Spin-transfer Oscillators Research Articles

Spin momentum transfer effects observed in electrodeposited Co/Cu/Co nanowires

Spin-transfer torque effects are reported in nanowires consisting in Co/Cu/Co trilayers electrodeposited on an anodic alumina template. Using a nanolithography process based on electrically controlled nanoindentation of the alumina template, we are able to investigate the spin transport properties of single nanowires at room temperature. For small applied magnetic fields, we have measured resistance changes above a critical direct-current (dc) injected current that corresponds to the change in resistance observed in the magnetoresistance curves at low current. We conclude that magnetic reversals are driven by a spin-polarized current. The critical current densities needed for the magnetization reversals are in the 107 A/cm2 range and the dependence of the critical currents with the applied field is consistent with the spin-transfer mechanism. For large applied magnetic fields, the differential resistance exhibits some peaks that we attribute to the onset of high-frequency excitations of the free-layer magnetization. According to the high density of electrodeposited nanowires in alumina templates, our results are promising for synchronized spin-transfer oscillators.

Nanocontact spin-transfer oscillators based on perpendicular anisotropy in the free layer

Micromagnetic simulations are used to predict the behavior exhibited by spin-transfer oscillators when materials with perpendicular anisotropy are introduced in the “free” layer of nanocontact devices. Under a perpendicular-to-plane bias field, the frequency exhibits nonlinear dependence on the anisotropy field, mostly originated by the exchange-dominated propagating nature of spin-wave modes. The increase of frequency without using large bias fields makes it suitable for potential technological applications. A study of the feasibility of bias-field-free devices has been also performed deriving multiharmonic signals at gigahertz frequencies. Here, the magnetization describes a gyrotropic motion where both vortex-core polarization and rotation sense switch periodically.

Phase and amplitude noise spectra in spin-transfer nano-oscillators

Analytical expressions of the phase and the amplitude noise spectra under the thermal fluctuation fields (i=x, y, z) are derived for a simplified model of spin-transfer oscillators within a small amplitude approximation. The line broadening caused by the transverse fluctuations hxT and hyT is calculated by using the linear phase-noise theories developed for electronic oscillators and that caused by the longitudinal fluctuation hzT is treated as a random walk problem well known in the theory of magnetic resonance. It is found that the linewidth caused by hxT and hyT is proportional to (αT∕MV)(1∕A2), where T is the temperature, α, M, and V are the Gilbert damping parameter, the saturation magnetization, and the volume of the free layer, respectively, and A is the amplitude of oscillation. On the other hand, the width caused by hzT is independent of the amplitude and is simply proportional to αT∕MV.

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

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.

Influence of the Oersted field in the dynamics of spin-transfer microwave oscillators

This paper focuses on the magnetization dynamics induced by the balance between the physical positive damping and the negative induced by spin-transfer torque in point-contact devices. We consider an applied field perpendicular to the device plane which both saturates the magnetization of the free layer and tilts the one of the pinned layer about 30° out-of-the-film plane. The influence of the nonuniform current-induced magnetic (Oersted or Ampere) field on the magnetization dynamics of such oscillators has been taken into account within a micromagnetic framework. Results of micromagnetic calculations show that the Oersted field yields spatial asymmetries in the magnetization configuration, which do not introduce any modifications in the frequency domain. Finally, Slonczewski’s analytical formulation [J. Magn. Magn. Mater. 159, L1 (1996)] about the spatial geometry of the current-excited spin-wave modes has been validated numerically.

Spin-torque oscillator using a perpendicular polarizer and a planar free layer

Spintronics materials have recently been considered for radio-frequency devices such as oscillators by exploiting the transfer of spin angular momentum between a spin-polarized electrical current and the magnetic nanostructure it passes through. While previous spin-transfer oscillators (STOs) were based on in-plane magnetized structures, here we present the realization of an STO that contains a perpendicular spin current polarizer combined with an in-plane magnetized free layer. This device is characterized by high-frequency oscillations of the free-layer magnetization, consistent with out-of-plane steady-state precessions induced at the threshold current by a spin-transfer torque from perpendicularly polarized electrons. The results are summarized in static and dynamic current-field state diagrams and will be of importance for the design of STOs with enhanced output signals.

Spin transfer oscillators emitting microwave in zero applied magnetic field

Using pillar-shaped spin valves with the magnetization of the reference layer being pinned perpendicularly to the easy axis of the free layer, we show that spin-transfer-induced microwave emission can be obtained at exactly zero applied magnetic field and in its vicinity. The frequency tunability (typically 150 MHz/mA), the spectral purity (quality factor up to 280), and the power (up to 5 nV/Hz1/2) of the emission compares well with other spin-transfer oscillators based on spin-valve nanopillars. This ability to get satisfactory microwave emission without needing bulky magnetic field sources may arise from a small nonvanishing field-like term in the current-induced torque. It may be of interest for the design of submicron microwave sources.

Mode Locking of Spin Waves Excited by Direct Currents in Microwave Nano-oscillators

A spin-wave theory is presented which explains the frequency pulling and mode locking observed when two closely spaced spin-transfer nanometer-scale oscillators with slightly different frequencies are separately driven in the same magnetic thin film by spin-polarized carriers at high direct-current densities. The theory confirms recent experimental evidence that the origin of the phenomena lies in the nonlinear interaction between two overlapping spin waves excited in the magnetic nanostructure.

