Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W8: Focus Session: Ferromagnetic Resonance and Damping |
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Sponsoring Units: GMAG Chair: Hans Nembach, University of Colorado Boulder Room: 104 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W8.00001: Correlation of spin-wave mode structure and shape imperfections in individual Ni80Fe20 nanomagnets via heterodyne magneto-optic microwave microscopy Hans Nembach, Justin Shaw, Carl Boone, Robert McMichael, Tom Silva It was recently shown that modes localized at the edges are sensitive to presumed defects. We measured localized spin-wave modes of individual Ni80Fe20 nanomagnets (NMs) with sizes ranging from 100 nm to 400 nm via heterodyne magneto-optical microwave microscopy. Comparison of field-swept spectra with micromagnetic simulations allows for identification of the observed spin-wave modes. One of the modes, the ``center-mode'', extends throughout the NM. The lowest order (highest resonance field) ``end-modes'' are localized at the ends of the nanomagnet. As such, it is expected that the end modes are more susceptible to edge defects. Spectra from nominally identical nanomagnets show that the resonance fields of the two end-modes vary substantially between nanomagnets.. We measured the lateral shape of the NMs with scanning electron microscopy, and then used the measured shapes to simulate the mode-spectra, but shape distortions cannot explain the observed mode distortions. Sidewall angle, re-deposition, and mill-induced edge-damage might also be important to accurately model end-mode distortions. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W8.00002: Splitting of spin-wave modes in magnetic nanostructures under axial symmetry violation Martha Pardavi-Horvath, Olena Tartakivska, Olga Salyuk During the last two decades the spin wave dynamics in magnetic nanostructures is a topic of intensive study. However, the analytical theoretical description of spin waves in such confined structures is a complicated problem. Due to the inhomogeneity of the internal demagnetizing field, it is possible to find exact eigenfunctions for only selected cases with the simplest geometries. For practical applications the direction of the external magnetic field may deviate from the symmetry axis. Additional spin-wave modes can be the source of magnetic noise. Therefore, it is important to investigate the evolution of the spin wave spectra in the case of symmetry violation. In this work the evolution of spin wave spectra of submicron circular dots and cylindrical nanowires have been studied for the case when the magnetic field deviates from the symmetrical (parallel to the normal) direction. It is shown that for such geometry the symmetry violation leads to a splitting of spin-wave modes, and that the number of the split peaks depends on the mode number. A quantitative description of the spectra is given using a simple perturbation theory. The role of boundary conditions (pinned, mixed or free) is discussed. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W8.00003: Wavenumber dependent Gilbert damping in metallic ferromagnets Yi Li, William Bailey New terms to the dynamical equation of magnetization motion, associated with spin transport, have been reported over the past several years. Each newly identified term is thought to possess both a real and an imaginary effective field leading to fieldlike and dampinglike torques on magnetization. Here we show that three metallic ferromagnets possess an imaginary effective-field term which mirrors the well-known real effective-field term associated with exchange in spin waves. Using perpendicular standing spin wave resonance between 2-26 GHz, we evaluate the magnitude of the finite-wavenumber ($k$) dependent Gilbert damping of the uniform mode ($\alpha_u$) and the first spin wave mode ($\alpha_s$) in three typical ferromagnets, Ni$_{79}$Fe$_{21}$, Co, and Co$_{40}$Fe$_{40}$B$_{20}$. By taking the difference of $\alpha _s$ and $\alpha _u$ and excluding the eddy current damping $\alpha_E$ ($\Delta\alpha_k=\alpha_s-\alpha_u+\alpha_E$), we find the presence of a $k^2$ term, as $\Delta\alpha_k=\Delta\alpha_0+A_{k}\cdot k^2$ in all three metals. We interpret the new term as the continuum analog of spin pumping, predicted recently, and show that its magnitude, $A_{k}$=0.07-0.1 nm$^2$, is consistent with transverse spin relaxation lengths (1-3 nm) as measured by conventional spin pumping. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W8.00004: Nonlinear ferromagnetic resonance shift in nanostructures Feng Guo, Lyuba Belova, Robert McMichael In dynamic magnetic systems, various experiments have shown that the ferromagnetic resonance frequency can shift up or down with increasing driving power in the nonlinear regime. The resonance shift is important in understanding nonlinear physics in nanomagnets and for applications of spin-torque oscillators. Here, we present a systematic study on the sign of the nonlinear coefficient, i.e. the direction of the resonance field/frequency shift. We use ferromagnetic resonance force microscopy (FMRFM) to measure the ferromagnetic resonance of a series of submicron NiFe ellipses with varying aspect ratios. We find the sign of the resonance shift is determined by both the applied field and the anisotropy field. Our measurement and micromagnetic modeling results are in qualitative agreement with a macro-spin analysis developed by Slavin and Tiberkevich [1]. However, both measurement and modeling results exhibit values of the nonlinear coefficient that are more positive (meaning that the resonance tends to shift toward low field direction) than are predicted by the macrospin model. We attribute the difference to the non-uniformity of the precession modes in the ellipses. By analogy with standing spin waves, we show that nonuniform precession tends to increase the nonlinear frequency coefficient through a magnetostatic mechanism. [1] A. Slavin and V. Tiberkevich, IEEE Trans. Mag., 45, 1875 (2009). V. Tiberkevicha, I. Krivorotovb, G. Gerhartc and A. Slavin, J. Magn. Magn. Mater., 321 (2009) L53 [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W8.00005: Ferromagnetic Resonance in a Height Modulated Permalloy Film Joseph Sklenar, Seongjae Lee, Phillip Tucciarone, Rok-Jun Lee, Daniel Tice, Ivan Nevirkovets, Olle Heinonen, John Ketterson We have performed ferromagnetic resonance experiments on permalloy films that are deposited on a colloidal crystal template. The colloidal crystal substrates we used consisted of polystyrene spheres that were hexagonally close packed with sphere diameter varying between 100-300 nm. On a single substrate only one sphere diameter was used. When sputtered onto the colloidal crystal the permalloy film is no longer uniform and obtains a periodic height modulated perturbation from the underlying spheres. When performing FMR experiments we varied the in-plane magnetic field and observed two main anisotropic modes with an angular dependence obeying an expected six-fold symmetry from the underlying perturbations. To explain the origin of these modes we will also present micromagnetic simulations. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W8.00006: Damping of perpendicular standing spin waves via VNA-FMR in sputtered Py/Ta films Thomas Silva, Martin Schoen, Hans Nembach, Justin Shaw, Carl Boone, Mathias Weiler, Mikhail Kostylev Bar'yakhtar first proposed that non-local damping processes $\propto \nabla ^{2}m$ are intrinsic in systems with exchange splitting. A recent theory from Tserkovnyak, \textit{et al.}, substantiates Bar'yakhtar's claim with quantitative estimates based upon s-d exchange and diffusive spin transport models. We measured mode-dependent damping in 10-nm-thick nanomagnets. Data were in qualitative agreement with Bar'yakhtar/Tserkovnyak theory, though the magnitude of the effect was far greater than expected, suggestive that spin-orbit/interface effects are important (PRL 110, 117201). To test the theory further, we measured field-swept spectra of perpendicular standing spin waves up to wavenumber $k=2\times 10^{6}$ cm$^{-1}$ ($\approx 30$ GHz) in Permalloy/Ta films with Py thicknesses from 50 nm to 200 nm by use of vector-network-analyzer FMR. The spectra are fitted simultaneously with multiple complex susceptibilities to account for inter-mode interference. To account for eddy currents, results are compared to a 1-d electrodynamic model that solves the Maxwell and Landau-Lifshitz equations. The damping data shows no clear trend with $k$, indicating that nonlocal effects are too small to observe for bulk spin waves over the accessible range in $k$-space. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W8.00007: Detection of microwave phase variation in nanometer-scale magnetic heterostructures Invited Speaker: Cheng Cheng The internal phase profile of electromagnetic (EM) radiation determines many functional properties of metal, oxide, or semiconductor heterostructures. In magnetic heterostructures, emerging spin electronic phenomena depend strongly upon the phase profile of the magnetic field $\tilde{H}$ at gigahertz frequencies. Here we demonstrate nanometer-scale, layer-resolved detection of EM phase through the rf magnetic field $\tilde{H}_{\textrm{rf}}$ in magnetic heterostructures. Time-resolved x-ray magnetic circular dichroism reveals the local phase of $\tilde{H_{\textrm{rf}}}$ acting on individual magnetizations $\tilde{M_{i}}$ through the susceptibility as $\tilde{M}=\tilde{\chi}\tilde{H}_{\textrm{rf}}$. An unexpectedly large phase variation, $\sim$ 40$^{\circ}$, is detected across spin-valve trilayers driven at 3 GHz. The results have implications for the identification of novel effects in spintronics and suggest general possibilities for EM phase profile measurement in heterostructures. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W8.00008: Amplitude and phase of perpendicular standing spin waves via VNA-FMR in sputtered Py/Ta films Martin A. Schoen, Hans T. Nembach, Mathias Weiler, Justin M. Shaw, Carl T. Boone, Mikhail Kostylev, Thomas J. Silva The first observation of perpendicular standing spin waves via ferromagnetic resonance in thin films dates back to the mid `50s. However, phase-sensitive, broad-band FMR methods have only recently been developed with sufficient signal-to-noise to examine the dependence of spin-wave phase on mode index. We measured field-swept complex spectra of perpendicular standing spin waves (PSSWs) up to wavenumber 2x10$^{6}$ cm$^{-1}$ (30 GHz) in Py/Ta films of different thickness with vector-network-analyzer FMR with coplanar waveguide excitation. The spectra are fitted simultaneously with multiple complex susceptibilities to account for inter-mode interference. The spin-wave stiffness constant D and the pinning-parameter $\varepsilon $ are determined from a quadratic fit of the exchange field to the mode number. For all film thickness, $\varepsilon $ is indicative of weak pinning. Mode amplitude vs. excitation direction points to a dead layer at the Py/Ta interface. We observe strong inter-mode phase variations, especially for thinner (\textless 100 nm) films. Results are compared to a 1-d electrodynamic model that simultaneously solves the Maxwell and Landau-Lifshitz equations. The observed phase shifts are not expected if substrate conductivity is ignored. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W8.00009: Phase-sensitive detection of both inductive and non-inductive ac voltages in ferromagnetic resonance Mathias Weiler, Justin M. Shaw, Hans T. Nembach, Martin A. Schoen, Carl T. Boone, Thomas J. Silva Spin pumping causes significant damping in ultrathin ferromagnetic/normal metal (NM) multilayers via spin-current generation of both dc and ac character in the NM system. While the nonlinear dc component has been investigated in detail by utilization of the inverse spin Hall effect (iSHE) in NMs, much less is known about the linear ac component that is presumably much larger in the small-excitation limit. We measured generated ac voltages in a wide variety of Permalloy/NM multilayers via vector-network-analyzer ferromagnetic resonance. We employ a custom, impedance-matched, broadband microwave coupler that features a ferromagnetic thin film reference resonator to accurately compare ac voltage amplitudes and phases between varieties of multilayers. By use of the fact that inductive and ac iSHE signals are phase-shifted by $\pi$/2, we find that inductive signals are major contributors in all investigated samples. It is only by comparison of the phase and amplitude of the recorded ac voltages between multiple samples that we can extract the non-inductive contributions due to spin-currents. Voltages due to the ac iSHE in Permalloy(10nm)/platinum(5nm) bilayers are weaker than inductive signals, in agreement with calculations based upon recent theoretical predictions. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W8.00010: Ferrimagnetic resonance in thin film organic-based magnets Howard Yu, Rohan Adur, Yu Lu, Megan Harberts, Arthur J. Epstein, P. Chris Hammel, Ezekiel Johnston-Halperin Recent advances in spintronics suggest that the high frequency response of ferromagnetic materials is an attractive path to generating pure spin currents. Further, experiments in inorganic systems indicate that the linewidth of the ferromagnetic resonance (FMR) is an important metric for FMR driven spin injection. Here we perform magnetic resonance measurements on the organic-based ferrimagnetic semiconductor V[TCNE]$_{\mathrm{x\sim 2}}$, consisting of vanadium ions in a network of organic linking molecules. We observe a single resonance with an extremely sharp linewidth, on the order of 1 Oe (yttrium iron garnet, YIG, has a comparable linewidth). Previous studies of V[TCNE]$_{\mathrm{x\sim 2}}$ show similar linewidths but with many peaks in the spectrum, indicating that our results represent a significant improvement in sample homogeneity. Finally, we also demonstrate the ability to manipulate the magnetic properties through chemical modification of the organic linker, yielding thin films of V[MeTCEC] and V[EtTCEC]. These studies demonstrate the potential for high frequency all-organic spintronic and magnetoelectronic devices. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W8.00011: Yttrium Iron Garnet Thick Films Formed by the Aerosol Deposition Method for Microwave Inductors Scooter Johnson, Harvey Newman, E.R. Glaser, Shu-Fan Cheng, Marko Tadjer, Fritz Kub, Charles Eddy, Jr. We have employed the aerosol deposition method (ADM) to direct-write 40 $\mu$m-thick polycrystalline films of yttrium iron garnet (YIG, Y$_3$Fe$_5$O$_{12}$) at room temperature onto patterned gold inductors on sapphire substrates at a deposition rate of 1--3 $\mu$m/min as a first step toward integration into microwave magnetic circuits. A challenge to integrating magnetic films into current semiconductor technology is the high-temperature regime (900--1400$^\circ$C) at which conventional ferrite preparation takes place. The ability of the ADM to form dense, thick films at room temperature makes this a promising approach for integrated magnetics where low-temperature deposition and thick films are required. The ADM YIG film has an rms roughness of 3--4 $\mu$m, is comprised of nano-crystalline grains with a density 50\% of the theoretical value. XRD patterns of the as-deposited film and starting powder indicate a polycrystalline single-phase film. In-plane VSM and FMR measurements reveal a saturation of 22 emu/g, coercivity of 27 Oe, and linewidth of 360 Oe. Early measurements of air-filled and YIG-filled gold inductors between 0.01--10 GHz indicate an improved inductance of nearly a factor of 2 at low frequency. At higher frequency, resonance effects diminish this improvement. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W8.00012: Magnon excitation and decay in Ferromagnetic Insulator/metal multilayers Tao Liu, jiaxi Li, Jianwei Zhang We studied magnon excitation in a Ferromagnetic Insulator(FI) layer(such as YIG), which connected two Ferromagnetic/normal metal multilayers in two sides. In our modeling, we adopted self-consistent spin dependent Boltzmann equations in metal layers and magnon Boltzmann equation in FI layer. When applying an in-plane current in first FM layer, a transverse spin current was generated due to Anomalous Hall effect(AHE), after crossing normal metal layer, this transverse spin current will produce magnon excitation at N/FI interface. With carrying spin information, magnon excitations in FI can eventually excite a new spin current at second F/N interface. Although the FI cannot support any spin current propagation across it, but spin polarization information was passed through FI with propagation of magnon. Finally, the transverse spin current in second FM layer can also generate another in-plane spin current by AHE. The magnon excitation in FI layer is dominated by the interfacial interaction at Normal/FI boundary. Our results show the magnon in FI layer have decay behaviors to low energy model. We also showed that when applying a magnetic field on FI layer, spin current in final FM layer can be manipulated by varying magnon excitation. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W8.00013: Resonant damping in $Fe_{3} O_{4} $/Ag below the Verwey Transition Michael Sinko, Daniel Stanley, Michael Pechan, P.B. Jayathilaka, Casey Miller The temperature dependence of damping in epitaxial $Fe_{3} O_{4} $/Ag bilayers grown on [001] MgO substrates was investigated as a function of Ag thickness. The $Fe_{3} O_{4} $ layers were fixed at 350 nm thick, while the Ag thicknesses ranged from 0 to 500 nm. The epitaxial nature of the films was verified by in plane XRD of the $Fe_{3} O_{4} $ (311) and (220) directions. Ferromagnetic resonance (FMR) measurements at 9.2 GHz were carried out with the sample film normal to the applied magnetic field and at temperatures ranging from 30 to 295 K. All samples exhibited easy-plane anisotropy consistent with thin-film shape effects and a sample magnetization of approximately 400 $emu/cm^{3}$. Room temperature resonance line-widths were largely independent of Ag layer thickness and remained fairly constant with decreasing temperature until the Verwey transition ($T_{V} \simeq 110 K$), below which damping increases dramatically for all samples. Of particular note is the influence of the Ag layer thickness on the low temperature damping, wherein a peak in magnitude is observed at approximately 50 nm of Ag. This unexpected damping resonance will be discussed in terms of spin pumping into the Ag in conjunction with the changing $Fe_{3} O_{4} $magnetodynamics associated with the Verwey transition. [Preview Abstract] |
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