Session W29: Focus Session: Current-Induced Oscillations

Competition between orbital torques and spin polarization in controlling FMR linewidths

We have investigated temperature dependent dynamic magnetic properties of rare earth (Gd, Tb, Sm)/Ag/transition metal (Fe, Co, Ni and Py) trilayers by ferromagnetic resonance technique. We found that Fe and Co among TM (transition metals) show narrower magnetic resonance linewidths in rare earth (RE)/Ag/TM/Ag thin film trilayers compared to the values for Ag/TM/Ag, while Ni and Py in the trilayer films show equal or larger linewidths. We attribute this behavior to the relative contributions of intraband and interband scattering to the Gilbert damping parameter. The Y/Ag/(Fe, Co) trilayers seems not to change the resonance linewidth from the bulk value, suggesting that the magnetic moments for the f-electrons play a significant role.   

Current-induced dynamics in almost symmetric magnetic nanopillars

Magnetic nanodevices usually include a free layer whose configuration can be changed by spin-polarized current via the spin transfer (ST), and a fixed reference (polarizing) layer. The polarizer is usually made much larger than the free layer to minimize the effects of ST. However, it is presently not known what makes a specific magnetic layer behave as a fixed polarizer or a free layer driven by ST. Little is also known about the dynamics in bilayers with thin polarizers, where the effects of ST on both layers are significant. We will discuss our spectroscopic measurements of current-induced dynamics in nanopillars with similar thicknesses of the extended polarizer and the nanopatterned free layer. We demonstrate coherent precession for both polarities of current in symmetric devices. However, even slightly asymmetric devices exhibit a rapid suppression of precession for one of the current polarities. We interpret our results in terms of the dynamical coupling between magnetic layers due to spin transfer, completely suppressing precession of the thicker layer.   

Measurements of out-of-plane dynamics induced by spin transfer in magnetic nanopillars

Current-induced spin transfer (ST) can induce dynamical states in magnetic multilayer nanopillars not accessible by any other techniques. For-in plane magnetic field, the predicted dynamical regimes include elliptical, clamshell, and out-of-plane precession. The first two regimes have been demonstrated and extensively analyzed. However, the out-of-plane precession has so far been elusive. Calculations [1] show that dynamical coupling between ferromagnets due to ST can result in suppression of coherent out-of-plane precession in nanopillars with a patterned polarizing layer, which is the geometry studied so far. We will discuss our measurements of current-induced dynamics in nanopillars with extended polarizer, in which the decoherence caused by the coupling between magnetic layers is minimized. We demonstrate coherent out-of-plane precession, whose dependence on current and the direction of the magnetic field is consistent with micromagnetic simulations. Most surprisingly, our data are asymmetric with respect to reversal of the magnetic field, which is explained by a combination of the Oersted field and sample shape imperfections. [1] S. Urazhdin, Phys. Rev. B 78, 060405(R) (2008).   

Spin Torque Dynamics of Nanomagnets with Weak Magnetic Anisotropy

We study switching and persistent precession of magnetization induced by spin transfer torque in Co(4 nm)/ Cu(6 nm)/ Co(0.7 nm)/Pt nanopillar spin valves where perpendicular magnetic anisotropy at the Co/Pt interface nearly cancels the easy-plane shape anisotropy of the free Co layer. We find that in this system with weak total magnetic anisotropy, spin torque can switch magnetization of the free layer between the in-plane and the out-of-plane static magnetic states. In the regime of current-driven persistent magnetization precession, we observe unusual non-monotonic dependence of the precession frequency on current. Simulations show that these unusual features of spin torque dynamics are due to the second-order perpendicular magnetic anisotropy term at the Co/Pt interface. Our work demonstrates a method for controllable switching of magnetization of a nanomagnet between stable in-plane and out-of-plane magnetic configurations by spin-polarized current.   

Slonczewski windmill with dissipation and asymmetry

J. Slonczewski invented spin-transfer effect in layered systems in 1996. Among his first predictions was the regime of the ``windmill motion'' of a perfectly symmetric spin valve. In this regime magnetizations of the layers rotate in a fixed plane keeping the angle between them constant. Since ``windmill'' was predicted to happen in the case of zero magnetic anisotropy, while in most experimental setups the anisotropy is significant, the phenomenon was not a subject of much research. However, the behavior of the magnetically isotropic device is related to the interesting question of current induced ferromagnetism and is worth more attention. Here we study the windmill regime in the presence of dissipation, exchange interaction, and layer asymmetry. It is shown that the windmill rotation is almost always destroyed by those effects, except for a narrow interval of electric current, determined by the parameters of the device.   