Comparison of frequency, linewidth, and output power in measurements of spin-transfer nanocontact oscillators

We have measured the detailed dependence of the oscillation frequencies, linewidths, and output powers of spin-transfer nanocontact oscillators as functions of applied field strength, bias current, and angle of the applied magnetic field. For fields applied only moderately out of the plane of the film, the evolution of these properties is continuous. However, for fields applied more strongly out of plane they exhibit discontinuous evolution in both current and applied field. These discontinuities typically correlate with changes in the device resistance, changes in device output power, and a broadening of their spectral linewidths. However, away from these discontinuities, the oscillator output powers are larger and the linewidths narrower when compared to geometries having the fields applied at smaller angles. Our measurements suggest that the discontinuous evolution of the frequency with current and applied field results from an abrupt change in the precessional mode of the free layer.

Electrical Measurement of Spin-Wave Interactions of Proximate Spin Transfer Nanooscillators

We have investigated the interaction mechanism between two nanocontact spin transfer oscillators made on the same magnetic spin valve multilayer. The oscillators phase lock when their precession frequencies are made similar, and a giant magnetoresistance signal is detectable at one contact due to precession at the other. Cutting the magnetic mesa between the contacts with a focused-ion beam modifies the contact outputs, eliminates the phase locking, and strongly attenuates the magnetoresistance coupling, which indicates that spin waves rather than magnetic fields are the primary interaction mechanism.

Stochastic theory of spin-transfer oscillator linewidths

We present a stochastic theory of linewidths for magnetization oscillations in spin-valve structures driven by spin-polarized currents. Starting from a nonlinear oscillator model derived from spin-wave theory, we derive Langevin equations for amplitude and phase fluctuations due to the presence of thermal noise. We find that the spectral linewidths are inversely proportional to the spin-wave intensities with a lower bound that is determined purely by modulations in the oscillation frequencies. Reasonable quantitative agreement with recent experimental results from spin-valve nanopillars is demonstrated.

Synchronization of spin-transfer oscillators driven by stimulated microwave currents

We have simulated the non-linear dynamics of networks of spin-transfer oscillators. The oscillators are magnetically uncoupled but electrically connected in series. We use a modified Landau-Lifschitz- Gilbert equation to describe the motion of each oscillator in the presence of the oscillations of all the others. We show that the oscillators of the network can be synchronized not only in frequency but also in phase. The coupling is due to the microwave components of the current induced in each oscillator by the oscillations in all the other oscillators. Our results show how the emitted microwave power of spin-transfer oscillators can be considerably enhanced by current-induced synchronization in an electrically connected network. We also discuss the possible application of our synchronization mechanism to the interpretation of the surprisingly narrow microwave spectrum in some isolated spin-transfer oscillators.

Injection Locking and Phase Control of Spin Transfer Nano-oscillators

We have directly measured phase locking of spin transfer oscillators to an injected ac current. The oscillators lock to signals up to several hundred megahertz away from their natural oscillation frequencies, depending on the relative strength of the input. As the dc current varies over the locking range, time-domain measurements show that the phase of the spin transfer oscillations varies over a range of approximately +/-90 degrees relative to the input. This is in good agreement with general theoretical analysis of injection locking of nonlinear oscillators.

Finite-temperature modeling of nanoscale spin-transfer oscillators

Magnetization dynamics induced by spin-polarized currents in magnetic nanodevices have been numerically simulated using a single-domain model proposed by Slonczewski extended to include temperature effects. For currents with a spin polarization antiparallel to the device easy axis and for fields above the magnetostatic anisotropy field, transfer of spin momentum from one layer to an adjacent layer can cause the layers to undergo sustained oscillations. Here we numerically calculate the expected excitation spectra and linewidths of spin-transfer oscillators and explain observed variations in excitation linewidths. The linewidth arises from thermal excitations that give rise to disorder in the orbits and, in certain regimes, hopping between nearly degenerate orbits. The excitation spectra for a device show transitions from thermally activated elliptical motion, at zero and low currents, to a bent elliptical motion at intermediate currents, and finally to tilted out-of-plane orbits. At the transition between in-plane and out-of-plane orbits, there is a region of low-frequency noise due to thermal hopping between degenerate orbits and a shift in the spectral behavior. The linewidth arising from thermal interactions is a sensitive function of the device volume and varies from for devices to for devices. The modeling explains the difference in linewidths observed for nanopillar and point-contact geometries as a natural consequence of inherent thermal fluctuations and the difference in excitation volumes.

Frequency modulation of spin-transfer oscillators

Spin-polarized dc electric current flowing into a magnetic layer can induce precession of the magnetization at a frequency that depends on current. We show that addition of an ac current to this dc bias current results in a frequency modulated (FM) spectral output, generating sidebands spaced at the modulation frequency. The sideband amplitudes and shift of the center frequency with drive amplitude are in good agreement with a nonlinear FM model that takes into account the nonlinear frequency-current relation generally induced by spin transfer. Single-domain simulations show that ac current modulates the cone angle of the magnetization precession, in turn modulating the frequency via the demagnetizing field. These results are promising for communications and signal processing applications of spin-transfer oscillators.