Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A19: Focus Session: Spin Transport & Magnetization Dynamics in Metals I |
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Sponsoring Units: GMAG DMP Chair: Dario Arena, Brookhaven National Laboratory Room: D170 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A19.00001: Electrically detected ferromagnetic resonance studies of individual Permalloy nanowires Zheng Duan, Carl Boone, Ilya Krivorotov, Nathalie Reckers, Juergen Lindner, Michael Farle We report measurements of electrically-detected broadband ferromagnetic resonance (FMR) of individual lithographically-defined Permalloy nanowires. For these measurements, the nanowire is placed near a shorted end of a coplanar strip waveguide, and magnetization precession in the nanowire is excited by microwave power applied to the waveguide. Resistance of the nanowire is measured via four leads attached to the nanowire. The amplitude of the magnetization precession as a function of dc bias magnetic field and microwave frequency is electrically detected via two independent methods: (i) measurements of time-average resistance (photoresistance) arising from anisotropic magnetoresistance (AMR) and (ii) measurements of the rectified voltage (photovoltage) arising from AMR resistance oscillations and inductive microwave current in the nanowire. Using these complementary techniques, we make ferromagnetic resonance measurements as a function of frequency and magnetic field applied to the nanowire. We will discuss the dependence of the resonance frequency and linewidth on the nanowire width and compare our results to the results of conventional ferromagnetic resonance measurements of unpatterned Permalloy films. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A19.00002: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 8:24AM - 8:36AM |
A19.00003: Parametric excitation of a magnetic nanocontact by a microwave field Sergei Urazhdin, Vasil Tiberkevich, Andrei Slavin We demonstrate that magnetic oscillations of a current-biased magnetic nanocontact can be parametrically excited by a microwave field applied at twice the resonant frequency of the oscillation. The threshold microwave amplitude for the onset of the oscillation decreases with increasing bias current, and vanishes at the transition to the auto-oscillation regime. The dependence of parametric excitation on the driving frequency is strongly asymmetric, which is caused by the nonlinearity of the studied dynamical system. Based on our observations and analysis, we propose a simple quantitative method for the characterization of magnetic nanoelements. We show that by measuring the threshold and frequency range of parametric excitation, it is possible to determine damping, spin-polarization efficiency, and coupling coefficient to the microwave signal. In addition, by measuring the frequency range of parametric synchronization in the auto-oscillation regime, one can independently determine the dynamic nonlinearity of the nanomagnet. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A19.00004: Spin wave emission patterns from a point source in a magnonic crystal Glade Sietsema, Michael E. Flatt\'e We have calculated the spin-wave emission patterns for a point source of spin waves, such as a spin-torque oscillator, embedded within a two-dimensional magnonic crystal. The magnonic crystal consists of cylinders of one magnetic material embedded within another, in a square or hexagonal lattice. Spin wave frequencies and linewidths have been calculated using the Landau-Lifshitz-Gilbert equation[1] and the emission patterns calculated from the resulting Green's function of the spin-wave system. As the frequency of the spin torque oscillator increases the emission patterns change from roughly isotropic to highly anisotropic, demonstrating efficient spin-wave transfer along certain crystallographic directions.\\[4pt] [1] Tiwari and Stroud, PRB 81, 220403 (2010) [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A19.00005: Microwave Measurements of Giant Magnetoresistance Daniel Endean, James Heyman, Stefan Maat, E. Dan Dahlberg ~A measurement of the GMR effect seen in the reflection and transmission coefficients of a film placed in a rectangular microwave waveguide bridge is presented. ~The relative change in transmission coefficient is found to be nearly a factor of 2 larger than the transport GMR effect while the change in reflection coefficient is of opposite sign and an order of magnitude smaller. ~A full treatment of the reflection and transmission coefficients, considering both the interfaces and the decay of the fields within the layers, provides a quantitative relationship to the transport GMR with no free parameters. ~We describe this model and show agreement with experiment. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A19.00006: Nanoscale spin wave localization using magnetic resonance force microscopy Han-Jong Chia, Robert McMichael We report on a novel technique for exciting localized spin wave modes in ferromagnetic thin films using the magnetostatic field from a soft magnetic tip on a scanned probe. Unlike previous studies [1,2] that used permanent magnet tips and applied fields at or near the film normal, we use a configuration of ferromagnetic resonance force microscopy (FMRFM) where the applied field, sample magnetization and probe magnetization are all aligned parallel to the film plane. In this configuration, the dipole field of the tip creates a minimum in the net applied field where spin waves are localized. Our experiments confirm the presence of localized spin waves. Micromagnetic modeling is used to generate images of the localized spin waves at various tip-sample separations. These images indicate that the localized modes exist in a region that is smaller than the tip diameter and that they have the form of standing waves with wave vectors parallel to the applied field. Our technique combined with micromagnetic modeling presents a pathway for obtaining magnetic resonance imaging (MRI)-like spatial maps in ferromagnetic films with submicron resolution. \\[4pt] [1] I. Lee et al., \textit{Nature} \textbf{466}, 845 (2010). \\[0pt] [2] E. Nazaretski et al., \textit{Phys. Rev. B}, \textbf{79}, 132401 (2009). [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:48AM |
A19.00007: Perpendicular ferromagnetic resonance measurements of damping and the Land\'{e} g-factor in sputtered (Co$_{2}$Mn)$_{(1-x)}$Ge$_{x}$thin films Invited Speaker: We analyzed vector network analyzer-ferromagnetic resonance data for sputtered polycrystalline (Co$_{2}$Mn)$_{(1-x)}$Ge$_{x}$ thin films measured in a perpendicular configuration to minimize two magnon scattering. The films were grown with varying Ge content and subjected to post-deposition annealing at 200, 245, and 300$^{\circ}$C. We can adequately fit the data with the slow relaxing impurity model for damping, similar to what was successfully used to explain enhanced damping in RE- doped Permalloy films. However, it was required to generalize the theory to include coherence effects that modify the original fluctuating field correlation function from a damped exponential to an exponentially damping cosine. We find that the spectroscopic splitting factor $g$ is a clearly decreasing function of Ge content for 245 and 300$^{\circ}$C anneal samples. Analysis of the content dependence for $g$ provides strong evidence of a significant negative spin polarization between -0.15 and -0.35 spins at the Ge sites. This is consistent with our analysis of magnetometry data in the context of generalized Slater Pauling (GSP) theory, which presumes that the minority band density of states has a deep minimum at the Fermi energy. GSP analysis yields a spin polarization of -0.25 at the Ge sites. The substantial antiferromagnetic spin polarization of the Ge sites, in addition to the correlation of the slow relaxing damping strength with Ge content, suggests that Ge atoms, perhaps in the form of point defects in the Co sub-lattice, are acting as the slow relaxing impurities. Finally, successful fitting of linewidth data with a model that includes coherence during the relaxation process indicates slight transverse as well as longitudinal exchange coupling between the Ge ``impurities'' and the magnetization, giving rise to mixing of the electronic energy levels responsible for the relaxation process. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A19.00008: Dependence of Gilbert damping on number of bilayers in perpendicularly magnetized Co/Ni multilayers See-Hun Yang, Weifeng Zhang, Hyonseok Song, Sung-Chul Shin, Georg Woltersdorf, Markus Haertinger, Christian Back, Stuart Parkin Magnetic materials in which their magnetic moment direction is oriented perpendicular to the plane of the magnetic layers in thin film heterostructures have been much studied for their potential application to spintronic devices. In particular, theories of current induced excitation, via the phenomenon of spin torque transfer, show that perpendicularly magnetized layers can be more easily excited or their magnetization direction switched than in-plane magnetized layers. The current density required for switching is directly proportional to the Gilbert damping within the magnetic layers. We have studied the dependence of Gilbert damping on the number of bilayers in multilayers formed from alternating Co and Ni layers. We compare results from time-resolved, ultrafast pump-probe magneto-optical Kerr effect measurements with those from strip-line and cavity ferromagnetic resonance techniques. We find that the Gilbert damping parameter has a weak dependence on the number of bilayers. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A19.00009: Amorphous Gd-Fe-Co as Prospective Material for Perpendicular STT-MRAM Manli Ding, Joseph Poon, Jiwei Lu, Tim Mewes A number of Rare-Earth-Fe-Co alloys are known to have strong magneto-crystalline anisotropy, giving rise to out-of-plane easy magnetization in thin films. Since Gd is in the L=0 state and there is no spin-orbit coupling, Gd alloys are favored to have low Gilbert damping. The Gd ferromagnetic sublattice couples antiferromagnetically with the Fe(Co) ferromagnetic sublattice. Perpendicular anisotropy exists near the compensation point, where the magnetization is small. Two magnetization compensation ranges are found in GdFeCo, with one existing at high Gd content and one at low Gd content. At higher Gd{\%}, a high coercive field $\sim $ 5 kOe and a low Gilbert damping $\sim $ 0.03 are measured. At lower Gd{\%}, a much lower coercive field $\sim $ 300 Oe is measured. High temperature treatment causes a degradation of the perpendicular anisotropy. