Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z18: Focus Session: Spin-torque and Related Magnetic Oscillations |
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Sponsoring Units: DMP FIAP GMAG Chair: Robert McMichael, National Institute of Standards and Technology Room: 320 |
Friday, March 22, 2013 11:15AM - 11:51AM |
Z18.00001: Decoherence and mode-hopping in spin-torque oscillators Invited Speaker: Pranaba Kishor Muduli A Spin Torque Oscillator (STO) is a nano-sized magneto-resistive device that can produce microwave signals in the GHz range as a result of spin transfer torque [1, 2]- a phenomena which is receiving increasing importance in contemporary spintronics research both for fundamental spin physics as well as a number of possible microwave applications e.g., oscillator, detectors and modulators. A very important question, both for fundamental physics as well for applications, is what limits the coherence time of the STO. This is a subject of significant interest recently. Until now theoretical studies have investigated decoherence through thermal noise assuming that only a single mode is excited [3]. On the other hand, experiments clearly show both the existence of multiple modes and persistent mode-hopping between several modes. The impact on coherence time of such mode-hopping has been largely unexplored and a theoretical study of its origin is entirely lacking. In this work, we will present first ever systematic experimental investigations of mode hopping, and its impact on the coherence time in a magnetic tunnel junction based spin torque oscillator [4]. We will discuss micromagnetic simulations and a theoretical treatment to show that the non-conservative fields due to finite damping-either positive or negative (spin torque) -couple individual modes and, in the presence of thermal noise, govern the experimentally observed mode-hopping. Using quantitative analysis of both coherence and dwell times, we will show that mode-hopping could be a limiting factor for STO coherence. Finally we show how our theoretical treatment can be extended to the case of a metallic nanocontact based STO, where anomalous temperature dependence of linewidth is found as result of the mode coupling [5].\\[4pt] References:\\[0pt] [1] L. Berger, Phys. Rev. B 54, 9353 (1996).\\[0pt] [2] J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [3] A. Slavin and V. Tiberkevich, IEEE Trans. Magn. 45, 1875 (2009).\\[0pt] [4] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. Lett. 108,207203 (2012).\\[0pt] [5] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. B 86, 174408 (2012). [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z18.00002: Influence of Interlayer Exchange Coupling and Exchange Bias on the Ferromagnetic Resonance Spectra Dirk Backes, Bartek Kardasz, Juergen Langer, Andrew D. Kent We present a study of the influence of exchange bias and exchange coupling on the shape and width of ferromagnetic resonance (FMR) spectra. Such interactions are employed in pinned synthetic antiferromagnets (SAF), layer stacks in which two ferromagnetic layers are antiferromagnetically coupled due to interlayer exchange coupling (IEC). One of the ferromagnetic layers shares an interface with an antiferromagnet, thus pinning its magnetization due to the exchange bias (EB) effect. It has been shown that quantitative values for the IEC and EB interactions can be determined from FMR dispersion relations [1]. In this work we study how these interactions manifest themselves in the peak intensities and line widths of FMR spectra. For this we adjust the strength of exchange bias and IEC by varying the thickness of PtMn and Ru in PtMn/ 2 CoFe/ Ru/ 2.3 CoFeB layer stacks (thicknesses in nm). We investigate various cases: i) presence or absence of an exchange bias field, combined with different kinds of IEC: ii) strong and weak antiferromagnetic, weak parallel, and no coupling. [1] D. Backes et al., JAP 111, 07C721 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z18.00003: Spin torque nanooscillators: new applications in information processing Ferran Macia, Andrew D. Kent, Frank C. Hoppensteadt Nanonometer scale electrical contacts to ferromagnetic thin films (STNOs) can provide sufficient current densities to excite magnetic-moment dynamics resulting in emission of short wave-length spin waves. We discuss several applications of spin-wave patterns created from STNOs and their interaction with background oscillations. We review how to encode information in STNOs signals $-$modulating their amplitude, frequency or phase$-$ and stability against noise. We first model arrays of STNOs in extended ferromagnetic thin films and define conditions to control spin-waves emission directions. We also study arrays of oscillators in patterned ferromagnetic thin films and we put forward a method to build an STNO lookup tables or an STNO based network analyzer. Using spin waves complements digital semiconductor technologies and offers new possibilities for increased memory capacity and computation performance.