Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E6: Spin Excitations in Ultrathin Films, Nanostructures and Domain WallsFocus Industry
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Sponsoring Units: GMAG DMP FIAP Chair: Behrouz Khodadadi, University of Alabama Room: 302 |
Tuesday, March 15, 2016 8:00AM - 8:36AM |
E6.00001: Nonlinear spin-wave excitations at low magnetic bias fields Invited Speaker: Georg Woltersdorf We investigate experimentally and theoretically the nonlinear magnetization dynamics in magnetic films at low magnetic bias fields. Nonlinear magnetization dynamics is essential for the operation of numerous spintronic devices ranging from magnetic memory to spin torque microwave generators. Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance. In the experiments we use X-ray magnetic circular dichroism to determine the number density of excited magnons in magnetically soft Ni$_{80}$Fe$_{20}$ thin films. Our data show that the common Suhl instability model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behavior in the low magnetic field limit. Here we derive a model of parametric spin-wave excitation, which correctly predicts nonlinear threshold amplitudes and decay rates at high and at low magnetic bias fields. In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes. For these modes, we also find pronounced frequency locking effects that may be used for synchronization purposes in magnonic devices. By using this effect, effective spin-wave sources based on parametric spin-wave excitation may be realized. Our results also show that it is not required to invoke a wave vector-dependent damping parameter in the interpretation of nonlinear magnetic resonance experiments performed at low bias fields. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E6.00002: Spin torque ferromagnetic resonance in Heusler based magnetic tunnel junctions Jie Zhang, Timothy Phung, Aakash Pushp, Jaewoo Jeong, Yari Ferrante, Charles Rettner, Brian P. Hughes, See-Hun Yang, Stuart S.P. Parkin Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to high spin polarization and high tunneling magnetoresistance. Whilst much work has focused on the influence of the half metallic character of the Heusler compounds on the magnetoresistance of MTJs, there is much less work investigating the influence of this electronic structure on the spin transfer torque. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components as a function of the bias voltage in symmetric (CoMnSi/MgO/CoMnSi) and asymmetric (CoMnSi/MgO/CoFe) structure magnetic tunnel junctions using spin transfer torque ferromagnetic resonance. Lastly, we report on the effect of asymmetric bias dependence of the differential conductance on the spin transfer torque. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E6.00003: Parametric excitation of magnetization by electric field Yu-Jin Chen, Han Kyu Lee, Roman Verba, Jordan Katine, Vasil Tiberkevich, Andrei Slavin, Igor Barsukov, Ilya Krivorotov Manipulation of magnetization by electric field is of primary importance for development of low-power spintronic devices. We present the first experimental demonstration of parametric generation of magnetic oscillations by electric field. We realize the parametric generation in CoFeB/MgO/SAF nanoscale magnetic tunnel junctions (MTJs). The magnetization of the free layer is perpendicular to the sample plane while the magnetizations of the synthetic antiferromagnet (SAF) lie in the plane. We apply microwave voltage to the MTJ at 2$f$, where $f$ is the ferromagnetic resonance frequency of the free layer. In this configuration, the oscillations can only be driven parametrically via voltage-controlled magnetic anisotropy (VCMA) whereby electric field across the MgO barrier modulates the free layer anisotropy. The parametrically driven oscillations are detected via microwave voltage from the MTJ near $f$ and show resonant character, observed only in a narrow range of drive frequencies near 2$f$. The excitation also exhibits a well-pronounced threshold drive voltage of approximately 0.1 Volts. Our work demonstrates a low threshold for parametric excitation of magnetization by VCMA that holds promise for the development of energy-efficient nanoscale spin wave devices. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E6.00004: Control of Spin Wave Band Structure in YIG using Electric Fields Glade Sietsema, Michael E. Flatt\'e It has previously been shown that a uniform electric field can be used to modify the dispersion relations of spin waves in a YIG slab \footnote {X. Zhang et al., Phys. Rev. Lett. \textbf{113}, 037202 (2014)}. The application of the electric field results in a Dzyaloshinsky-Moriya interaction, which then produces a linear shift of the spin wave frequencies \footnote{T. Liu et al., Phys. Rev. Lett. \textbf{106}, 247203 (2011)}. In this work we consider the effects of a periodically varying electric field on a slab of YIG. The spin wave frequencies and linewidths of the system are obtained from the Landau-Lifshitz-Gilbert equation using the plane-wave expansion method. We demonstrate that the periodic variation of the electric field opens band gaps in the spin wave dispersion relations. A band gap width of several hundred MHz is observed when the electric field strength is alternating between 0 and $8 \cdot 10^7$ V/m over a length scale of 200 nm. The frequency and width of these band gaps can be tuned by adjusting the electric field strength and the lattice constant associated with the periodicity, and quality factors on the order of 100 can be achieved. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E6.00005: Measuring magnon propagation in magnonic crystals at millikelvin temperatures Alexy Karenowska, Arjan van Loo, Richard Morris, Sandoko Kosen, Andrii Chumak, Alexander Serga, Burkard Hillebrands Magnon systems are increasingly widely recognized as a potential basis for solid-state quantum information processing. Propagating magnons are readily excited over the same range of microwave frequencies as are used in established quantum circuit technologies, and couple readily to electromagnetic fields. These facts, in combination with the relatively slow speeds and highly tunable dispersion of the excitations, make them a particularly interesting proposition in the context of quantum devices. Here, we present the first experimental study of microwave-frequency magnonic crystals (magnon systems with an artificially engineered bandgap) at millikelvin temperatures. Our magnonic crystals were prepared by etching grooves into a magnon waveguide made from a film of the magnetic insulator, yttrium iron garnet. The high signal-to-noise afforded by our low-temperature measurement environment makes it possible to make detailed observations of the dispersion of externally excited propagating magnon modes within the crystals. Our results lead us to suggest a range of device applications of dispersion-engineered magnonic systems in the context of microwave-circuit based quantum information processing. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E6.00006: Torque-mixing Magnetic Resonance Spectroscopy Joseph Losby, Fatemeh Fani Sani, Dylan Grandmont, Zhu Diao, Miro Belov, Jacob Burgess, Shawn Compton, Wayne Hiebert, Doug Vick, Kaveh Mohammad, Elham Salimi, Gregory Bridges, Douglas Thomson, Mark Freeman A universal, mechanical torque method for magnetic resonance spectroscopy is presented. In analogy to resonance detection by induction, a signal proportional to the transverse component of a precessing dipole moment can be measured as a pure mechanical torque in broadband, frequency-swept spectroscopy. Comprehensive electron spin resonance of a single-crystal, mesoscopic yttrium iron garnet disk at room temperature are presented to demonstrate the method. The rich detail allows analysis of even complex 3D spin textures. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E6.00007: Ferromagnetic resonance of a YIG film in the low frequency regime John Ketterson, Scott Grudichak, Joseph Sklenar, C. C. Tsai, Moongyu Jang, Qinghui Yang, Huaiwu Zhang, Seongjae Lee An improved method for characterizing the magnetic anisotropy of films with cubic symmetry is described and is applied to an yttrium iron garnet (111) film. Analysis of the FMR spectra performed both in-plane and out-of-plane from 0.7 to 8 GHz yielded the magnetic anisotropy constants as well as the saturation magnetization. The field at which FMR occurs is sensitive to anisotropy constants in the low frequency (< 2 GHz) regime and when the orientation of the magnetic field is nearly normal to the sample plane; the restoring force on the magnetization arising from the magnetocrystalline anisotropy fields is then comparable to that from the external field, thereby allowing the anisotropy constants to be determined with greater accuracy. Work at Northwestern was supported by the US DOE, Office of Basic Energy Sciences, Materials Science and Engineering Division under grant number DE-SC0014424. The film growth was supported by the National Natural Science Foundation of China (NSFC) under Grants 51272036 and 51002021 and 51472046. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E6.00008: Current driven asymmetric domain wall propagation Chirag Garg, Aakash Pushp, Timothy Phung, See-Hun Yang, Brian P. Hughes, Charles Rettner, Stuart S.P. Parkin In ultrathin magnetic heterostructures, the presence of spin-orbit coupling gives rise to chiral Neel walls which are stabilized by the Dzyaloshinskii-Moriya Interaction (DMI), and also to a highly efficient chiral spin torque mechanism. In straight nanowires, the current-driven propagation of alternating N\'{e}el DWs without the presence of an in-plane field is equivalent, leading to the lock-step motion of several DWs in a nanowire. Here, we show that by engineering the structure in which the domain walls propagate, which in our case is in the shape of a Y-shaped junction, the DW propagation process becomes selective to the polarity of the DWs even in the absence of any externally applied magnetic fields. We remarkably find that after splitting at the Y-shaped junction, the DW velocity in one branch remains largely unaffected compared to its initial velocity whereas simultaneously the DW velocity in the other branch decreases by as much as 10-90{\%}. We show that this large change in the DW velocity in a particular branch depends on the relative angle between the local magnetization of the DW and the spin current emanating from the underlying heavy-metal layer in these nanowires. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E6.00009: Notch-Boosted Domain Wall Propagation in Magnetic Nanowires Xiang Rong Wang, Hauiyang Yuan Magnetic domain wall (DW) motion along a nanowire underpins many proposals of spintronic devices. High DW propagation velocity is obviously important because it determines the device speed. Thus it is interesting to search for effective control knobs of DW dynamics. We report a counter-intuitive finding that notches in an otherwise homogeneous magnetic nanowire can boost current-induced domain wall (DW) propagation. DW motion in notch-modulated wires can be classified into three phases: 1) A DW is pinned around a notch when the current density is below the depinning current density. 2) DW propagation velocity above the depinning current density is boosted by notches when non-adiabatic spin-transfer torque strength is smaller than the Gilbert damping constant. The boost can be many-fold. 3) DW propagation velocity is hindered when non-adiabatic spin-transfer torque strength is larger than the Gilbert damping constant. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:48AM |
E6.00010: Magnetic domain walls as reconfigurable spin-wave nano-channels Invited Speaker: Kai Wagner Research efforts to utilize spin waves as information carriers for wave based logic in micro- and nano-structured ferromagnetic materials have increased tremendously over the recent years [1,2]. However, finding efficient means of tailoring and downscaling guided spin-wave propagation in two dimensions, while maintaining energy efficiency and reconfigurability, still remains a delicate challenge. \newline Here we target these challenges by spin-wave transport inside nanometer-scaled potential wells formed along magnetic domain walls. For this, we investigate the magnetization dynamics of a rectangular-like element in a Landau state exhibiting a so called 180° Néel wall along its center. By microwave antennae the rf-excitation is constricted to one end of the domain wall and the spin-wave intensities are recorded by means of Brillouin-Light Scattering microscopy revealing channeled transport. Additional micromagnetic simulations [3] with pulsed as well as cw-excitation are performed to yield further insight into this class of modes. \newline We find several spin-wave modes quantized along the width of the domain wall yet with well defined wave vectors along the wall, exhibiting positive dispersion. In a final step, we demonstrate the flexibility of these spin-wave nano-channels based on domain walls. In contrast to wave guides realised by fixed geometries, domain walls can be easily manipulated. Here we utilize small external fields to control its position with nanometer precision over a micrometer range, while still enabling transport. Domain walls thus, open the perspective for reprogrammable and yet non-volatile spin-wave waveguides of nanometer width. \newline \newline [1]: A. V. Chumak, V. I. Vasyuchka, and B. Hillebrands, Nat. Phys. 11, 453-461 (2015). \newline [2]: D. Grundler, Nat. Phys. 11, 438-441 (2015). \newline [3]: A. Vansteenkiste, J. Leliaert, M. Dvornik, M. Helsen, F. Garcia-Sanchez and B. van Waeyenberge, AIP Advances 4, 107133 (2014). \newline [4]: K. Vogt, F. Y. Fradin, J. E. Pearson, T. Sebastian, S. D. Bader, B. Hillebrands, A. Hoffmann and H. Schultheiss H. , Nat. Comm. 5, 3727 (2014). [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E6.00011: Studying Kittel-like modes in a 3D YIG disk using Torque-mixing Magnetic Resonance Spectroscopy Fatemeh Fani Sani, Joseph Losby, Dylan Grandmont, Zhu Diao, Miro Belov, Jacob Burgess, Shawn Compton, Wayne Hiebert, Doug Vick, Kaveh Mohammad, Elham Salimi, Gregory Bridges, Douglas Thomson, Mark Freeman We report a study of ferrimagnetic resonance in a mesoscopic, single-crystalline YIG disk using torque-mixing magnetic resonance spectroscopy (TMRS). The Kittel model for magnetic resonance is a touchstone in measuring fundamental magnetic properties for magnetic films, which does not significantly depend on the film size. In 3D structures, ladders of confined resonance modes are observed, and these can exhibit the non-monotonic evolution of frequency with field familiar from Kittel modes. TMRS is a tool uniquely suited for observing this physics in individual 3D structures, on account of its combination of high sensitivity and broadband capability coupled with fine frequency resolution. [Preview Abstract] |
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