Spin-dependent tunneling effects in magnetic tunnel junctions

It has long been known that current extracted from magnetic electrodes through ultra thin oxide tunnel barriers is spin polarized. This current gives rise to two important properties: tunneling magnetoresistance (TMR) when the tunnel barrier is sandwiched between two thin magnetic electrodes and, spin momentum transfer, which can be used to manipulate the magnetic state of the magnetic electrodes. In the first part of my talk I show how the structure of thin CoFe layers can be made amorphous by simply sandwiching them between two amorphous layers, one of them the tunnel barrier. No glass forming elements are needed. By slightly changing the thickness of these layers or by heating them above their glass transition temperature they become crystalline. Surprisingly, the TMR of the amorphous structure is significantly higher than of its crystalline counterpart. The tunneling anisotropic magnetoresistance, which has complex voltage dependence, is also discussed. In the second part of my talk I discuss the microwave emission spectrum from magnetic tunnel junctions induced by spin torque from spin polarized dc current passed through the device. We show that the spectrum is very sensitive to small variations in device structures, even in those devices which exhibit similarly high TMR ($\sim $120{\%}) and which have similar resistance-area products ($\sim $4-10 $\Omega \mu $m$^{2})$. We speculate that these variations are due to non-uniform spatial magnetic excitation arising from inhomogeneous current flow through the tunnel barrier. [In collaboration with Xin Jiang, M. Hayashi, Rai Moriya, Brian Hughes, Teya Topuria, Phil Rice, and Stuart S.P. Parkin]   

Frequency-doubling spin-torque microwave oscillator

We describe a new type of spin torque oscillator with two free layers that is capable of emitting high microwave power ($>$ 1 $\mu $W) at high frequency ($>$ 50 GHz) in zero external field. This device has two perpendicular-anisotropy fixed ferromagnetic layers and two easy-plane free layers sandwiched between the fixed layers, with all of the magnetic layers separated from each other by non-magnetic spacers. We simulate current-driven magnetization dynamics in this structure in the macrospin approximation, taking into account spin-torque interactions between adjacent ferromagnetic layers. Our simulations show that for both fixed layers magnetized in the same direction perpendicular to the plane of the sample, spin-torque induces clockwise rotation of one of the layers and counterclockwise rotation of the other. This type of current-driven dynamics gives rise to large-amplitude microwave signal with the frequency that is the sum of the precession frequencies of the free layers. We study the effect of dipolar coupling, shape anisotropy and external field on the dynamics of this spin torque oscillator and determine the optimal device parameters for high-amplitude high-frequency microwave signal generation.   

Spectral Line Shape and Line Width of a Single-Mode Spin Torque Oscillator

Spin torque auto-oscillators are strongly nonlinear dynamical systems that are highly susceptible to external perturbations such as spin-polarized current and temperature. To understand the effect of thermal fluctuations on the oscillator dynamics, we measure power spectrum of single-mode spin torque oscillators based on a GMR nanocontact to a permalloy nanowire. Our measurements reveal deviations of the power spectral line shape from a simple Lorentzian. These deviations can be understood in terms of dephasing induced by the oscillator amplitude fluctuations. The measured spectral line shape is in a good agreement with a recent analytic theory of spin torque oscillator dynamics at a non-zero temperature [1]. We show that precise measurements of the line shape give information on important oscillator parameters such as Gilbert damping in the large-amplitude regime of current-driven magnetization dynamics. [1] V. S. Tiberkevich, A. N. Slavin, J.-V. Kim, Phys. Rev. B 78, 092401 (2008).   

Time domain studies of aperiodicity in spin-torque driven vortex oscillations

Previous studies of current-driven magnetic vortex oscillations in nanopillars [1] and point contact geometries [2] have been restricted to detection of the \textit{average} envelope of the oscillations. In this talk we discuss aperiodic features of the vortex oscillations that were studied based on single-shot time domain measurements of the oscillating GMR signal. These measurements reveal stochastic mode jumping at 10's of $\mu $s mean duty cycles between several closely spaced frequencies. The power spectrum of the time traces indicates that the shape and amplitude of the oscillation's spectral peaks change abruptly as the function of time, corresponding to aperiodic modulation of these oscillations on the $\mu $s time scale. Due to the very narrow \textit{long-time }linewidths of the oscillations it is possible to detect clearly these fine modulations of the peak shape, frequency and amplitude. From these studies of the spin-torque-driven vortex oscillator stability we seek to obtain insights for the design and fabrication of spin-torque vortex oscillators with even narrower linewidths. [1] V.S. Pribiag \textit{et al.,} \textit{Nature Phys.} \textbf{3}, 498 (2007). [2] Q. Mistral \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{100}, 257201 (2008).   