\\[4pt] [1] F. Maci\`a \emph{et al.} Nanotechnology 22 095301 (2011)\\[0pt] [2] F. Maci\`a \emph{et al.} Journal of Applied Physics 109, 07C733 (2011). [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z18.00004: Microwave emission by a spin Hall nano-oscillator Ronghua Liu, Weng L. Lim, Sergei Urazhdin In a recently developed class of spintronic devices, the spin Hall effect (SHE) produces a pure spin current controlling the magnetization of ferromagnets. While SHE opens possibilities for new material combinations and device geometries, it also requires innovative approaches to device engineering. We demonstrate coherent microwave generation by a spintronic nanodevice that utilizes local injection of spin current generated by SHE into an extended magnetic film to generate magnetization oscillations, and anisotropic magnetoresistance of the magnetic layer to convert the oscillations into a microwave signal. We will describe our measurements of the dependence of spectral characteristics of the oscillations on current, temperature, and magnetic field. The dependence on current was remarkably similar to the spin-valve nano-oscillators. However, the dependence on temperature was different from the traditional magnetic nano-oscillators, indicating a significant temperature dependence of both the magnetization dynamics and the SHE. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z18.00005: Spintronics r.f. oscillator driven by magnetic field feedback Ashwin Tulapurkar, Dinesh Dixit, Katsunori Konishi, C.V. Tomy, Yoshishige Suzuki Magnetic tunnel junctions (MTJ) can be used as nano-scale rf oscillators using spin-transfer torque effect. Here we present an alternative novel mechanism of ``magnetic field feedback'' for driving MTJs into precessional states. To realize this effect, MTJ needs to be fabricated on top of a co-planar wave-guide. A dc current is passed through MTJ to produce a fluctuating voltage across it as a combination of thermal fluctuations of free layer and magneto-resistance effect. This voltage is applied across co-planar wave-guide to create a fluctuating magnetic field which acts on the free layer to enhance its fluctuations. If the dc current exceeds a critical value, precessional states of free layer are excited. We have derived expression for the critical current using lineralized LLG equation, modified to include the ``feedback'' magnetic field. We have verified the feedback effect by numerical simulation of stochastic LLG equation including random magnetic field: we find that the damping of the free layer can be increased/decreased by applying --ve/$+$ve dc current. Simulations show that by applying dc current more than critical current, large amplitude oscillations with high quality factors are possible. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z18.00006: Parallel pumping instabilities of spin wave modes in a nanodisk Robert McMichael, Feng Guo, Lyuba Belova The confined spin wave modes in a magnetic nanostructure are fundamental to the magnetization dynamics, and the majority of studies of these spin waves have used uniform transverse pumping fields to excite the modes. However, our recent ferromagnetic resonance force microscopy measurements have shown that parallel pumping reveals a richer set of resonances than the more conventional transverse pumping. This talk presents measurements and micromagnetic modeling of the parallel pumping process in a 500 nm diameter 25 nm thick Permalloy disk with fields applied in plane. In the experiments, the linear, transversely pumped spectrum at 5.2 GHz is simple, with a main resonance at 38 mT applied field and a weaker mode at 54 mT. At a doubled excitation frequency of 10.4 GHz and high pumping power, five resonances are excited by parallel pumping. Two of these resonances occur at the same fields as the modes observed under transverse pumping, but the most intense mode is one that does not appear in the transverse spectrum. The modeling results show similar behavior, and also provide images of the excited modes. The lowest thresholds for parallel pumping belong not to the nearly uniform ``main'' precession mode, but to standing waves that propagate along the field direction. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z18.00007: Evidence for non-local damping in individual nano structures with a single magnetic layer Hans Nembach, Justin Shaw, Carl Boone, Tom Silva The spin excitation damping $\alpha$ in nanomagnets is a critical parameter for switching in STT-MRAM because the required power depends on $\alpha^2$. We experimentally demonstrate that intralayer spin-pumping is a significant source of damping. Ferromagnetic resonance spectra were measured by heterodyne magneto-optical microwave microscopy for individual Ni80Fe20 nanomagnets down to 100 nm. Micromagnetic simulations show that one spin-wave mode, i.e. the ``center-mode,'' is distributed throughout the nanomagnet, whereas the two ``end-modes'' are localized at the ends. $\alpha$ is found to increase for the ``center-mode'' with decreasing nanomagnet size but shows the opposite trend for the ``end-modes.'' It was proposed that dissipative transverse spin-currents can increase $\alpha$. Calculations of this additional damping are in agreement with the experimental data. We also used micromagnetics to test the hypothesis that an area of increased damping close to the edges of the nanomagnets forms during patterning. Such simulations predict that $\alpha$ for both spin-wave modes increases with decreasing size of the nanomagnets, contrary to our experimental observations. Thus, we conclude that non-local contributions to $\alpha$ are the dominant mechanism for size-dependence of $\alpha$. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z18.00008: Ferromagnetic resonance (FMR) spin-pumping in FM/I/NM heterostructures Yong Pu, C. Du, H. Wang, R. Adur, A. Berger, J. Beardsley, A. Hauser, P. Odenthal, A. Swartz, R. Kawakami, J. Pelz, E. Johnston-Halperin, F. Yang, P.C. Hammel The recent demonstration of the injection of a pure spin current via ferromagnetic resonance (FMR) in the FM electrode, spin-pumping, with no need for an accompanying charge current, promises low-power high-efficiency spin injection in a wide variety of materials. Here we report the demonstration of FMR spin-pumping in Ferromagnet/Insulator/Non-magnetic materials heterostructures via different spin detection techniques, and characterizations of the dynamically injected spin. Our investigation proves the possibility that one can both utilize the advantages of FMR spin-pumping, and simultaneously overcome the well-known resistance mismatch problem, which usually happens for spin injection through a FM/NM direct contact and drastically suppresses the efficiency of spin injection into NM. Furthermore, by individually and systematically varying the magnetic, electrical and mechanical properties of each element of the FM/I/NM heterostructures, we are able to study the fundamental mechanisms for FMR spin-pumping, e.g. coupling range and strength, and role of and interplay between spin, charge, lattice, magnon and phonon degree of freedoms. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z18.00009: Numerical study of spin-dependent transition rates within pairs of dipolar and exchange coupled spins with (s=1/2) during magnetic resonant excitation Mark Limes, Jinqi Wang, William Baker, Sang-Yun Lee, Brian Saam, Christoph Boehme The effect of dipolar and exchange interactions within pairs of paramagnetic electronic states on Pauli-blockade-controlled spin-dependent transport and recombination rates during magnetic resonant spin excitation is studied numerically using the superoperator Liouville-space formalism. The simulations reveal that spin-Rabi nutation induced by magnetic resonance can control transition rates which can be observed experimentally by pulsed electrically (pEDMR) and pulsed optically (pODMR) detected magnetic resonance spectroscopies. When the dipolar coupling exceeds the difference of the pair partners' Zeeman energies, several nutation frequency components can be observed, the most pronounced at $\sqrt{2}\gamma B_1$ ($\gamma$ is the gyromagnetic ratio, $B_1$ is the excitation field). Exchange coupling does not significantly affect this nutation component; however, it does strongly influence a low-frequency component $< \gamma B_1$. Thus, pEDMR/pODMR allow the simultaneous identification of exchange and dipolar interaction strengths. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z18.00010: Configurational Dependence of the Magnetization Dynamics in Spin Valve Systems Ruslan Salikhov, Radu Abrudan, Frank Bruessing, Kurt Westerholt, Hartmut Zabel, Florin Radu, Ilgiz A. Garifullin Spin current related phenomena in F1/N/F2 spin valve heterostructures, where F is a ferromagnetic layer and N is a nonmagnetic metal layer, are important in modern magnetism. Spin valve theory predicts a spin pumping effect with a precessional relaxation rate that depends on the configuration of F1 and F2 [1]. Using time-resolved x-ray resonant magnetic scattering we report on the precessional dynamics of spin valve systems with parallel (P) and antiparallel (AP) orientation. We observe in Co/Cu/Py spin valve systems an increase of the magnetic damping parameter in Py with changing magnetization direction of Py and Co layers from P to AP orientation [2]. Furthermore we studied the temperature dependence of the spin pumping effect and possible other causes for the configurational dependence of the damping parameter, such as domain wall induced coupling or magnetic dipole coupling [3]. The main focus is on Co/Cu/Py and on Co$_2$MnGe/V/Py trilayers with spin valve properties.\\[4pt] [1] J.-V. Kim, C. Chappert, JMMM \textbf{286}, 56 (2005)\\[0pt] [2] R. Salikhov \textit{et al.}, APL \textbf{99}, 092509 (2011)\\[0pt] [3] R. Salikhov \textit{et al.}, PRB \textbf{86}, 144422 (2012) [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z18.00011: Domain-wall-controlled transverse spin injection Rembert Duine We propose an effect whereby a charge current accross a domain wall in a magnetic wire injects a transverse pure spin current in an adjacent normal metal. We compute how this effect may be measured via inverse spin Hall effect detection, and consider its effect on enhancement of spin transfer. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z18.00012: Magnetic Bloch Oscillations in 1D ferromagnets Olav Sylju{\aa}sen, Sergey Shinkevich Domain-walls in certain 1D ferromagnets can oscillate when exposed to a static magnetic field. Such magnetic Bloch oscillations have however not been observed experimentally to date. We have calculated neutron scattering signatures of magnetic Bloch oscillations for the material ${\rm{CoCl}_2\cdot 2\rm{H}_2\rm{O}}$, and investigated numerically the possibility of using a laser to generate such oscillations at low temperatures. Our results are positive, and may be used to assist the experimental search for magnetic Bloch oscillations. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z18.00013: Spin-Wave Generation by DW Collision Seonghoon Woo, Tristan Delaney, Geoffrey Beach Spin waves (SWs) in nanoscale metallic ferromagnets have generated much recent interest. Micromagnetic simulations have shown that SWs can couple to and propel DWs by exciting internal resonances, and this effect could be used as a means of low-power DW manipulation. However, generating and detecting large-amplitude exchange-mode SWs is challenging due to their very short wavelengths, which cannot be directly excited by. Here we show, through micromagnetic (OOMMF) simulations, that DWs can be used both to efficiently generate and detect exchange-mode SWs. We first examine SW emission resulting from field-driven DW collisions in Permalloy nanowires. DW annhilation generates intense SW bursts that almost uniformly populate the available SW spectrum across a broad frequency range. The SW power spectrum was characterized as a function of nanowire width, DW topology, and driving field used to induce DW collision. SW bursts were detected through their influence on a third DW pinned at a notch a fixed distance from the DW collision point. SWs induced DW depinning in the presence of background field significantly below the DW depinning field in the absence of SW excitations. The reduction in depinning field dropped with distance between the collision point and the pinned DW, consistent with the decay length due to Gilbert damping. These results show DWs can act as efficient sources of large-amplitude SWs, which can be detected by their influence on a nearby DW. The design of experiments to test these predictions will be discussed. [Preview Abstract] |
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