to an external signal, 13 self-synchronization of several oscillators 14-16 and phase-locked loop techniques. 17 In this study, we calculate the effect of delayed self-injection on the critical current, frequency response, and linewidth of STNOs. This strategy, where the oscillating output current is re-injected at the 2 input of the oscillator has been shown efficient at improving phase noise in other types of oscillators. 18,19 Using numerical simulations for the dynamics, we find that both the critical current and linewidth of STNOs can be reduced with this technique, while still allowing for frequency tunability. Additionally, we develop simplified analytic expressions that are in good agreement with numerical simulations of the frequency response, critical current, and linewidth -simplifying future experimental and theoretical work. We focus our numerical results on vortex MTJs because they exhibit good output power spectral purity, but emphasize that the analytic results are general to any kind of STNO. The main result i...

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Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars

Cited by 124 publications

References 23 publications

Linewidth of power spectrum originated from thermal noise in spin torque oscillator

Linewidth of power spectrum originated from thermal noise in spin torque oscillator

Quantized spin-wave modes in magnetic tunnel junction nanopillars

Critical current and linewidth reduction in spin-torque nano-oscillators by delayed self-injection

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