The Role of Spin-Motive Forces in Spin-Valve Dynamics

A spin-motive force (smf) is the counterpart of an electro-motive force, which couples to the spin rather than charge degrees of freedom of electrons. Here we discuss how smfs work in spin-valves. When the magnetization makes a sudden change, there often appears a large peak in dV/dI, i.e., a voltage jump that is better interpreted in terms of smfs. To see this, we model spin-valves using an equivalent circuit that involves magnetic dissipation represented by smfs as well as electric dissipation through ordinary resisters for both majority and minority currents. There are four possible conduction paths, e.g., the majority electrons hop to the majority band, or to the minority band and vice versa. The first path adds an up electron to the free layer and causes a rotation in a certain sense, while the second path adds a down electron and a rotation in the opposite sense. Since the rotations are in opposite senses so is the work done on the free layer and hence the smf. By solving the circuit problem and the Landau-Lifshitz equations supplemented with the Slonczewski torque-transfer term simultaneously we find the spin-transfer effect is dramatically modified by smfs. With the relevant parameters a stable large angle precession and a voltage signal are predicted.   

Mutual phase-locking and frustration in arrays of interacting spin-torque nano-oscillators

We developed a perturbation theory describing collective dynamics of spin-torque nano-oscillator (STNO) arrays in a weak-coupling limit. In this limit each STNO is described by a single dynamical variable -- effective phase $\phi _j $, which satisfies the equation ${d\phi _j } \mathord{\left/ {\vphantom {{d\phi _j } {dt}}} \right. \kern-\nulldelimiterspace} {dt}=\omega _j +\sum\nolimits_k {\lambda _{j,k} \sin (\phi _k -\phi _j +\beta _{j,k} )} $. Here $\omega _j $ is the free-running (unperturbed) frequency of the $j$-th oscillator, $\lambda _{j,k} $ is the effective coupling amplitude of $j$-th and $k$-th oscillators, and $\beta _{j,k} $ is the \textit{frustration angle} of the oscillators' interaction. The frustration angles $\beta _{j,k} $ are determined by the intrinsic nonlinearity of STNO and by the delay of coupling signals. The frustration angles can be controlled by changing the distance between STNOs and/or by adding reactive elements to the STNO circuit. We have analyzed collective dynamics of STNO arrays in the case of global coupling, i.e. when coupling amplitudes and frustration angles for all STNOs are equal, $\lambda _{j,k} =\lambda $, $\beta _{j,k} =\beta $. We have shown that STNO array mutually phase-locks only when $\cos (\beta )>0$. The critical coupling amplitude $\lambda _{cr} $, at which phase-locking starts, has a minimum for $\cos (\beta )=1$ (i.e., for $\beta =2n\pi )$ and increases with the decrease of $\cos (\beta )$. For $\cos (\beta )<0$ the mutual phase-locking of more than two STNOs is impossible, and the STNO array enters a \textit{frustrated state}, in which the output power becomes vanishingly small due to the destructive interference between individual STNOs.   

Spin-torque-driven ferromagnetic resonance in a nonlinear regime

Spin-torque-driven ferromagnetic resonance (ST-FMR) is a quantitative tool for studying spin-transfer interactions in nanojunctions. Using this method we have studied Co/Cu/CoNi spin valves, in which the CoNi synthetic free layer has perpendicular magnetic anisotropy. Perpendicular field swept resonance lines were measured under a large amplitude GHz current excitation, which drove ST-FMR into a nonlinear regime and produced a large angle precession of the free layer magnetization. With increasing rf power, the resonance lines deviate from a Lorentzian shape and became asymmetric, with a lower resonance field and a larger linewidth. A non-hysteretic step jump in ST-FMR voltage signal was also observed at high powers. The comparison of the experimental results to the foldover and the nonlinear damping theories will be presented.   

Microwave oscillation generation in a Co/Cu/Co nano-contact without external magnetic field

Using spin-transfer-torque effect to generate microwave oscillation at zero magnetic- field is of recent interest. Here, we report the observation a resistive oscillation at microwave-frequencies ranging from 1.5 to 3 GHz in a nano-contact formed on a Co/Cu/Co tri-layer structure without any external field. The observed oscillation modes have frequencies that are much higher than that reported in other similar experimental systems [1,2]. We have studied the evolution of the oscillation as a function of the DC excitation current and the effect of a small in-plane field. Micromagnetic simulations support the notion that the oscillation is as a result of the translational motion of a vortex-core underneath the nano-contact, due the competition of the circular Orstead field and the spin-transfer torque, both induced by the DC current passing through the nano-contact. The work was supported by the Western Institute of Nanoelectronics (WIN). [1] M. R. Pufall et al., Phys. Rev. B 75, 140404(R) (2007). [2] Q. Mistral et al, Phys. Rev Lett. 100, 257201 (2008).