Approximate theory of microwave generation in a current-driven magnetic nanocontact magnetized in an arbitrary direction

Slavini, A.N.; Kaboš, Pavel

doi:10.1109/tmag.2005.845915

Cited by 161 publications

(148 citation statements)

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“…Among them, a strong interest of spin transfer effects is the observation of a steady precession of a magnetization, generating a microwave power due to the magnetoresistive response of the devices [3,4,5]. Numerous efforts were made to understand the microwave emission under magnetic field and current bias in a single spin transfer nanooscillator (STNO) [6,7,8,9,10,11]. Most of the characteristics presently observed in STNOs are indeed very attractive for future applications in telecommunication devices.…”

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confidence: 99%

Coupling Efficiency for Phase Locking of a Spin Transfer Nano-Oscillator to a Microwave Current

et al. 2008

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Coupling Efficiency for Phase Locking of a Spin Transfer Nano-Oscillator to a Microwave Current

et al. 2008

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“…This point is especially significant for STNOs in which the influence of noise on the device performance is of a qualitative importance. The theory is compared to recent experiments on STNO nanopillars for which good qualitative agreement is found.Our theory is based on a classical Hamiltonian formalism for spin-waves in magnetic multilayers with the spin-transfer effect [7,8]. It is assumed that only one spin-wave mode -the mode having the lowest relaxation rate -is excited at the auto-oscillation threshold, and dominates the oscillation dynamics both below and just above this threshold.…”

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Line Shape Distortion in a Nonlinear Auto-Oscillator Near Generation Threshold: Application to Spin-Torque Nano-Oscillators

Kim

Mistral²,

Chappert³

et al. 2008

Phys. Rev. Lett.

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The lineshape in an auto-oscillator with a large nonlinear frequency shift in the presence of thermal noise is calculated. Near the generation threshold, this lineshape becomes strongly non-Lorentzian, broadened, and asymmetric. A Lorentzian lineshape is recovered far below and far above threshold, which suggests that lineshape distortions provide a signature of the generation threshold. The theory developed adequately describes the observed behavior of a strongly nonlinear spin-torque nano-oscillator.PACS numbers: 75.30.Ds, 75.40.Gb, 05.10.Gg Thermal noise plays a key role in the dynamics of autooscillatory systems, especially for the nano-sized systems in which the characteristic energy of auto-oscillation is comparable with the thermal noise energy k B T . One important consequence of noise is the broadening of the power spectrum. It is well-established, for example, that white Gaussian noise leads to a broadened Lorentzian shape of the power spectrum both far below and far above the generation threshold in conventional auto-oscillators (for which the frequency is weakly dependent on the amplitude) [1]. In addition, the presence of noise also blurs the threshold between the damped and auto-oscillatory states.In the immediate vicinity of the auto-oscillation threshold, a non-Lorentzian spectrum appears: the lineshape can be described by a sum of several Lorentzian profiles corresponding to several simultaneously excited statistical modes. These modes have the same central frequency for a conventional auto-oscillator and, therefore, the contributions of higher-order modes do not lead to large qualitative changes in the overall spectral line profile [1]. In contrast, the central frequencies of the partial Lorentzian profiles are shifted in frequency by different amounts in a nonlinear auto-oscillator in which the frequency is strongly dependent on the amplitude. In the vicinity of the auto-oscillation threshold at which these shifts are of the order of the oscillation linewidth, the frequency nonlinearity leads to important qualitative changes in the lineshape: the spectral line is asymmetric and significantly broadened.An example of an auto-oscillating system with strong frequency nonlinearity is the spin-torque nano-oscillator (STNO), which is of considerable interest at present. This oscillator consists of two ("free" and "fixed") ferromagnetic layers separated by a nonmagnetic (metallic or dielectric) spacer. When driven by a spin-polarized direct current, an effective negative damping results in the "free" layer due to the spin-transfer effect [2,3].For a sufficiently large direct current, the induced negative damping overcomes the natural positive damping of the "free" layer, and auto-oscillations of magnetization of this layer result [4,5,6]. These and related phenomena are subjects of intensive research at present due to their possible applications in microwave nano-electronics.In this Letter, we present a theory of lineshape distortion in auto-oscillators with a strong nonlinear frequency shift in t...

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“…First the spin transfer torque acting on a magnetic element is related to the transverse spin polarisation of the current (transverse meaning perpendicular to the magnetization axis of the element) and can be derived from spin-dependent transport equations [8][9][10][11][12][13][14][15][16][17]. On the other hand, the description of the magnetic excitations generated by the spin transfer torque raises problems of non-linear dynamics [8,[18][19][20]. For example, in the simple limit where the excitation is supposed to be a uniform precession of the magnetization (macrospin approximation), this precession can be determined by introducing the spin transfer torque into a Landau-LifshitzGilbert (LLG) equation for the motion of the magnetic moment.…”

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Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

et al. 2007

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The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin transfer phenomenon. We report transport and microwave power measurements on specially designed nanopillars for which a non-standard angular dependence of the spin transfer torque (wavy variation) is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field, as this is also predicted by simulation with such a wavy angular dependence of the torque. This type of non-standard nanopillars can represent an interesting way for the implementation of spin transfer oscillators since they are able to generate microwave oscillations without applied magnetic field. We also emphasize the theoretical implications of our results on the angular dependence of the torque. 2The magnetization of a ferromagnetic body can be manipulated by transfer of spin angular momentum from a spin-polarized current. This is the concept of spin transfer introduced by Slonczewski [1] and Berger [2] in 1996. In most experiments, a spin-polarized current is injected from a spin polarizer into a "free" magnetic element, for example in pillar-shaped magnetic trilayers [3][4][5][6]. The phenomenon of spin transfer has a great potential for applications. It can be used either to switch a magnetic configuration (the configuration of a magnetic memory for example) [3][4][5] or to generate magnetic precessions and voltage oscillations in the microwave frequency range [6][7]. In the most usual situations, such oscillations are observed in the presence of a magnetic field.From a fundamental point of view, spin transfer effects raise two different types of problems [8]. First the spin transfer torque acting on a magnetic element is related to the transverse spin polarisation of the current (transverse meaning perpendicular to the magnetization axis of the element) and can be derived from spin-dependent transport equations [8][9][10][11][12][13][14][15][16][17]. On the other hand, the description of the magnetic excitations generated by the spin transfer torque raises problems of non-linear dynamics [8,[18][19][20]. For example, in the simple limit where the excitation is supposed to be a uniform precession of the magnetization (macrospin approximation), this precession can be determined by introducing the spin transfer torque into a Landau-LifshitzGilbert (LLG) equation for the motion of the magnetic moment. However, the determination of the spin transfer torque and the description of the magnetization dynamics cannot be regarded as independent problems. In standard trilayered structures with in-plane magnetizations and with the usual angular dependence, a switching regime is found at zero and low magnetic field and the precession regime with generation of voltage oscillations is mainly observed above some threshold field [8]. We will show that a new behavior, characterized by large a...

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Approximate theory of microwave generation in a current-driven magnetic nanocontact magnetized in an arbitrary direction

Cited by 161 publications

References 17 publications

Coupling Efficiency for Phase Locking of a Spin Transfer Nano-Oscillator to a Microwave Current

Coupling Efficiency for Phase Locking of a Spin Transfer Nano-Oscillator to a Microwave Current

Line Shape Distortion in a Nonlinear Auto-Oscillator Near Generation Threshold: Application to Spin-Torque Nano-Oscillators

Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

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