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A19.00010: Direct observation of four-magnon scattering in spin-wave micro-conduits Helmut Schultheiss, Katrin Vogt, Philipp Pirro, Thomas Braecher, Burkard Hillebrands We report on experiments which demonstrate the intrinsic nonlinear damping of spin waves due to four-magnon scattering processes in a micrometer sized permalloy stripe. The magnetization is excited by a microwave current transmitted through the shorted end of a coplanar waveguide. The excitation spectrum of the spin waves is locally probed by Brillouin light scattering microscopy for different excitation frequencies covering a wide range of excitation powers over three orders of magnitude. We find a transition from a pure and clean monochromatic excitation of spin waves at low microwave powers to a large broadening above a certain threshold power. The spectral distribution of the measured spin-wave intensities shows a unique profile which is in good agreement with theoretical expectations for four-magnon scattering processes. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A19.00011: Elastically driven ferromagnetic resonance in nickel thin films M. Weiler, C. Heeg, H. Huebl, R. Gross, S.T.B. Goennenwein, L. Dreher, M.S. Brandt Due to magneto-elastic coupling, magnetic degrees of freedom are influenced by elastic deformation. We here demonstrate that the magneto-elastic interaction of a radio frequency (RF) surface acoustic wave (SAW) with a ferromagnetic thin film enables an all-elastic excitation and detection of ferromagnetic resonance (FMR). We have measured the SAW magneto-transmission at room temperature in Ni/LiNbO$_3$ hybrid devices as a function of SAW frequency, external magnetic field magnitude and orientation. Our data are consistently described by a modified Landau-Lifshitz-Gilbert approach [1], in which the magnetization precession is not driven by a conventional, external RF magnetic field, but rather by a purely virtual, internal tickle field stemming from RF magneto-elastic interactions. This causes a distinct magnetic field orientation dependence of elastically driven FMR, which we observe in both simulations and experiment. \newline This work is financially supported by the Deutsche Forschungsgemeinschaft via project GO 944/3-1, SFB 631, and the excellence cluster Nanosystems Initiative Munich (NIM). \newline [1] M. Weiler \textit{et al.} arXiv:1009.5798 [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A19.00012: Time-resolved probing of magnon mass renormalization in epitaxial Fe films Vladimir Stoica, Christian Schlepuetz, Donald Walko, Yuelin Li, Eric Dufresne, Eric Landahl, Roy Clarke Irradiation of ferromagnetic metals with femtosecond laser pulses leads to sub-picosecond ultrafast demagnetization, followed by coherent spin wave dynamics on the picosecond to nanosecond timescales. Presently, it is of high interest to develop a cohesive picture that consistently accounts for these experimental observations. One way to address this is to refine the experimental techniques for improving the quantitative comparison with theory. Here, we present in-detail investigations of the coherent exchange spin waves in epitaxial Fe films, which are used for accurate determination the spin wave stiffness constant, D. These studies enabled to detect the effect of femtosecond laser excitation on D and correlate the results with time-resolved X-ray diffraction measurements of the thermal relaxation. Our data provide evidence for the magnon mass renormalization induced by electron-magnon interaction. Preliminary data obtained in Ni and Co seem to indicate the same effect. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A19.00013: Taking Ferromagnetic Resonance to Millikelvin Temperatures Hans Huebl, Christoph Zollitsch, Fredrik Hocke, Mathias Weiler, Martin Radlmeier, Karl Neumaier, Sebastian T.B. Goennenwein, Rudolf Gross Ferromagnetic Resonance (FMR) is a sensitive tool for the investigation of magnetic anisotropy and magnetization damping in thin magnetic films. Broadband FMR based on coplanar waveguide technology hereby is particularly attractive as it allows for the investigation of plain films as well as of single magnetic nanostructures. We here present broadband FMR data of 50~nm thick nickel and cobalt thin films, recorded at temperatures range from 4.2~K down to 50~mK. The excellent sensitivity of our setup allows for the detection of FMR with an incident microwave power of 100~fW at the base temperature of the dilution refrigerator. Our FMR measurements in Co and Ni reveal no distinct temperature dependence of the anisotropy and damping parameters in the temperature regime below 4.2~K, which appears consistent with the trend observed in measurements from room temperature down to 4.2~K. Our proof-of-principle experiments open the path for broadband FMR studies of magnetic anisotropy and magnetization damping at millikelvin temperatures a regime so far very scarcely explored. This project is financially supported by the Deutsche Forschungsgemeinschaft via SFB 631 and the Cluster of Excellence Nanosystems Initiative Munich (NIM). [Preview Abstract] |
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